TableofContentsCoverTitlePageCopyrightPrefaceChapter1:IntroductiontoSoil

WhatIsSoil?NatureandUsesofSoilHowBigIsanAcre?AHectare?

Chapter2:SoilFormationTheRockCycleCompositionoftheEarth'sCrustProcessesofRockWeatheringFactorsofSoilFormationSoilHorizonDevelopmentLet'sTakeaTrip

Chapter3:SoilPhysicalPropertiesSoilPhasesSoilSeparatesSoilTextureSoilStructureBenefitsofAggregationPorosityandDensityCompositionofSoilPoresSoilConsistenceSoilColor

Chapter4:SoilBiologicalPropertiesOrganicMatterandHumusTheCarbonCycleFactorsAffectingSoilOrganicMatterLevelsTheDecompositionProcessFactorsAffectingtheRateofDecomposition

ImportanceofSoilOrganicMatterCarbonSequestrationPlantRootsandtheRhizosphereMicroorganismsTypesofMicroorganismsTheNitrogenCycleImmobilizationandMineralizationDenitrificationBiologicalDecompositionofRocksMacroorganismsPesticideUseandSoilOrganisms

Chapter5:SoilChemicalPropertiesSoilColloidalSystemSilicateClaysOxideClaysCationExchangeAnionExchangeSoilReaction(pH)SoilAggregation

Chapter6:SoilWaterHydrologicCycleSoilWaterStorageandMovementWaterUsebyPlantsDrainageIrrigationWaterConservation

Chapter7:SoilTemperatureImportanceofSoilTemperatureFactorsAffectingEnergyInputsEnergyInputsandTemperatureChangeHeatTransferinSoilsSoilTemperatureFluctuationsManagingSoilTemperature

Chapter8:SoilFertilityandPlantNutrition

SoilFertilityConditionsAffectingLevelandAvailabilityofPlantNutrientsNutrientMobilityinSoilsMethodstoIncreasetheAvailabilityofAddedNutrientsPlantNutritionDeterminingNutrientNeedsAddingPlantNutrientsTimingofFertilizerApplicationPrecisionFarmingOrganicFarming/GardeningCompostingBiosolids

Chapter9:SoilManagementPhysicalConditionTillagePracticesChemicalCharacteristicsBiologicalCharacteristicsCropProductionFactors

Chapter10:SoilConservationandtheEnvironmentErosionProcessesErosionbyWaterErosionbyWindErosionbyMassWastingSedimentasaPollutantExtentoftheProblem

Chapter11:ConservationAgricultureCAPrinciplesCAAdoptionSummaryofCABenefitsBibliography

Chapter12:SoilClassificationandSurveysTheSoilClassificationCategoriesThe12SoilOrdersTimeIstooShortforStrongSoilDevelopment

ClimateIstheDominantFactorinSoilDevelopmentParentMaterialIsSpecificVegetationIsaGrassland(Prairie)Mollisols:GrasslandSoils(6.9%)ClimateandVegetationCombinationDominatesVegetationandParentMaterialDominateSoilHorizonsDescriptionoftheDiagnosticSurfaceHorizons(Epipedons)DescriptionsofSubsurfaceHorizonsOtherDiagnosticSubsurfaceHorizonsSoilMoistureandTemperatureRegimesSoilSurveysLandCapabilityClassesSoilLandscapeAppreciation

Chapter13:SoilandItsUsesUrbanSoilsEngineeringUsesMunicipalWasteDisturbedorContaminatedLands

GlossaryIndexEndUserLicenseAgreement

ListofIllustrationsChapter1:IntroductiontoSoil

Figure1.1Roadbankscanrevealthecomplexityofthesoil.

Figure1.2EarthenhousesarecommoninWestAfrica.

Figure1.3Livingorganismssoonerorlaterbecomeapartofthesoilonceagain.

Figure1.4Anacreis208.7ftonaside;ahectareis328ftonaside.

Chapter2:SoilFormation

Figure2.1Therockcycleshowshowheatandpressure,meltinganderosioncauserockstochangeinformthroughgeologictime.

Figure2.2Igneousrocks.

Figure2.3Sedimentaryrocks.

Figure2.4Exposuretoweatheringcausestinycrackstodevelopinthesurfaceofrocks,whichallowsforchemicalreactionswiththepenetratingsolutions.

Figure2.5Soilsarenaturalfeaturesofthelandscape.

Figure2.6Parentmaterialinatopographiclocationisactedonovertimebyorganismsandclimate.

Figure2.7Bedrockmaybeblanketedbysedimentfromseveralsources.

Figure2.8Representativelandforms.

Figure2.9Grassleavesarenormallyhighestinbases,broadleavesoftreesareintermediate,andconiferneedlesarethelowest.

Figure2.10Inadrainagecatena,thesoilreflectstheeffectsoflong-termmoistureconditions.

Figure2.11Landsurfacestendtobecomesmootherovertimeashillsareworndownandvalleysarefilled.

Figure2.12Theprofileontheleftillustratesasoilfromasubhumidgrassland;theoneontherightshowsasoilsfromahumidhardwoodforestregion.

Figure2.13Bioticcyclinghelpstoconcentratenutrientsnearthesoilsurface.

Figure2.14AtripthroughdifferentclimaticvegetationregionsoftheUnitedStateswouldrevealmanykindsofsoil.

Chapter3:SoilPhysicalProperties

Figure3.1Theapproximateproportionsofvariousphasesbyvolumeinamoistsurfacesoil.

Figure3.2Thesamemassofmineralhasmuchgreatersurfaceareawhenpulverized.

Figure3.3Alayeredclaycrystalissimilarinnaturetoastackofthinsheetsofdough.

Figure3.4Atexturaltriangleshowsthelimitsofsand,silt,andclaycontentofthevarioustextureclasses.

Figure3.5Soilstructuralunitsareclassifiedaccordingtoshapeandsize.

Figure3.6Soilwithouthumusbecomescloddy(left),whereashumus-richsoilisgranular(right).

Figure3.7Whenrocksweather,theybecomeloosenedandlessdenseassoilisformed.

Figure3.8Thezoneofcompactionhasahigherbulkdensityandlowerpermeability.

Chapter4:SoilBiologicalProperties

Figure4.1Atopographicsequenceofsoilsinahumidtemperateclimaticzone.

Figure4.2Thesurfacesoilcontainsmineralparticlesandorganicmatter.

Figure4.3Humus,shownasadarklayer,canbederivedfromleaflitterontheforestfloororfromrootsinsurfacesoil.

Figure4.4Carbonentersthebiospherethroughphotosynthesisandiscycledbackintotheatmospherebydecomposersandbyburning.

Figure4.5Therhizosphereisthevolumeofthesoil,water,andairimmediatelyaroundtheplantroot.

Figure4.6Microorganismsinthesoilareinstrumentalindecomposingplantmaterial,resultingintheformationofhumus.

Figure4.7Thenitrogencycle.

Figure4.8Nodulesontherootsofsomeplants(legumes)containbacteriathatarecapableoftakingnitrogenfromtheairtothebenefitoftheplant.

Figure4.9Nematodesareusuallymicroscopic.Theycanbedestructivetocrops.

Figure4.10Earthwormsareessentialformixingorganicmaterialwithmineralsinthesoil.

Figure4.11Springtailsandmitesplayanimportantroleinthedecompositionofdeadleavesandstems.

Figure4.12Antsareactiveintunnelinginthesoilandenrichingitwithorganicmaterial.

Figure4.13Intropicalregions,termitesbuildhugemoundsinwhichtheyconcentratecalciumaswellasorganicmaterialintheirnests.

Figure4.14Theburrowingactivitiesofanimalscontributetotheporosityandenrichmentofsoils.

Chapter5:SoilChemicalProperties

Figure5.1Clayparticlesareextremelysmallandinsometypesthelayerstendtocurl.

Figure5.2Ionsinsilicateclaysformageometricpatternsuchasinthiskaolinite.

Figure5.3Developmentofanegativechargeonasilicateclaylattice.

Figure5.4Layerlatticecrystalsofmontmorilloniteclayhaveahighcapacitytoholdplantnutrients,absorbwater,andswell.

Figure5.5Layerlatticecrystalsofhydrousmicaclayshavealowercapacitytoholdplantnutrientsandtoabsorbwater.

Figure5.6Layerlatticecrystalsofkaoliniteclayhaveaverylowcapacitytoholdplantnutrientsandtoabsorbwater.

Figure5.7SilicateclayscanbeidentifiedbyuseofX-rays.

Figure5.8Particleofoxideclayhaslittleornocrystallinityandaverylowcapacitytoholdplantnutrients.

Figure5.9Soilcolloidalparticlesattractionswiththeoppositeelectroncharge.

Figure5.10A“swarm”ofpositivelychargedionsaroundanegativelychargedsoilparticleresemblesbeesaroundahive.

Figure5.11Acalciumion(Ca2+)(left)migratesinsolutiontowardanegativelychargedsoilparticletowhichtwopotassiumions(K+)havebeenpreviouslyattracted.TheCa2+ion(right)changesplaceswiththetwoK+ions,whichmoveonintothesoilsolution.Aninstanceofcationexchangehasoccurred.

Figure5.12Cationsmovefromamineral,intosolution,tothecolloidsurface,andonintotherootletbyionexchange.

Figure5.13Developmentofanegativechargeonahumuscolloidparticle.

Figure5.14HydrogenionconcentrationisexpressedaspH.

Figure5.15SoilreactionisusuallylessthantwopHunitsoneithersideofneutral.

Figure5.16Acolloidalclayparticlehasexchangeablecationsaroundit.Each•(acid)or□(base)representsbillionsofions.

Figure5.17Aspoonfulofsoilweighing10g(dry)containsabout1.2quintillion(1.2×1021exchangesitestowhichplantnutrients(Ca,K,etc.)canbeheldavailableforplantroots.

Figure5.18Leachingofthesoilultimatelyreturnsbasestothesea.

Figure5.19Soiliswellaggregatedbyactionofcolloidsrichincalciumions(left).Soilrunstogetherinadensemassbyactionofcolloidscontainingabundantsodiumions(right).

Figure5.20Corngrowthispooreronsodium-richsoilthanoncalcium-richsoil.

Chapter6:SoilWater

Figure6.1Thehydrologiccycledescribestheflowofwaterintheenvironment.

Figure6.2Waterthatentersthesoilmaypercolateorevaporateoritmaybetranspiredorstored.

Figure6.3Soilswithlargeporespaces,suchassandysoilsandwell-granulatedtypes,usuallyhavehighinfiltrationandpercolationrates,whereasthosethathavesmallporespacesorareinpoorphysicalconditionhavelowinfiltrationandpercolationrates.Runoffoccursiftherateofrainfallexceedsthewaterinfiltrationrate.

Figure6.4Runoffandinfiltrationfora1.5-in.(38-mm)rainfallin1h.Theinfiltrationratedecreasesasthesoilwetsuntilrunoffbeginsafter10min.Lateinthestorm,therunoffandinfiltrationratesaresteady.Runoffwouldhavebegunlaterandbeenlessif

thesoilhadahigherinfiltrationrate.

Figure6.5Ifaplantseedlingisnotstrongenoughtoliftthesoilcrust,itdies.

Figure6.6Soilwaterreturnstotheatmospherebyevaporationfromthesoilsurfaceandbytranspirationfromplantleaves.

Figure6.7Amulchhelpspreventevaporationofwaterfromthesoil.

Figure6.8Blackplasticortarpapercontrolsweedsandevaporation.

Figure6.9Watermovesfromsoilparticleswiththethickestwaterfilmstosoilwiththethinnest.Astheplantrootabsorbsmoisture,watertendstomovetowardit(capillarymovement).Plantrootsalsogrowandextendintozoneswithmoremoisture.

Figure6.10ThewaterfilmsinAarethickestandthesoilisnearlysaturated;atBitisaboutatfieldcapacity;andthethinfilmsinCrepresentthewiltingpoint.

Figure6.11Soilwaterbetweenfieldcapacityandthewiltingpointisavailabletotheplant.

Figure6.12Watermovesintotherootsandthroughtheplantprimarilybycapillaryaction.

Figure6.13Thewatertablecanbeloweredtothelevelofthesubsurfacedrainagenetwork.

Figure6.14Differenttypesofsubsurfacedrainagesystems.

Figure6.15Channelizationisneededtocarrywaterfromsubsurfacedrains.

Figure6.16Examplesoftypesofirrigationsystems:(A)surfaceorflood,(B)sprinkler,(C)sub-irrigation,and(D)drip.PhotoscourtesyofUSDANRCS.

Chapter7:SoilTemperature

Figure7.1Thethermalconductivityofasoildependsonitsporosityandwetness.

Figure7.2Heatcanbetransferredfromwarmsoiltocoolairbyforcedorfreeconvection.

Figure7.3Thesurfaceenergybudgetsummarizesheatflowinthesoil–plant–atmospheresystem.Incomingsolarradiationevaporateswater,warmstheair,andwarmsthesoilthatemitslong-waveradiation.

Figure7.4Variationsofsurfaceandsubsoiltemperaturesthroughouttheday—warmingduringtheday,coolingatnight.

Figure7.5IntheNorthernHemisphere,solarradiationatmiddayproducesthehighesttemperatureondarksoil,butsoiltemperatureisalsoinfluencedbyseveralotherfactorsshownhere.

Chapter8:SoilFertilityandPlantNutrition

Figure8.1Phosphorusexistsoriginallyasacomplexmineralwithverylowsolubility.Weatheringbreaksitdownintolesscomplexforms,someofwhichcanbeusedbyplants.

Figure8.2TheinfluenceofsoilpHonnutrientavailability.Thewiderthebar,thegreatertheavailability.

Figure8.3Carbonandoxygencomefromcarbondioxideintheair,hydrogenfromwaterinthesoil,andotherelementsareabsorbedbyplantsfromthesoil.

Figure8.4Thesulfurinfossilfuelssuchascoalisthesourceofsulfurdioxide(SO2)emissionintotheatmospherewhenitisburned.

Figure8.5Propercollectionofsoilsamplesisextremelyimportant.Testsmadeoncarelesslytakensamplescanbemisleadingandcostly.

Figure8.6Gridsamplingisanalternativemethodofsamplingwheresoilsarequitevariable.

Figure8.7Sometypicalnutrientdeficiencysymptomscausedbylackofaspecificnutrient.

Figure8.8AcompletecommercialfertilizerisreportedintermsofvaryingpercentagesofN,P2O5,andK2O.

Figure8.9Nitrogenmaybeappliedasanhydrousammonia(NH3)gasfedfromapressuretankthroughhollowknivesthatcutintothesoil.

Figure8.10Mostnitrogenfertilizersstartwithammonia,whichreactswithvariousacids.Theyexistingaseous,dry,orliquidforms.

Figure8.11Liquidfertilizermaybeappliedtothesoilor,ifsufficientlydiluted,itcanbeusedasafoliarapplication.

Figure8.12Rockphosphatesformakingfertilizerareminedfromopenpits.

Figure8.13Potash,apotassiumcompound,isminedfromdepositsintheearth.

Figure8.14Animalmanureimprovessoilstructureaswellassupplyingnutrients.

Figure8.15Lagoonsprovidestorageandmaintainthenutrientvalueofmanure.

Figure8.16Atractor-poweredmobiletankandpumpunitforinjectingliquefiedmanureintothesoil.

Figure8.17Cropscanbeplowedunderasgreenmanuretoprovideorganicmatter.

Figure8.18Fertilizerdistributionusinga)broadcasttopdressedandb)broadcastincorporatedmethodsofplacement.

Figure8.19Pop-upordirectseedcontactmethodoffertilizerplacement.

Figure8.20Bandfertilizerplacementmethod.

Chapter9:SoilManagement

Figure9.1Grassesarelowinnitrogenatmaturityandaremoreslowlydecomposedthanlegumes,whichcontainmuchmorenitrogen.

Figure9.2Minimumtillageorno-tilloftenmeansplantingwhileresiduefromthepreviouscropisstillinthefield.

Figure9.3Diskingincorporatescropresiduetoashallowdepth,amoldboardplowcoverstheresidue,andachiselplowgoesdeepbutleavesnoresidueonthesurface.

Figure9.4Plowpanscanformatthedepthoftillageandinhibitrootpenetrationbecauseoftheirincreaseddensity.Chiselingorperiodicdeepplowingcanpreventthiseffect.

Figure9.5Implementsusedforfarming.

Figure9.6Agriculturallimeisproducedfromlimestonequarriedfrombedrock.

Figure9.7Manyhumid-regionsoilsneedregularapplicationsoflimetocombatacidity.

Figure9.8Anillustrationofhowprofitfromfertilizerismaximized.Inthishypotheticalexample,themostprofitablerateoffertilizationis100poundsperacre(about110kg/ha).Notethatthemaximumyielddoesnotcorrespondtothemostefficientrateofapplication.

Figure9.9Salinesoils(A)usuallyhave“whitecaps”ofsaltinthetopsofthebeds.Growthofcropsnormallyisspotted.Sodicsoils(B)areusuallydarkcolored(oftencalled“blackalkali”)andaregummyandslickwhenwetandcrackedwithapowderysurfacewhendry.

Figure9.10ThegeneralizedpHmanagementconsiderationsforsoilsoftheUnitedStates.RegionAsoilsaregenerallyabovepH7.0-soilsmaybesalineorsodic.InregionBtheacid-baserelationshipsarecommonlyfavorable,andinregionCthebaseshavebeenleachedsothatlimeandfertilizerareneededinhighamounts.

Figure9.11Forsomecropssuchasrice,thestrawremainingafterharvestissothickthatburningmaybetheonlypracticalwaytomanageit.

Chapter10:SoilConservationandtheEnvironment

Figure10.1Loosesubstratum(A)slowlydevelopsintosoilifsurfaceerosiontakesplaceataslowrate.Wheresoilisthinoverbedrock(B),erosionofthesurfaceleavesabarrenlandscape.

Figure10.2Agriculturalsystemscommonlyaccelerateerosion.

Figure10.3Theimpactofraindropscontributestoerosionbybreakingupsoilaggregatesandsplashingsoildownslope.

Figure10.4Gullyerosioncanbespectacular.

Figure10.5Rillandsheeterosioncanresultingreatsoilloss.

Figure10.6Twosafeguardsagainstsoilerosionarevegetativecoverandwell-aggregatedsoil.

Figure10.7Contouringisveryhelpfulincontrollingrunoff.

Figure10.8Agrassedwaterwayofferserosionprotection.

Figure10.9Adropspillwayisanerosioncontrolstructurethatpreventsgullyerosion.

Figure10.10Furrowdikestrapmostofthewaterthatfallsasrainorbysprinklerirrigationsoitcanbeusedbythecrop.

Figure10.11AgriculturalterracesforriceproductioninthehighlandsofVietnam.Theseterracesareirrigatedandhavebeenproductiveformorethan1,000years.

Figure10.12Parallelterracesmaybedrainedbyburiedtiles.

Figure10.13Althoughlesscommonthanwatererosion,winderosioncanbedevastating.

Figure10.14Winderosiontransportssoilparticlesbycreep,saltation,andsuspension.

Figure10.15Anunprotectedsoilsurface(A)inviteserosion,butcropresidueonthesurface(B)givesprotectionfromwindandwatererosion.

Figure10.16Stubble-mulchingloosensthesoilbutleavesmostoftheplantresidueonthesurface.

Figure10.17Shelterbeltsandroughsoilsurfacescanreducewinderosion.

Figure10.18Erosionlosscanbegreatfromcave-insalongriverbanks.

Figure10.19Cattlepathsaccentuateripplesmadebymasswasting.

Figure10.20Erosionnotonlyreducesthevalueofcroplandbutalsocausesserioussedimentationproblems.

Chapter12:SoilClassificationandSurveys

Figure12.1Worldsoilmap.

Figure12.2Entisolsareweaklydeveloped.

Figure12.3Inceptisolsarerelativelyimmature.

Figure12.4Aridisolsareveryfragile.

Figure12.5Gelisolshavepermanentlyfrozensubsoil.

Figure12.6MostOxisolsareintropicalregions.

Figure12.7Andisolshavemanylayersofvolcanicash.

Figure12.8Histosolsareaccumulationsoforganicmatter.

Figure12.9OneauthorstandsbesideasubsidencepostatBelleGlade,Florida.

Figure12.10Vertisolsarerichinclay.

Figure12.11Mollisolsareveryproductive.

Figure12.12Alfisolshaveahighbasecontent.

Figure12.13Ultisolslackbasesandquicklybecomeimpoverishedundercultivation.

Figure12.14Spodosolsareveryacid.

Figure12.15Anillustrationofapolypedon,pedon,andasoilprofile.

Figure12.16Ochricepipedon.

Figure12.17Mollicepipedon.

Figure12.18HisticepipedonontheOhorizon.

Figure12.19CambicBhorizon.

Figure12.20ArgillicBhorizon.

Figure12.21SpodicBhorizon.

Figure12.22OxicBhorizon.

Figure12.23PetrocalcicBhorizon.

Figure12.24Asoillandscapecanbebrokenintoseveralcomponents.

Figure12.25Varioussoilbodiesfittogethertoformthelandscape.

Figure12.26Mappingunitsymbol.

Figure12.27Asoilmappermakesaugerholestoinvestigatethesoilandrecordsthefindingsonanaerialphotograph

Figure12.28AdetailedsoilmapofonesectionoflandinRandallCounty,Texas.Itis1mile(1.6km)oneachside.

Figure12.29AgeneralizedmapofRandallCounty,Texas.

Chapter13:SoilandItsUses

Figure13.1Soilclassificationsystemsusedbyengineers(AASHTOandUSC)havedifferentrangesforparticlesizedistributionsthantheUSDAsystem.

Figure13.2Useofsandandgravelprovidesastablebaseforastructureonpotentiallyunstableground.

Figure13.3Variouskindsofsoilmaterialsareusedinconstructionofanearthwork.

Figure13.4Twoexamplesofearthshelteredinstallations.

Figure13.5Cross-sectionofaburiedpipeandtheeffectofseverecorrosioninawetlandposition.

Figure13.6Residentialwastewatersgeneratedinruralhomesarerecycledbysoilabsorptionofseptictankeffluent.

Figure13.7Irrigationoffarmlandisbeingtriedonalimitedbasisasameansofdisposalforwastewatergeneratedbysomesmallindustriessuchascanneries.

Figure13.8Cross-sectionofalandfillcellwhenfilled.

Figure13.9Cross-sectionofpartsoftwofields.Totheleftofthepost,thesoilisundisturbed.Totheright,thetopsoilwasremovedandsaved,andthenthedesiredsubsoilwasremoved.Theoriginaltopsoilwasreplaced,therebypermittingcropproduction.

ListofTablesChapter2:SoilFormation

Table2.1Compositionofearth'ssurfacecrust

Chapter4:SoilBiologicalProperties

Table4.1Essentialfunctionsperformedbysoilorganisms

Chapter5:SoilChemicalProperties

Table5.1Therangeincationexchangecapacityofsomecommonclayminerals

Chapter8:SoilFertilityandPlantNutrition

Table8.1Elementsrequiredforplantgrowthandprincipalformsinwhichtheyaretakenupbyplants(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Table8.2Essentialplantnutrients,functioninplantgrowth,anddeficiencysymptoms(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Table8.3Fertilizergrades(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Table8.4Combinationusedtoproducenitrogenfertilizers(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Table8.5Averagecontentofessentialelementsinbeeffeedlotmanure(basedon30samplesfromTexasHighPlainsfeedlots,figuredat30%moisturecontent)(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Chapter11:ConservationAgriculture

Table11.1Areaoflandunderno-tillinthetopcountriesasreportedtoFAOfrom2009to2014

Chapter12:SoilClassificationandSurveys

Table12.1The12soilordersusedinSoilTaxonomy,1theirformativeelements,correlatingFAOclassification,andU.S.andworldwidedistribution

Table12.2SoilTaxonomyclassificationscheme

Table12.3Examplesofsoilhorizons

Table12.4TwosoilsofColumbiaCounty,Wisconsin

Chapter13:SoilandItsUses

Table13.1SuitabilityorlimitationratingforsoilsoftheClarion-Nicollet-Websterassociation

Table13.2Characteristicsofsoilsforengineeringpurposes

SoilScienceSimplifiedSixthEdition

NealS.EashProfessorofBiosystemsEngineering&SoilScienceUniversityofTennesseeinKnoxville,Tennessee.

ThomasJ.SauerResearchSoilScientistintheSoil,Water,andAirResearchUnitattheUSDAAgriculturalResearchServiceinAmes,IA,USA.

DebO'DellDoctoralCandidateintheBiosystemsEngineering&SoilScienceDepartmentattheUniversityofTennesseeinKnoxville,TN,USA.

EvahOdoiAdjunctAssistantProfessorintheBiosystemsEngineering&SoilScienceattheUniversityofTennesseeinKnoxville,TN,USA.

IllustratedbyMaryC.Bratz

Copyright©2016byJohnWiley&Sons,Inc.Allrightsreserved

PublishedbyJohnWiley&Sons,Inc.,Hoboken,NewJerseyPublishedsimultaneouslyinCanada

Nopartofthispublicationmaybereproduced,storedinaretrievalsystem,ortransmittedinanyformorbyanymeans,electronic,mechanical,photocopying,recording,scanning,orotherwise,exceptaspermittedunderSection107or108ofthe1976UnitedStatesCopyrightAct,withouteitherthepriorwrittenpermissionofthePublisher,orauthorizationthroughpaymentoftheappropriateper-copyfeetotheCopyrightClearanceCenter,Inc.,222RosewoodDrive,Danvers,MA01923,(978)750-8400,fax(978)750-4470,oronthewebatwww.copyright.com.RequeststothePublisherforpermissionshouldbeaddressedtothePermissionsDepartment,JohnWiley&Sons,Inc.,111RiverStreet,Hoboken,NJ07030,(201)748-6011,fax(201)748-6008,oronlineathttp://www.wiley.com/go/permission.LimitofLiability/DisclaimerofWarranty:Whilethepublisherandauthorhaveusedtheirbesteffortsinpreparingthisbook,theymakenorepresentationsorwarrantieswithrespecttotheaccuracyorcompletenessofthecontentsofthisbookandspecificallydisclaimanyimpliedwarrantiesofmerchantabilityorfitnessforaparticularpurpose.Nowarrantymaybecreatedorextendedbysalesrepresentativesorwrittensalesmaterials.Theadviceandstrategiescontainedhereinmaynotbesuitableforyoursituation.Youshouldconsultwithaprofessionalwhereappropriate.Neitherthepublishernorauthorshallbeliableforanylossofprofitoranyothercommercialdamages,includingbutnotlimitedtospecial,incidental,consequential,orotherdamages.

Forgeneralinformationonourotherproductsandservicesorfortechnicalsupport,pleasecontactourCustomerCareDepartmentwithintheUnitedStatesat(800)762-2974,outsidetheUnitedStatesat(317)572-3993orfax(317)572-4002.Wileyalsopublishesitsbooksinavarietyofelectronicformats.Somecontentthatappearsinprintmaynotbeavailableinelectronicformats.FormoreinformationaboutWileyproducts,visitourwebsiteatwww.wiley.com.

LibraryofCongressCataloging-in-PublicationData:Coverimage:Mycola,wetspotoncrackedearthunderdramaticsky,iStock/GettyImagesPlus

PrefaceSoilScienceSimplified,SixthEditionexplainssoilscienceinaneasilyunderstandablemanner.Students,professionals,andnonprofessionalsalikewillgainanaccurateworkingknowledgeofthemanyaspectsofsoilscienceandbeabletoapplytheinformationtotheirendeavors.Thebookisaprovenandsuccessfultextbookandworkswellasassignedreadingforuniversitystudentsinthenaturalsciencesandearthsciences.Agriculturalsciencecoursestaughtatthehighschoolorposthighschoollevelcanalsousethiseditionasaresource.

Soilsciencehasbeenlargelydirectedtowardagriculturalproduction.Farmingremainsattheforefrontoffoodandfiberproductionandis,morethanever,concernedwithsoilanditsproperties.

Anyonewhoworkswithsoilcanbenefitfromanunderstandingofsoilanditsproperties.Horticulturists,foresters,landscapearchitects,andsimilarprofessionalscanbenefitfromanin-depthunderstandingofsoils.Homegardenerscanlikewisebenefit.Thosewhoconstructhousesandotherstructuresneedtounderstandthatthesoil'sphysicalandchemicalpropertiescanimpactfoundationproblems.Engineersneedthesameunderstandingofsoilpropertiesastheybuildroads,bridges,dams,levees,andsimilarstructures.Environmentalistsandpeopleinrelatedareasfindaworkingknowledgeofsoilsuseful.

Therearemanyusesofsoil-farmorethanforproductionagriculture.Andeverybodywhoworkswiththelandinanywayneedstoknowhowtotakefulladvantageoftheinformationinasoilsurveyreport.Theneedforanunderstandingofsoilisever-present.Ifyourprofessionwillinvolvetheuseofsoil,readandunderstandtheinformationinthissixtheditionofSoilScienceSimplified.Keepareferencecopyinahandyspotinyourbookcase.

Thissixtheditionexpandsandupdatesseveralchapters.Newapproachestothecontenthavebeenincorporatedtoprovideinformationneededbythoseprofessionalslistedpreviously.Achapteronconservationagriculture(CA)hasbeenaddedthatdescribestheevolutionofagriculturalmanagementpracticesthatsupportandstrengthenbothfoodproductionandenvironmentalresources.Theillustrationsandphotosdemonstratetheprinciplesdescribedinthetextandenhancecomprehension.

Drs.Eash,Sauer,andOdoiareexperienceduniversityprofessorsofsoilsciencewhohavetaughtandconductedresearchinsoils.Throughexperienceinthefield,classroom,andlaboratory,theyhavegainedabasic,hands-onappreciationoftheimportanceofappliedsoilscience.Thisbookrepresentstheirmanyyearsofexperienceandthedesiretoprovideaworkingknowledgeofsoilandhowitspropertiesinfluencedecisionsonthebestuseofsoil,whetheritisusedasamediumforplantgrowth,asabaseforthefoundationofbuildings,orforanyotherpurpose.

ThisbookhasbeenusedsuccessfullyasaresourceincertificationprogramsintheagriculturalindustrysuchastheCertifiedCropAdvisorprogramsponsoredbytheAmericanSocietyof

Agronomy.WeonceagainusemanyofthelineillustrationsbyMaryC.Bratzthathaveappearedinearliereditionsofthebookwhichcontinuetobeusefulincommunicatingessentialideasandprocessesinsoilscience.

Chapter1IntroductiontoSoilSoilisanaturalresourceonwhichpeoplearedependentinmanyways.Sincethebirthofthesoilconservationmovementinthe1930s,therehasbeenanincreasedinterestinconservingthesoil.Theenvironmentalawarenessandconcernsthathaveoccurredoverthepastseveraldecadeshavefocusedattentionontheneedtoconservesoilasafundamentalpartoftheecosystem.Thereis,however,littlepublicunderstandingofthesoil'scomplexity.

Carefulobserversmayseesoilexposedinroadbanksorexcavations,anditmaybenoticedthatthesoildoesnotlookthesameinalllocations(Fig.1.1).Sometimesthedifferencesareapparentinthefewinchesofsurfacesoilthatthefarmersplow,butgreatervariationscanusuallybeseenbylookingatacrosssectionofthetop3or4ft.(0.9or1.2m)ofsoil.Thequalityandquantityofvegetativegrowthdependsonthepropertiesofthesoillayers.

Figure1.1Roadbankscanrevealthecomplexityofthesoil.

Roadsandstructuresmayfailiftheyareconstructedonsoilswithundesirablecharacteristics.Specialcaremustbetakentoovercomesoillimitationsforspecificengineeringuses.Satisfactorydisposalofhumanwasteandlivestockmanureisbecominganincreasingconcern,particularlywheresoilsareusedasadisposalsite.

Pooryieldsofagriculturalcropsandpoorgrowthoftreesmayresultfromamismatchingofcropsandsoils.Thismismatchingmayhappenbecausethelandownerhasnotexaminedthesoilhorizonsorunderstoodtheirlimitations.Soilscientistsstudythefactorsnecessaryforpropersoilmanagementandplantgrowth.

WhatIsSoil?Thetraditionalmeaningofsoilisthatitisthenaturalmediumforthegrowthoflandplants.TheSoilScienceSocietyofAmericahaspublishedtwodefinitions.Oneis"The

unconsolidatedmineralororganicmaterialontheimmediatesurfaceoftheearththatservesasanaturalmediumforthegrowthoflandplants."

AmoreinclusivedefinitionbytheSocietyis"Theunconsolidatedmineralororganicmatteronthesurfaceoftheearththathasbeensubjectedtoandshowstheeffectsofgeneticandenvironmentalfactorsof:climate(includingwaterandtemperatureeffects)andmacro-andmicroorganisms,conditionedbyrelief,actingonparentmaterialoveraperiodoftime."TheeffectofeachofthesegeneticandenvironmentalfactorswillbediscussedinChapter2onsoilformation.

Soildiffersfromthematerialfromwhichitisderivedinmanyphysical,chemical,biological,andmorphologicalpropertiesandcharacteristics.Thedifferencesinthesepropertiesandcharacteristicswillbediscussedinsubsequentchapters.Theireffectonsoilmanagementdecisionsisimportantwhetherthesoilistobeusedforcropproduction,inanurbansetting,orforroads,dams,wastedisposal,anditsmanyotheruses.

Mostsoilconsistsoffragmentedandchemicallyweatheredrockwhichincludessand,silt,andclayseparates,anditusuallycontainshumus,whichispartiallydecomposedorganicmatter.Soilisverydiverseoverthefaceoftheearth,anditvariesconsiderably.Ifpropertiesofasoilareknown,thesoilcanbeproperlymanaged,anditwillservequitewellforthepurposeforwhichitisused.

NatureandUsesofSoilSoilisamediuminwhichplantsaregrownforfoodandfiber.Itisfortunatethatovermostofthelandareaoftheearth,soilcoversbedrocktoaconsiderabledepth.Iftherewerenosoil,thecontinentswouldbewastelandsofbarrenrock.Insoil,seedsgerminateandplantsgrowastheyobtainwaterandnutrientsfromthesoil.Cropsofthefieldsandforestsproducefoodandfiber.

Soilgivesmechanicalsupportforplantrootssothateventalltreesstandfordecadesagainststrongwinds.Soilalsophysicallysupportsstructuressuchashouses,buildings,sidewalks,streets,andhighways.Sometimesthepropertiesofsoilsareundesirableandbuildingsandpavementwillcrackduetotheinstabilityoftheunderlyingsoil.Abandonedroadsmaybeburiedbysoilcarriedupwardbyants,earthworms,andothercreatures.

Inintertropicalregions,millionsofpeoplelivecomfortablyinplacesbuiltchieflyfromlocallyexcavatedsoil.SuchearthenhousesarecommoninWestAfrica(Fig.1.2).Acompoundearthendwellingforanextendedfamilymaybequiteanimpressivestructure.TheadobehousesofthesouthwesternUnitedStatesandthepioneersodhousesoftheprairiesareotherexamplesofearthenhouses.Modernearth-shelteredhomesareshownwithpridebytheownersandbuilders.Formaximuminsulation,housesoftenfeatureanearthenembankmentcoveringallbutthesideexposedtothesun.

Figure1.2EarthenhousesarecommoninWestAfrica.

Soilisinvolvedinseveralprocessesinthehydrologiccycle.Waterintheformofrain,dew,fog,irrigation,orsnowmeltmaymoveintothesoil(infiltrate)orevaporateorrunoffofthesoilsurfaceintotheareadrainagesystemintolakesorstreams.Thewaterthatinfiltratesintothesoilmayevaporateorbeutilizedbyplants.Itmaybeusedbytheplantstoformcompoundsoritmaybetranspiredfromtheleavesbackintotheatmosphere.Ifthereismorewaterthanthesoilwillhold,itmaypercolatedownwardtobecomepartofthegroundwaterreservoirandeventuallybecomepartofstreams,rivers,andsprings.

Manyorganicandinorganicpollutantsinwastewateraresorbedastheypassthroughthesoil,therebypartiallycleansingthegroundwater.Ifpotentialpollutantsthatareverysolubleareaddedtothesoil,theymaybecarriedbythesoilwaterintothegroundwatertoourdetriment.

Soilisanair-storagefacility.Plantrootsandbillionsofotherorganismslivinginthesoilneedoxygen.Theporesysteminsoilprovidesaccesstoair,whichmovesintoanddrawnoutofthesoilbychangesinbarometricpressure,byturbulentwind,bytheflushingactionofrainwater,andbydiffusion.Someplants,suchasrice,havethecapacitytoconductoxygenintowaterloggedsoil.Soilaircontainsconsiderableamountsofcarbondioxide.

Soilisevenusefulasamineralsupplementforpeople.InsomeimpoverishedAfricancountries,selectedtypesofsoilcontaininghighcalcium,forexample,havebeenusedasaspecialfoodsupplement.Specifically,pregnantwomenandtheirbabieshavebenefitedfromthemother'singestionofsoilfromtermitemoundsthatareenrichedwithcalcium.Byusingthisnaturalresource,thesewomenmayhaveadequatecalciumintheirsystems.

Soilacceptsbackthatwhichcamefromit.Whenplantsdie,itisnotlongbeforeorganismsthatcausethemtodecomposewillbeactiveandtheplantwilleventuallybecomeapartofthesoil.Evenhugelogsontheforestfloorsoondisappear(Fig.1.3).Animalsthatliveinthewildaswellasotherformsoflifealsoreturntothesoilwhentheydie.Societyproducesvastamountsofwasteofeverysize,shape,anddescription,whichisoftenburiedinlandfillswhereitwilldecomposeifitisorganic.

Figure1.3Livingorganismssoonerorlaterbecomeapartofthesoilonceagain.

Soilisbeautiful;itisanaestheticresource.Peoplemaybecomefondoftheirnativesoil,whetheritisblackandbrownorredandyellow.Thereisarainbowofvarioushuesofsoilunderourfeet.

Changesinbothsoilandvegetationthroughtheseasonsareobservedwithgreatinterest.Somesoilsformwidecracksindryseasonsandswellwhentherainsreturn.Frostactionmaycreatelittleicepillarsthatliftthesurfaceofthegroundinwinter.Thesmelloffreshlytilledsoilseemsgoodtofarmersandgardenersastheyplanttheircropswithhighexpectationsforanabundantharvest.Somepeoplelovetheirnativesoilsomuchthateventodaytheystillperformtheancientritualofkneelingtokissitwhentheyreturnhome.

HowBigIsanAcre?AHectare?LandmeasurementsintheUnitedStatesareintheEnglishsystemwhileformostoftherestoftheworldthemetricsystemisused.TheprincipalmeasureoflandintheEnglishsystemistheacrewhilethehectareisusedinthemetricsystem.Oneacreequals0.4047ha;hence,anacreisonlyabout40%thesizeofanhectare.Anacrehas43,560sqft,whileahectarecontains107,628sqft.

IntheUnitedStates,soilamendmentsareusuallyappliedinunitsofpounds(ortons)peracre.Intherestoftheworld,applicationsareusuallyinmetricunitsofkilograms(ormetrictons)perhectare.Poundsperacrecloselycorrespondtothemetricunitsofkilogramsperhectare.Tonsperacrecloselycorrespondtometrictonsperhectare.Lossofsoilbyerosionisratedintonsperacre.

Comparingahouselotsizetoanacrewillgiveaperspectiveonthesizeofanacre.Mosthouselotswillbeapproximately10,000sqftinsize(aboutone-fourthofanacre),whereasanacrehas43,560sqft.SeeFigure1.4foraperspectiveontheseunitsofmeasurements.

Figure1.4Anacreis208.7ftonaside;ahectareis328ftonaside.

Chapter2SoilFormationManypeoplethroughouttheworlddependuponsoilfortheirsubsistence.Soilisadynamicfeatureonthelandscapeandfewrealizetheimportanceoftheparentrocktosoilfertilityandproductivity.Althoughtheparentrocksdeterminemanysoilcharacteristics,soilismuchmorethanjustweatheredrock.

Pedogenesisisthetermusedtodescribetheformationanddevelopmentofthesoilprofile.The"pedon"isthethree-dimensionalbodyofsoilusedasthesoilbaseofreferenceand"genesis"isoftendefinedas"beginning."Throughpedogenicprocessesthesoilprofiledevelopsfromtheverythin—maybeseveralinchesthickwhichiscommoninyoungsoils—tosoilthatisgreaterthan80in.(2m)thick,whichiscommoninoldersoils.

Tounderstandsoilpedogenicprocesses,itisessentialtoconsiderhowrocksareformed,howthatformationinfluencestheirmineralogy,andsubsequentbreakdownintosoilparentmaterial.Themineralsinrocksstronglyinfluencethecompositionofthesoilderivedfromthem.Theotherfactorsinnaturethatinfluencethespecificpropertiesofsoilinagivenlocationwillalsobediscussedinthischapter.

Oneofthebasicrulesofnatureisthatnothingremainsthesameoverlongperiodsoftime.Astronomerstellusthatevenstarssuchasoursunhaveafinitelifespan.Theycoalescefromcosmicdust,formintoshiningsolarbodies,finallyexpendtheirenergy,collapse,andreturntocosmicdust.ThesecretsoftheseprocesseshaveonlyrecentlybeenrevealedbytheHubbletelescope.Onearth,thealterationofrocksfromoneformtoanotherismuchmoreeasilyunderstoodbecausewecanstudyspecimensofrocksandrelatethemtotheirpositionintheearth'scrust.

Rocksaremerelycombinationsofminerals.Mineralshavespecificchemicalcompositionswhereasarockreferstoamaterialwithinaspecifiedrangeofmineralogicalcompositionthatisofappreciableextentinthecrustoftheearth.Someofthemostcommonrocksaregranite,basalt,sandstone,andlimestone.Roundedpiecesofrock—socommoninglaciatedregions—areboulders,stones,cobbles,andgravelsindescendingorderofsize.

TheRockCycleTounderstandtheformationofsoil,considerfirsttherocksfromwhichthemineralparticlesinthesoilwerederived.Astheearthcooled,themoltenmagmacrystallizedintoigneousrocks.Aslongastherehasbeenwaterontheearth,flowingwaterhasbeenerodingrocksandthefineparticlesproducedhavebecomesediments,whichmaysolidifyintosedimentaryrocks.Underconditionsofextremeheatandpressure,bothigneousandsedimentaryrocksmaybemodifiedandatleastpartiallyrecrystallizedintometamorphicrocks.

Theshiftingofcontinentscauseslandmassestoslideoverandburyotherlandmassestotheextentthattheburiedonesmaybecomemoltenagain.Wherethisoccursthereisevidenceofgreattectonicactivityintheformofearthquakes,volcanoes,faults,andrelatedphenomena.Therefore,overgeologictimetherocksoftheeartharecycledfromoneformtoanother(Fig.2.1).Rocksaretheevidencefortheseactionsinthepast,andthesameprocessescontinuetoday.

Figure2.1Therockcycleshowshowheatandpressure,meltinganderosioncauserockstochangeinformthroughgeologictime.

CompositionoftheEarth'sCrustChemistsrecognizeafewover100elementsthatmakeupeverythingtangibleonearth.Ofthese,theeightlistedinTable2.1arethemostabundantelementsintheearth'scrust.Theothersarenolessimportant,butarepresentinmuchsmallerquantities.

Table2.1Compositionofearth'ssurfacecrust

Element IonOxygen O2–

Silicon Si4+

Aluminum Al3+

Iron Fe2+,Fe3+

Calcium Ca2+

Magnesium Mg2+

Potassium K+Sodium Na+

SilicateMineralsIfmoltenmagmafromwithintheearthcoolsveryrapidly,theseelementssolidifyrandomlyintoaglasssuchasobsidian,amaterialcommonlyusedinjewelry.Ifthecoolingisslower,theelementswillassemblethemselvesintocrystallinesilicateminerals.Theslowerthecoolingis,thelargerthecrystals.

Silicatesaremineralsmadeup,inlargemeasure,ofcombinedsiliconandoxygen.Theyarethemostcommonmineralsinrocks.Whenonlysiliconandoxygenionsareinvolved,theyformafour-sidedstructurewithoxygenionsatthepointsandasiliconioninthecenter.Itcanbecomparedtoathree-sidedpyramid,withthebasebeingthefourthside.Thisiscalledatetrahedron.IftheO2–oneachcornerissharedwithanothertetrahedron,averystrongframeworkstructureresults.Amineralwiththisformisquartz,anditissoresistantthatitissaidtobenonweatherable.Hence,thebeachesalongouroceansconsistmainlyofsand.

Ofthesilicates,mostarealuminosilicates;feldsparsaretheclassicexample.Feldsparsalsohaveaframeworkstructurebutfromone-fourthtoone-halfoftheSi4+wasreplacedwithAl3+duringtheoriginalcrystallizationofthefeldspar.SinceAl3+hasalowerpositivechargethanSi4+,theunsatisfiednegativebondsfromtheO2–aresatisfiedprimarilybyK+andCa2+inthecrystal.Feldsparsarequitestablebutarelessresistanttoweatheringthanisquartz.Theweatheringoffeldsparaccountsformuchofthepotassiumandcalciumfoundinthesoil,theoceans,andsedimentaryrocks.

Micasaretheothermaingroupofaluminosilicates.Thetetrahedraareformedintolayersthatcanbelifted,onefromtheother,likethepagesofabook.Whenseparatedfromtherocks,thesesmallflatparticleswillglisteninthesun,especiallyiftheysettledoutofflowingwaterandlayflatonthedriedsoilsurface.

Mostoftheverydarkcoloredmineralsinrocksareferromagnesiansilicates.Insteadoftheframeworksilicatestructurediscussedabove,thesemineralshavesingle,paired,orchainedsetsoftetrahedrathatarebondedtogetherbyaccessoryions,usuallyFe2+andMg2+,hence,thetermferromagnesian.Itisbywayoftheaccessoryionsthatweatheringgainsaccesstothesemineralsandtheintegrityofthemineralstructureisdestroyed.Twocommongroupsofthesedarkmineralsaretheamphibolesandpyroxenes.

IgneousRocksIgneousrocks(Fig.2.2),includinggranitesandtheirmetamorphicassociates,makeupthebedrockfoundationofthecontinents.Themineralsinthemarecrystallineinformand,ifthemagmacooledslowlyfarbelowthesurfaceoftheearth,thecrystalsarecomparativelylarge.Thisisthecasewithgranite.Ifthecoolingofthemagmatookplacemorerapidly,thecrystalsaresmall,suchasinrhyolite.Graniteandrhyolitemaybeidenticalinmineralogicalcompositionandarecharacterizedbyhavingabundantquartzduetothehighsilicacontentofthemagma.Inaparallelmanner,magmalowerinsilicamaysolidifyintoverydarkcoloredgabbroorbasalt,dependingontherateofcooling.

Figure2.2Igneousrocks.

Crystallineigneousrocklaysjustbelowtheunconsolidatedsurfacematerialonaboutone-quarteroftheearth'slandarea.Elsewhereitismoredeeplyburied.Itisquarriedforbuildingstonesandmonuments.Pinkandlight-coloredgraniteispopular.ItoutcropsdramaticallyintheBlackHillsofSouthDakotaatMountRushmore,wheretheheadsoffourU.S.presidentshavebeencarved.Gabbrocanbepolishedintoabeautifulbuildingstoneandissometimescalledblackgranite.Awell-knownexampleofitistheVietnamMemorialinWashington,DC.Blackishandfinelycrystallizedbasaltiswellknownbecauseofextensivevolcanicactivityonearth.

SedimentaryRocksSedimentaryrocks(Fig.2.3)arethebedrockforaboutthree-quartersofthelandareaoftheearth.Theserocksweredepositedaslooselayersofsedimentonthebottomsandedgesofancientseas.Sand,primarilyquartzgrains,wasdepositedneartheshores,graysiliceousmudfartherout,andlimy,whitishmudfromfossilshellsinthedeepwater.Theselayersgraduallyhardenedintorocktobecomesandstone,shale,andlimestone,respectively.Asthelandwasslowlyupliftedandtheseasreceded,sedimentaryrockcoveredmostofthecontinents.

Figure2.3Sedimentaryrocks.

MetamorphicRocksRockscanbealteredbyheatandpressurewithintheearth.Themetamorphicrocksthatresultmayhavebeenanyoftheigneousorsedimentaryrocks.Graniteiscommonlymetamorphosedintogneiss,abeautifullybandedrockwhereinlikemineralsbecameconcentratedduetosimilarviscosityanddensityintheshiftingmagma.Sandstoneiscementedbysilicafromsolutiontobecomequartzite,whichisthemostresistantrockthatiswidespreadontheearth.Shaleisconvertedintoslateandlimestoneintomarblebyheatandpressure.

ProcessesofRockWeatheringWhenlivingorganismssuchasplantsdie,theyarerottedbysaprophyticmicroorganisms.Inasimilarmanner,naturallyoccurringphysicalandchemicalforcescauserockstobeweatheredintosaprolite.Collectively,saproliteiscalledtheregolithoftheearth,whichiscomposedoftheloosemineralmaterialsabovesolidbedrock.Theeffectsofrockweatheringcanbeobservedbysplittingastonethathasbeenexposedonornearthegroundsurfaceforalongtime(Fig.2.4).Duringtheweatheringprocess,thealteredrockmaterialmayaccumulateinplaceoverthesolidrockoritmayslide,bewashed,orbeblowntoothersites.Soilformationbeginssoonafterlooserockmaterialisstabilized.

Figure2.4Exposuretoweatheringcausestinycrackstodevelopinthesurfaceofrocks,whichallowsforchemicalreactionswiththepenetratingsolutions.

BiologicalDecompositionofRocksIfyouhaveeverbeenonahikeintothemountainsandsitdownonarocktorest,ifyoulookcloselyattherockyouwillquicklyseethatitiscoveredbysmallmacroscopicorganismscalledlichens.Lichensarecomposedofanalgaandafungusandareoftenoneofthefirstorganismstocolonizeexposedrocks.

JustthreeyearsaftertheislandofKrakatoawaslargelyblownawaybyaviolentvolcaniceruptionin1883,scientistsvisiteditandfoundthatthesurfaceofthefreshbedrockwasalreadybeinginvadedbycyanobacteria,oneofthemostself-supportingformsoflifeonearth.Itcanbothphotosynthesizeandfixnitrogen.Growingalongwiththecyanobacteriawerenitrogen-andcarbon-fixingbacteriaaswellasfungiandlichens.Weakacidsproducedbythesemicroorganismsweredissolvingnutrients(phosphorus,calcium,etc.)fromtherocksandbuildingupahumicmatcapableofsupportingmossesandeventuallyhigherplants.Theweakacidsincludecarbonicacidformedbysolutionofcarbondioxidegasinwaterandlacticacidproducedbyfungi,andthestrongeracids(nitricandsulfuric)wereformedbybacteria.Certain

fungiandbacteriacanreleasephosphorusfrommineralparticles.Itisevidentthatmicroorganismsareinvolvedinrockweatheringfromthestart.

PhysicalWeatheringPhysicalweatheringofrocksistheirbreakdownintoprogressivelysmallerpieceswithnochangeinmoleculararrangementwithintheminerals.Anyoftheforcesthattransportsolidparticlescausesthemtowear.Sandonabeachthatisrolledbyeachincomingwaveisafamiliarexample.Strongwindspickupsandandblastitagainstobjectsthatsoonshowtheeffectsofabrasion.Treerootspenetratecracksinrocksandastherootsgrowtheycausethecrackstoexpandandeventuallybreaktherock.Intemperateregions,waterenterscracksinrocks,freezes,andcancausethesurfaceoftherocktopeeloffliketherindofanorange,whichiscalledexfoliation.Glaciersweretheultimateinphysicalweatheringastheybrokeloosemassivebouldersandmovedthemgreatdistanceswithagrindingaction.Hillswerelowered,valleyswerefilled,andtherewasagenerallevelingeffectexceptattheglacier'sedge,wheresmallhillscalledterminalmoraineswerecreated.Nomattertheextentofphysicalweathering,itdoesnotdirectlycausesignificantreleaseofionsfromthemineralsforthebenefitofplants.

ChemicalWeatheringChemicalweathering,asthetermimplies,resultsfromchemicalreactionsthatalterthemolecularcompositionofminerals.Thesechemicalforcesreactwiththesurfaceofminerals.Ifphysicalweatheringdidnotgreatlyincreasesurfaceareabybreakingdownrockintosmallerpieces,chemicalweatheringwouldprogressmuchmoreslowly.

Hydrolysis

Hydrolysisisimportantinmineralweathering.Ittakesplacewhenhydrogenions(H+)inwaterreplacemetallicionsinminerals.Allwaterisslightlyionized,sohydrolysisispervasive.Rainabsorbscarbondioxide(CO2)asitfalls,resultinginarelativelyweakcarbonicacid(H2CO3),whichgreatlyincreasesthereactionofhydrolysis.However,theancientstatuesfromtheGreekandRomanempiresdidnotshowmuchdegradationuntilsmokefrommodernindustryresultedinsulfuricacidandnitricacidintheprecipitation.Overmillionsofyears,mostoftheacidinthesoilresultedfromtherespirationofCO2bylivingorganisms.PlantrootsalsoreleaseH+duringnutrientuptake.

Inonesimplifiedexampleofhydrolysis,potassiumfeldsparreactswithwatertoyieldsilicicacidandpotassiumhydroxideasshowninfollowingequation:

Thesilicicacidisthebuildingblockofclay.Inthereaction,theprimarymineralisdestroyed,clayisformed,andpotassium(K+)isreleasedintothesoilforusebyplants.Ifprecipitationissufficienttoleachawaythebase(KOH),thelandwillbecomemoreacidandtheseawill

becomemorebasic.

OxidationOxidationtakesplacewhencertainmultivalentionsloseanelectron(anegativecharge)tobecomemorepositive.Acommonelementinrockscapableoftwovalencestatesisiron.Justasawrenchleftintherainwillrust,soalsoiron-bearingmineralsinrocksbecomeoxidized.Theequationsforthereactionare

HydrationByitself,oxidationwouldnotbeextremelydisruptivetothemineral,but,innature,itisfollowedbyhydration:

Inthisreaction,nwatermoleculesattachthemselvestoanironoxidemolecule.Thisresultsinconsiderableexpansion,whichgreatlydisruptsthemineralstructureoftherocksandcausesthemtocrumble.Forthisreason,whendigginginthesubsoilinahumidregionitiscommontoencounterstonesthatdisintegrateifstruckbyaspade.Manganeseisanotherelementinmineralsthatcanexistinmultivalentionicstates,butitismuchlessabundantthaniron.Saltmayalsohydratewithsimilarresults.

ReductionReduction,beingtheoppositeofoxidation,reflectsagainofelectronsinmultivalentions.Itisnotdisruptivetomostbedrock,butitdoeshaveamarkedinfluenceonsoilwhereoxygenhasbeendepletedbymicroorganismsinwetplaces.Underreducingconditions,ironandmanganesemaybedissolvedandremovedfromthesystemortranslocatedtoregionswithfreeoxygen.Heretheyprecipitateasnodules,concretions,orvarioustypesoflayersandcoatings.

SolutionWatercomesascloseasanythingtoauniversalsolvent.However,itisonlycapableofdissolvinglargequantitiesofsolublesaltsthatwereprecipitatedfromsolutionatsomeearliertime.Thecalciumcarbonateinlimestonecamefromtheshellsofseacreatures.Itisanexampleofasaltthatisslowlysolubleinpurewater,butwaterenrichedbycarbonicacid,duetobiologicalactivity,reactswithlimestone.Thisdissolutionisevidencedbymassivecaveswheretherockwasdissolvedbybiologicallyacidifiedwaterthatseepeddownfromthesoil.

FactorsofSoilFormationSoilscientiststhinkofsoilsasnaturalbodiesthathavelength,breadth,anddepth.Eachsoilbodyoccupiesaportionofthelandscape.Thismeansthatsoilsaremorethansimplythe

productofrockweathering;theyarecomponentsofthelandscape(Fig.2.5),justasarerivers,forests,marshes,andprairies.Thousandsofyearshavebeenrequiredtomakeourpresent-daysoils.Fivefactorsofsoilformationhavebeenidentified(Fig.2.6).Theyare(1)parentmaterial,(2)climate,(3)livingorganisms,(4)topography,and(5)time.

Figure2.5Soilsarenaturalfeaturesofthelandscape.

Figure2.6Parentmaterialinatopographiclocationisactedonovertimebyorganismsandclimate.

SoilParentMaterialParentmaterialofmineralsoilsistheweatheredrockthatwasslowlybrokenupatasiteorwastransportedtherebynaturalagents.Itcanbegroupedinto(1)crystallinerocks,suchasgraniteandgneiss,(2)sedimentaryrocks,suchassandstoneandlimestone,and(3)geologicallyrecentdeposits,suchasalluviumandglacialtill.

Soilsthathaveformedfromgranitecontainafullrangeofparticlesizes,fromgravelandsandtothefinestclay.Sincequartzgrains(somewhatlikebitsofglass)ingraniteareveryresistanttoweathering,theybecomethegrittysandinthesoil.Theless-resistantmineralsinrock—suchasfeldspar(awordmeaningfieldcrystal)anddarkmineralsrichinironandmagnesium(ferromagnesianminerals),includingblackmica—arealteredbyweatheringintofineclay

particles.

Blackanddarkgraycrystallinerocksincludegabbro(coarsegrained)andbasalt(finegrained).Becausetheserockscontainnoquartz,soilsformedfromgabbroandbasaltarenotsandybutareclayey,sticky,andratherfertile.

Soilsfromsandstonearesandy;thosefromshalearesiltyorclayey.Soilsfromlimestoneconsistlargelyofinsolubleshaleymaterialsthatwereincludedasgraymudintheotherwisemoreweatherablerockmass.Therefore,soilsfromlimestonecommonlyareclayey.

Recentdepositsareblanketsofgeologicallyyoungsedimentsthatoverliethetypesofbedrockjustdiscussed.Theyinclude(1)eolian(windblown)sand,(2)loess,(3)volcanicash,(4)glacialdrift,(5)alluvium,(6)andcolluvium(Fig.2.7).

Figure2.7Bedrockmaybeblanketedbysedimentfromseveralsources.

Eoliansandsaremostcommoninaridandsubhumidareas.Mostwereinitiallydepositedbywaterwhenmassiveexpansesofsandstonewerebeingerodedoveralongperiod.Windactionmayshiftthesecoversandsintoduneformations,whicharethenreferredtoaseoliandeposits.TheSandHillsofwesternNebraskaareagoodexampleofeoliandeposits.Whenviewedfromanairplane,theyareseenasanexpanseofcrescent-shapeddunes.Theyaredroughtyandnotveryproductiveforcropsorlivestock.

Loessisawind-transporteddepositthatmainlyconsistsofsiltthatwasderivedfromthefloodplainsofriversthatdrainedthemeltwaterfromglaciers.Thesesiltshavearichsupplyofplantnutrient-bearingminerals,andtheirsizeissuchthattheyholdasignificantquantityofwaterforcrops.Extensiveareasoffertileagriculturalsoilscanbefoundinloessdepositsinsuchplaces

asChina,theMississippi-MissouriValley,andtheDanubeValleyinEurope.

VolcanicashiswidespreadinHawaii,Oregon,andWashingtonintheUnitedStatesandinCentralAmerica,Japan,Indonesia,andmanyothermountainousareas.Themineralogyofvolcanicashisvariable,butmostofitdevelopsintohigh-qualitysoilforcropproduction.

Glacialdeposits,oftenwithacoveringofloess,areparentmaterialsofsoilsinmuchofthecornbeltinNorthAmericaandthewheatbeltofEurasia.Theywereleftbyglaciers(andtheirmeltwaters)thatadvancedandretreatedrepeatedlybetween1millionand10,000yearsago.Glacierscarriedalotofrockdebriscollectedbyagrindingactionontheterrainoverwhichtheypassedandthusweremadeof"dirty"ice.Anunsortedmixture(till)ofstones,sand,silt,andclaywasdepositedinbroadblanketsandridgescalledmoraines.Glacialtillissometimesstonyenoughtoinhibitcultivation,butitsfreshsupplyofmineralsprovidesanabundanceofmanyplantnutrients.Rapidlyflowingmeltwatersleftbehindextensivesheetsofsandandgravel,calledoutwash,thattendtobedroughtyforcrops.Wherehugeiceblocks,whichmeltedlater,weresurroundedbyglacialdrift(tillandoutwash),largepitsorpotholeswereformed.Manylakesonceexistedneartheglaciers.Today,theancientlakebottomsarealmostlevelfarmlandswithrichsiltyandclayeysoils.

Alluviumissedimentthatwasdepositedbyriversandstreamsinvalleysthroughouttheworld.Centuriesoferosionhavecreatedfertileareasofalluvialsoils:theBangkokPlain,theMekongDelta,theMississippiDelta,andthevastalluvialplainsofChina.Aboutone-thirdofthehumanpopulationissupportedonthesefertilefloodplainsthatarerichintopsoilmaterialsbroughtdownfromtheuplands.Althoughfloodingisamajorhazardtohumans,buildings,andcrops,itisamajoragentindepositingsoilmaterials.Alluvialsoilsarefinelylayered(stratified)togreatdepths.Eachlayermayrepresentthedepositofasingleflood.Thesesoilsshowmarkedchangeshorizontally,fromsomewhatsandynearriverbanksonnaturalleveesandalluvialfanstoclayeyandevenpeatyinremoteswampyareas.Oldersoilswithdistinctsubsoillayersmaybefoundonnaturalterraces,or"highbottoms,"thatnowstandabovetherestofthevalleyfloorbutweresubjecttofloodingatonetime(Fig.2.8).

Figure2.8Representativelandforms.

Colluvium,agravity-transporteddepositatthebaseoffoothillsormountains,movedfromabovetoitspresentlocation.Often,asinthecaseofmudflows,itwasinasomewhatfluidstateatthetimeoftransport.Thesedepositsareextremelyvariableincompositionbutarenotgeographicallyextensive.Colluviumincludestalus,whichconsistsofchunksofbrokenrockatthefootofamountain.

ClimateEveryplaceonearthhasclimatethatcanbedescribedbasedonitsmanycomponents.Thetwocomponentsthatmoststronglyinfluencesoilformationareprecipitationandtemperature.

Eachofthesoil-formingfactorsinteractswiththeothers,andthisisevidentwithclimate.Itstronglyinfluencestherateatwhichrocksareweatheredintoalooseregolith.Itcontrolsthesupplyofwaterforphysicalweatheringanddeterminesbreakupbyfreezingandthawing.Climaticchangeledtotheadvancementandretreatofglaciersandtheresultingglacialtill.

Itistheeffectofclimateonchemicalweatheringthathasthegreatestinfluenceontheweatheringofrocks.Precipitationprovidesthewaternecessaryforchemicalweatheringprocessesandmaybesufficienttocarryawaysolubleproducts,therebyallowingthereactiontocontinue.Withoutwater,therecanbenohydrolysisorhydration.Evenoxidation-reductionmaybedependentonthequantityofdissolvedoxygen.Thesolutionofmineralsincertainrocksisdependentonrainfallunlesstheyareadjacenttoabodyofwater.

Temperaturehasamarkedinfluenceontherateofsoilformation.Perhapsthemostobviouseffectisthatwhichoccursinthetemperatezone,whereessentiallynochemicalweatheringtakesplacewhilethegroundisfrozen.Thereisawell-establishedruleinchemistrythatforevery10°Criseintemperature,therateofchemicalreactionsincreasesbyafactorof2–3.Forexample,thesoilsofthewarmersouthernpartoftheUnitedStatesaremorehighlyweatheredthanthoseinthecoolernorthernstatesevenwhereglacierswerenotafactor.

Thecombinedinfluenceofprecipitationandtemperatureisprobablyasimportantaseitheroneofthemindividually.Ifthetemperatureiscool,waterdoesnotevaporatefast,sotheeffectivenessoftheprecipitationishigh.Ontheotherhand,somewarmareasreceivequitealotofprecipitation,butduetorapidevaporation,theyhavethepropertiesofamuchdrierclimate.Asanexample,St.Paul,Minnesota,andSanAntonio,Texas,eachreceiveabout28in.ofprecipitationannually,butbecauseofthecoolMinnesotatemperature,thesoilthereisnormallymoist,whereasintheSanAntonioarea,thesoilisusuallydry.Thiseffectisalsoreflectedinthenativevegetation,whichishardwoodforestintheSt.Paulareaanddrought-tolerantvegetationintheprairiesofSouthTexas.

LivingOrganismsTheinfluenceofalltheorganisms,plants,andanimals(bothlargeandsmall)isthebioticfactorofsoilformation.Chapter4isdevotedtosoilbiology,butinthissectionthewaysthatlivingorganismsareinvolvedinsoildevelopmentarediscussed.

Inanyparticularclimaticregion,theamountofhumusinthesoilisadirectresultofhowmuch

andwhattypeofplantresiduehasbeenincorporatedintoit.Thus,ifvegetationissparse,thesoilwillbelowinhumusandlessfertile.Grasseshaveafibrousrootsystemthatquitethoroughlyinvadesthetinyporesofthesoilsothatastherootslive,die,anddecayoverthousandsofyears,thesoilbecomeswellsuppliedwithhumus.Treerootsaremuchlarger,butbecausetheydonotinvadetheporesofthetopsoilascompletelyasthoseofgrasses,thehumuscontentofsoilsunderforestsisusuallylower.Mostofthetreesintheworld'sforestscanbedividedintotwogroups:thehardwoodswithbroadleavesandthesoftwoods(conifers)withneedles.Chemicalanalysesofbroadleavesandneedlesshowthatneedlesareusuallymoreacidbecausetheycontainfewerbase-formingelementssuchascalciumandmagnesium.Grassescontainevenmorebasesthaneitherbroadleavesorneedles(Fig.2.9).Therefore,soilsformedunderconiferforeststendtobethemostacidandleastwellbuffered(e.g.,againstacidrain).

Figure2.9Grassleavesarenormallyhighestinbases,broadleavesoftreesareintermediate,andconiferneedlesarethelowest.

Grasslandregionshavethemostfertilesoilforagriculture,butmostofthemaresubjecttoextendeddryperiods.Pioneerstendedtoselectthehardwoodforestsasplacestosettlebecausethesoilswerequitegood,andtheyneededtheforestproductsfortheirlivelihood.

TopographyThelayoftheland—itslevelnessorhilliness—iscalledtopography.Topographyinfluencestheformationofsoilprimarilyintwoways:(1)Erosioncarriestopsoilfromthehigherpositions,particularlythesideslopesofhills,anddepositsitinthevalleys.Thisresultsinrelativelythicker,morefertilesoilsinthevalleys.(2)Waterdrainsfromtheuplandstothevalleysandiftheexcessisremovedinatimelymanner,vegetationismoreabundantthere.The

abundantplantlife,whichdoesnotdecomposeasrapidlyinmoistvalleysasonthedrieruplands,alsocontributestotheformationofdeep,dark-colored,fertilesoils.Asaresult,muchoftheworld'spopulationreliesoncropsgrowninvalleysfortheirfood.

Climaticconditionsmodifytheeffectsoftopographyonsoildevelopment.Inthesubhumidanddrierclimates,thesoilsarewelldrainedinallpositionsinthelandscape,buttheydifferinthicknessbytheirlonghistoryoferosionordeposition.Inthehumidregionswitharollinglandscape,soilsmaybethinandexcessivelydrainedonthehillsandthickwithpoordrainageinthevalleys.Broad,nearlyleveltopographicpositionstypicallyhavedeeplydevelopedsoilseveniftheyliehighabovethedrainageways.Inthehumidregions,theseareaswillshowtheeffectsofexcessmoistureunlesstheparentmaterialiscoarsetextured,soitwillallowrapidinternaldrainage.Insemiaridregions,broadleveluplandstypicallyhavedeep,darkcoloredsoilsformedundergrasslandvegetation.

Topographyisastrongindicatorofsoilcharacteristicswithinaparticularregion.In1935anEnglishearthscientistnamedMilne,workinginEastAfrica,noticedthesequentialnatureofsoilsfromthetopofonehill,downthroughthevalley,upthenexthill,anddownagainrepeatedly.Beingascholarinclassiclanguages,MilneknewtheGreektermforthearcformedbyachainsuspendedbetweentwoposts.Fromthis,hederivedthetermcatena,meaningasequenceofsoilsdifferingfromeachotherduetotheirtopographicposition.Inatwo-dimensionalsense,hesaweachsoilasalinkinthechain.Acatenaisatoposequenceofsoilsthatmaydifferfromeachotherinavarietyofways,suchascompositionanddrainage.ThedrainagecatenarelationshipofahumidregionillustratedinFigure2.10ishorizontallycompressed.

Figure2.10Inadrainagecatena,thesoilreflectstheeffectsoflong-termmoistureconditions.

TimeTimeistypicallydiscussedasthelastofthefivesoil-formingfactors.Itisaconsiderationofhowlongtheotherfactorshavebeeninfluencingsoilformation.Theeffectsoftimecanbestbeseeninequatorialregions,wheretheextremesinagearewellexpressed.Geologicallyyoungareastypicallyhaveanirregulartopography,andtheyarecomparativelymorefertilebecauseyoungparentmaterialsusuallycontainanabundanceofweatherablemineralsthatslowlyreleaseplantnutrientsastheyweather.Geologicallyoldsurfaces,ontheotherhand,havelongsincelostmostoftheirweatherableminerals.Theirfertilityisfoundprimarilyintheorganic

matter,whichissubjecttorapiddepletionundercultivation.Sinceprehistorictimes,farmersinthetropicshavebeenattractedtoruggedlandscapesbecauseofthesuccessofgrowingbettercropsthere.SimilarcomparisonsofsoilfertilitycouldbemadebetweengeologicallyyoungregionssuchasthenorthernRockyMountainsandold,highlyweatheredportionsofthePiedmontPlateauinthesoutheasternUnitedStates.Inglaciatedregions,whichoccurinmuchofthenorthernpartoftheUnitedStates,thereisarelationshipbetweenthetimesincethelastglacialadvance,theirregularityofthelandscape,andthedegreeofsoildevelopmentasevidencedbytheconcentrationofclayinthesubsoil.Regionswithmorerecentglacialtill(<25,000years)havemanyundraineddepressionsthatmayformlakes.Moderatetosteepslopesarecommon,andtheleachingofclaytothesubsoilismoderate.Inregionswheretheglacialtillismucholder(>50,000years),moreofthedepressionshavebeenfilledandacompletedrainagepatternhasformed.Theslopesherearemoregentle,andthereisusuallyamuchgreaterconcentrationofclayleachedintothesubsoil(Fig.2.11).

Figure2.11Landsurfacestendtobecomesmootherovertimeashillsareworndownandvalleysarefilled.

SoilHorizonDevelopmentDuringsoilformationbothparentmaterialsandorganicmaterialsarealteredandtranslocatedsothatlayerscalledsoilhorizonsdevelop.Thelayersusuallycanberecognizedvisually.Acrosssectionofsoilhorizons,calledasoilprofile,isexposedwhenapitorroadsideisexcavated.TwoprofilesareillustratedinFigure2.12.Oneistypicalofsomeofthesubhumidgrasslandsandtheotherdepictsthesoilofhumidhardwoodforestregions.

Figure2.12Theprofileontheleftillustratesasoilfromasubhumidgrassland;theoneontherightshowsasoilsfromahumidhardwoodforestregion.

Althoughthenumberandpropertiesofthesehorizonsvarywidely,arathertypicalsoilprofileinahumidregionisdiscussedinthissection.Darkhumicmaterialscommonlyaccumulateinthetopsoil(theAhorizon),followedbyaleachedzone(Ehorizon—fromthewordeluvialmeaningwashedout).Thesubsoil(Bhorizon)commonlyhasanaccumulationofclay.ThedepthtothebottomoftheBhorizonistypicallythedepthtowhichthereareabundantplantrootsandbiologicalactivity.Certainlysomerootsmayextendmuchdeeper.

Theportionofthesoilprofilethathasbeenalteredbythesoil-formingfactorsiscalledthesolumandismadeupoftheA,E,andBhorizons.OnthesurfaceoftheAhorizon,theremaybealayerofplantresiduecalledanOhorizon.BelowtheBhorizontheunderlyingunconsolidatedmaterialiscalledtheChorizon.Ifbedrockiswithinafewfeetofthesurface,itiscalledtheRhorizon.Thesesymbolsmaybesubdividedwithsmalllettersandnumbersbecauseofthediversenatureofsoil.Thissystemprovidessymbolsusedinmakingdetailedsoilprofiledescriptions.Thesymbolsareatypeofshorthandusedbysoilscientists,andtheyrevealmuchaboutthesoilproperties.Theprincipalsoilhorizonscanbecategorizedintodiagnostichorizons,whichwillbediscussedinChapter11.

Leachingofplantnutrientssuchaspotassiumandcalciumtakesplaceaswatermovesthroughthesoil,butsomenutrientsareretainedbythefinelydividedhumusandclaymaterials.Plantstakeupthesenutrientsandtransportthemintotheirabovegroundparts.Thenutrientsarereturnedtothesoilastheseasonsprogress;thus,plantscontributetonutrientrecycling.Thisbioticcyclinghelpstokeepthesoilfrombecominginfertilebyfrequentleaching(Fig.2.13).

Weatheringisanongoingprocessinthesoilandtoalesserextentinthesubstratumbelow.Assoilages,itislikelytohaveahigherclaycontentbecauseclayresultsfromthephysicalandchemicalbreakdownoflargerparticles.

Figure2.13Bioticcyclinghelpstoconcentratenutrientsnearthesoilsurface.

Let'sTakeaTripAswetravelfromoneclimaticregiontoanother,therearedistinctchangesinthenativevegetation,andifthefarmfieldshavebeenplowed,therearedifferencesintheappearanceofthesoil,eventothecasualobserver.Ifthesoilisexposedtosomedepth,thereareevenmorechangesevidenttothosewhoexaminethesubsoilcarefully.

IfwetakeatripintheUnitedStatesfromthedesertsoftheWesttothehumidwoodlandsoftheEast,asuccessionofsoilscouldbeseen(Fig.2.14).Inthearidregions,thetan-coloredsoilisonlyalittledarkeronthesurfacethanitisdeeperdownbecausemeagerrainfallprovidesforonlysparsevegetation.Evenhere,however,therearedifferences.Saltsmaywhitenthesoilsurfaceinlowerareasifwatercontaininglargeamountsofsaltsevaporatesoffthesurface.Onveryoldgeologicsurfaces,carbonatesmayaccumulateinthesubsoiltoformrocklikelayers.Pebblesscatteredontheseancientsurfacesarelikelytohaveadarkreddish-brownvarnishfromoxidesofironandmanganese.

Figure2.14AtripthroughdifferentclimaticvegetationregionsoftheUnitedStateswouldrevealmanykindsofsoil.

Asourtriptakesusintothecentralmidwesternstates,weenteraregionwhererainismorecommonduringthegrowingseason,andwherethenativeprairiegrasseswiththeirabundantfibrousrootshavemadethetopsoilthick,dark,andrichinplantnutrients.ThesesoilsdonothavealeachedEhorizon.Theyare,inthemain,themostproductivesoilsintheUnitedStates.Whenfieldsareplowed,theyappearalmostblackfromtheabundanthumus,andifaroadiscutthroughthem,theyshowthatthehumuscommonlyextends2ormorefeet(61cm)belowthesurface.

Astheaveragerainfallandhumidityincreasetowardtheeasternone-halfoftheUnitedStates,forestsreplacethegrasslands,andthesoilsaremarkedlydifferent.Whentheyaretilled,thesesoilshaveagrayish-brownappearance,whichreflectstheirlowercontentofhumusandthepresenceofaleachedEhorizonbeneathathinAhorizon.Thesubsoilusuallyhasaconcentrationofclaythatshowsupasareddish-brownhorizoninroadcutsorotherexposures.Manyofthesesoilsareveryproductive,buttheyrequiremorefertilizerandlime

becauseleachingbygreaterrainfallhasoccurred.

IfweswingsouthacrosstheOhioRiver,wefindsoilsthataregeologicallymucholder,andsoilsinwhichtheeffectsofweatheringhavebeengreater.Herethecultivatedfieldsarequiteredinmostplacesasaresultofironfromthemineralsthathavebecomeoxidized.Inthesesoils,theclay-enrichedsubsoilformsamuchthickerzone,andtheirnativefertilityislow.

IfyoutravelfromsouthernTexastonorthernMinnesota,youwillfindthatsoilsinTexaswillgenerallyhavelessorganicmatterthanthoseinthenorth.InnorthernMinnesotathegrowingseasonismuchshorterduetothecoldtemperaturethatslowsthebreakdownortheorganicmatter.ThroughtimethisresultsinslowersoilcarboncyclingandanincreaseinsoilorganicmatterasyoumovenorthintheNorthernHemisphere.

Wheneveryouhavetheopportunitytotravel,bealerttothechangeinthesoilsandlandscapes.Ifyoulookcloselyyouwillseethatasthesoilchangessodothewayspeoplebuildroads,houses,andmanagethelandthroughtillageandcropselection.Youwillfindthatsomesoilscansupportalotpeoplewhereasothersoilscannot.

Chapter3SoilPhysicalPropertiesSoilphysicalproperties,thosepropertiesthatcanbeseenorfelt,arediscussedinthischapter.Chemicalpropertiescannotbeseenorfeltbutcanbedetectedwithsophisticatedscientificinstruments.Somechemicalpropertiescanbeeasilyalteredwithsoilamendments,butphysicalpropertiesareoftenmuchmoredifficulttochange.Thus,physicalpropertiesshouldreceivegreaterconsiderationinland-useplanning.

SoilPhasesFromaphysicalstandpoint,soilisathree-phasesystem:solid,liquid,andgas.Eachphaseisequallyessentialforgrowthofplants.Inatypicalsoilthesolidphaseismadeupprimarilyofmineralparticlesalongwithasmallamountofhumus(organicparticles).Organicsoilscommonlyfoundinwetlandsmayhaveahighamountofhumusparticlesinadditiontothemineralparticles.

Thesolidphaseisthesourceofnutrientsandprovidesanchorageforplantsandmakesupapproximatelyhalfofthesoilvolume.Theliquidandgasphasesareintheporesbetweenthemineralandorganicparticlesandoccupytheotherhalf.Theproportionofliquidandgasvariesasthesoilgainsorlosesmoisture.Plantsmustbeabletoabsorbwaterfromthesoil,andallexceptafewaquaticplantsdependuponthesoilporesfortheoxygenthatisessentialforeverycellintheirroots.Figure3.1illustratestheapproximateproportionofallthreephasesinamoistsoil.

Figure3.1Theapproximateproportionsofvariousphasesbyvolumeinamoistsurfacesoil.

SoilSeparatesThemineralfractionofthesoilconsistsofparticlesofvarioussizes.Soilseparatesaremineralparticlesthatareclassifiedonthebasisoftheirsize.Sand,silt,andclaymakeupthesoilseparates,whicharecollectivelyreferredtoasthe“fineearth”fractionandaresmallerthan2mmindiameter.The“coarseearth”fractionislargerthan2mmindiameterandconsistsofgravel,stones,andsoforth.The“fineearth”fractionplaysamajorroleinplantgrowthaswellasinfluencesland-useandmanagementdecisions.

IntheUSDAclassificationsystemsandparticlesrangeinsizefrom2to0.05mm;siltparticlesaresmallerthansandandrangeinsizefrom0.05to0.002mm;andclayparticlesaresmaller(lessthan0.002mm)thansiltparticles.Ifthediameterofmedium-sizedparticlesofclay,silt,andsandwereexpanded1,000times,theclaywouldhaveadiameteraboutthethicknessofthispage,thesiltabout1in.(2.5cm),andthesandabout40in.(1m).

SandSandformstheframeworkofsoilandgivesitstabilitywheninamixturewithfinerparticles.Puresand,however,doesnotclingtogether,soitiseasilyerodedbywaterandwind.Duringerosion,sandisnotsuspendedinthewaterorairbutbouncesalongthesurfaceandpilesupwherethevelocityofwindorwaterdecreases.Inthecaseofwinderosion,thiscausessandtoformintodriftslikesnow.

Quartzisusuallythedominantmineralinsandbecauseitisthemostresistanttoweatheringofthecommonmineralsinrocks;thus,itsbreakdownisextremelyslow.Manyothermineralsarefoundinsand,dependingontherocksfr-omwhichthesandwasderived.

Theshapeofsandgrainsismoreorlessspherical.However,theangularityofsandgrainsisvariableduetothedegreetowhichthespecificdepositwasrolledaroundbyflowingwater.

Sandcontributesverylittletoplantnutrition.Thequartzinsandcontributesnoplantnutrientstothesoilwhiletheotherminerals,suchasfeldspars,releasetheirnutrientsveryslowly.Nevertheless,soilsthathavealotoffeldsparandotherweatherablemineralsintheirsandfractiondevelopacomparativelyhigherstateoffertilityoverthethousandsofyearsofsoilformation.

SiltInmanyrespects,siltissimilartosandexceptthatitissmallerandistoosmalltobeseenwiththenakedeye.Itissphericalandmineralogicallysimilartosand.Siltistoofinetobegrittytothetouchbutimpartsasmoothfeelwithoutstickiness.Itisfineenoughtobesuspendedinflowingwater,butitdropsoutwhentheflowisreduced.Thisisthereasonthatharborsaresaidtobecome“siltedin.”Ifsiltisdisturbedbydriftingsand,itcanbepickedupandcarriedgreatdistancesbystrongwinds;thus,siltconstitutesthemainpartofthewind-depositedparentmaterial,loess.ThisconceptwillbediscussedfurtherinChapter10.

ClayThissoilseparateisforthemostpartmuchdifferent,particularlyinsizeandchemicalcomposition,fromsandandsilt.Sandandsiltareprogressivelyfinerandfinerpiecesoftheoriginalcrystalsintheparentrocks,whileclay,ontheotherhand,ismadeupofsecondarymineralsthatwereformedbythedrasticalterationoftheoriginalformsorbytherecrystallizationoftheproductsoftheirweathering.Clayissopowderyfinethat1gwouldhaveavolumeaboutequaltothatofapencileraserwhilethetotalsurfaceareawouldequalaboutone-fifthofafootballfield(Fig.3.2).Thistremendoussurfacearearesultsfromtheplatelikeshapeoftheindividualclayparticles.Themaximumdiameterofaclayparticleis0.002mm.Finerclaysintherangeof0.0001mmarecalledcolloidalclays.Theycanonlybeviewedclearlywithanelectronmicroscope.

Figure3.2Thesamemassofmineralhasmuchgreatersurfaceareawhenpulverized.

Toillustratesomecharacteristicsofclay,takealargeballofpiedoughandrollitintoathinsheetwitharollingpin(Fig.3.3).Piecescutfromthesheetcouldbestackedtomakeamodelofaclayparticle.Thepileofthinsheetswouldhaveamuchlargersurfacearea,insideandout,thantheoriginalballofdough.Similarly,eachclayparticleisactuallyastackofmanyverysmallsheets.Therearemanykindsofclay,eachwithdifferentinternalarrangementsofchemicalelementsthatgivethemindividualcharacteristics.ThemajorgroupsofclaysrelatedtotheirchemicalcharacteristicswillbediscussedinmoredetailinChapter5.

Figure3.3Alayeredclaycrystalissimilarinnaturetoastackofthinsheetsofdough.

SoilTextureSoiltextureisthedegreeoffinenessorcoarsenessofthesoil.Itisanexpressionoftherelativeamountsorpercentagesofsand,silt,andclay.Textureisapermanentpropertyofthesoil.Inageneralway,textureinfluencesthewaterandnutrientsupplyingpotentialforplants;theamountofhumus;thevolumeofpores;thebondingofparticlestoeachother;theabilityofthesoiltoadsorbandholdcertainchemicals;drainageofwater;andthesoil'sabilitytobearweight.Amongthesoilseparates,clayisthemostinfluentialonthesesoilproperties.Manyland-usedecisionsarebasedontextureofthesoil.

Allmineralsoilscanbeclassifiedinto12texturalclassesoftheUSDAclassificationsystemasrepresentedinthetexturaltriangle(Fig.3.4).Soilsthataredominatedbysandareconsidered“coarsetextured,”andthosedominatedbyclayareconsidered“finetextured.”Soilsthathavepropertiesstronglyinfluencedbymorethanonesoilseparateareconsidered“mediumtextured.”Additionsoforganicmattertoasoil(notshowninthetriangle)modifysoilbehavior;sandysoilsseemfinertexturedandclaysoilsseemcoarsertexturedthantheyreallyare.Chemicalandbiologicalpropertiesarealsochangedwiththeadditionoforganicmaterial.

Figure3.4Atexturaltriangleshowsthelimitsofsand,silt,andclaycontentofthevarioustextureclasses.

DeterminationofTextureTheproportionofsand,silt,andclaycanbeaccuratelydeterminedinthelaboratorybymeasuringthedensityofasuspensionofsoilparticlesinwaterwithahydrometer.Theresultingdata(sand,silt,andclaycontent)areplacedonatexturaltriangletodeterminethetexturalclassofasoil.Withpractice,texturecanalsobecloselyestimatedbythe“feelmethod,”whichiscommonlyusedinthefield.The“feelmethod”oftexturedeterminationrequiresrubbingamoistsoilbetweenthethumbandtheforefinger.Sandinasoilfeelsgritty.Ifmoistsoilfeelssmooth,butnotreallysticky,itisasiltysoil.Ifitisverystickyandcanberubbedintoacohesiveribbonthatextendsfromthefingerslikeabroadbladeofgrass,itisaclayeysoil.Laboratorydatacanbeusedtocalibrateone'sfingersastothefeelofeachoftheindividualsoilseparatessoastoplacethesoilinthepropertexturaldomainonthetexturaltriangle.

Asoilwithasignificantamountofsand,silt,andclayiscalledaloam.Variouskindsofloamsareclassifiedbyfeelaccordingtothedegreeofgrittiness,smoothness,andstickiness:sandyloam,siltloam,andclayloam.Asimpleloamwithoutexcessiveamountsofanysoilseparatehasabout20%clay,40%silt,and40%sand(Fig.3.4).Comparedtosiltandsand,clayissostickythatnotmuchisrequiredtogivethesoilaspecialtexture.Presenceoforganicmatterin

asoil(notshowninthetriangle)canmodifythefeelofasoil;sandysoilsseemfinertexturedandclaysoilsseemcoarsertexturedthantheyreallyare.

Thetextureofasoildoesnotindicatehowitwasformed.Didwind,water,orglacialicedroptheparticlesofsand,silt,andclayataparticularsite?Suchquestionsabouthowtheprocessesofsoilformationresultedinsand,silt,orclayfractionswerediscussedinChapter2.

SoilStructureIndividualparticlesofsand,silt,andclaytendtobecomeclusteredintounitsofvariousshapes.Thisclusteredunitisreferredtoassoilstructure,whichisdefinedasthearrangementofsoilparticles.Theresultingstructuralunitsarecalledaggregatesorpeds.Soilstructurecreatesarangeofdifferent-sizedpores.Withoutstructure,fine-texturedsoilswouldbeonemassivechunk(withmostlysmallerdiameterpores)orlikeloosebeachsand(withmostlylargediameterpores).Goodsoilstructuremeansalargevolumeofporesaswell.Goodstructureallowsthesoiltoretainadequatewateraswellasdrainexcesswater;promoteseaseofseedlingemergence,rootpenetration,andtubergrowth;airmovement;anderosioncontrol.

Structuralarrangementsresultfrombiological,chemical,andphysicalforcesthatcausethesoilparticlestobondwitheachother.Clayandhumusbecauseoftheirsmallsize,highsurfacearea,andelectricalchargesserveascementingagentsinthebondingofparticleswithintheaggregates.Microorganismsinthesoilalsoplayanimportantroleinproducingstickysubstancesthathelpcementparticlestogether.Oxidesofironandseveralcationsinsaltshelpthebondingprocess.Physicalforcesalsoplayasignificantroleinbringingparticlesclosertogethertoformaggregates;amongtheseforcesareshrinkingandswellingfromwettinganddrying,freezingandthawing,andtheactionsofexpandingrootsandofearthwormsandothersoilorganisms.Asandy-texturedsoildoesnothaveenoughcementingagentstoholdthesoiltogetherasaggregates.Asaresultthesesoilsarelikeasandybeachandareconsidered“structureless”or“singlegrained.”

Asstructureisformed,poresarecreatedwithinandbetweenaggregates.Thespaceswithintheaggregate(betweenindividualsoilparticles)aresmallporesormicropores,andthosebetweentheaggregatesarelargeporesormacropores.Thelargerporesallowwaterandairtomovethroughtheprofile,whilethesmallporesactasaspongeandretainwaterforusebyplants.Soilstructuralunitsareclassifiedaccordingtoshapeandsizeasgranular,platy,blocky,prismatic,andcolumnarstructures(Fig.3.5).

Figure3.5Soilstructuralunitsareclassifiedaccordingtoshapeandsize.

Granularstructureisbestrecognizedbyfarmersandgardenerswhostriveforamellowsoil.Themoreorlesssphericalclustersarecalledaggregates,andwhensoilistilleditcanbedeterminedifitiswellaggregatedbytheeaseofworkingit.Someclayandaplentifulamountoforganicmatterarethekeytostableaggregatesinthetopsoil.Theaggregatesincoarse-texturedtopsoilareusuallyratherporous,likebreadcrumbs,anditisdescribedashavingacrumbstructure.

Platystructurehasalonghorizontalandashortverticalaxis.Whenthisoccursinthesubsoil,waterpenetrationisrestricted.Forexample,on-sitewastedisposalsystemsforruralhomesarelikelytofailifsoilbeneaththeseepagebedhasplatystructure.

Blockystructureisthemostcommonstructureinthesubsoilinhumidregionsthathadforestasitsnativevegetation.Theverticalandhorizontalaxesareaboutthesamelength.Thisgivesasomewhatcubicalformthatallowsgoodwaterpercolationalongtheboundariesoftheblocks.Ifthereisplentifulclayinthesoil,theedgesoftheblocksarelikelytobeangular.Structureislesswelldevelopedincoarse-texturedsoilsandedgesoftheblocksarerounded.Thisisknownassubangularblockystructure.

Prismaticstructureisbestdevelopedinthesubsoilswithaplentifulamountofclayinregionswherethesoilbecomesperiodicallydesiccated.Theseconditionsaremostcommonwhere

prairiegrasseswerethenativevegetation.Thesidesoftheprismsactasanavenueforwatermovement.

Columnarstructureisanundesirablevariationofprismaticwhereinthetopsoftheprismsareroundedandusuallycoveredwithgraysoilparticles.Ifthetopsoilisclearedaway,thetopsofthecolumnslooklikethetopsofbakingpowderbiscuits.Thishappenswhenthereistoomuchsodiuminthesoil.Thisconditionisextremelyrestrictivetowaterpercolationbut,fortunately,itisusuallylocalizedinsemiaridregions.

Soilstructuralunitscanalsobeclassifiedbasedontherelativebondingstrengthoradherenceofindividualparticlestoeachotherwithinaped.Thebondingstrengthorstabilityofanaggregateisitsabilitytoresistbreakdownfromexternalforcessuchasraindropimpactandtillageactivities.Thebondingstrengthofsoilaggregatesisclassifiedasweak,moderate,orstrong.Theaggregatestabilityofthesurfacesoilisparticularlyimportantforminimizingsoilerosion.

BenefitsofAggregationAwell-aggregatedsoilisconsideredahigh-qualitysoil.Awell-aggregatedsoilisconsideredtohavegoodtilthbecauseitsloosenessallowsbetterwaterinfiltration,seedlingemergence,rootgrowth,andairandwatermovementintherootzone.

Aggregatesatthesurfaceofthesoilareconstantlysubjecttodestructiveforces(eitherphysicalorchemical)thatmayweakenthebondingorshearthesoilparticlesfromtheaggregate.Amongthephysicalforcesareraindropimpact,rapidwetting,rapidfreezing,intensivetillage,compactionduetotrafficandharvestingequipment,andsoforth.Oncetheaggregatesfallapart,itmaytakeseveralyearsofgoodsoilmanagementbeforethestructurewillreverttoitsoriginalstate.

Soilsundernaturalvegetation,orheavilymulchedsoils,tendtohavegoodstructurebecausetheyareprotectedfromthephysicalforcesofraindropimpact.Thesesoilstendtohavemorehumusthantheirtilledcounterparts,andthereforetheyarelikelytohaveahealthypopulationoforganisms,whichhelpsprotectthesoilfromfallingapart.Withouthumus,soilswithasignificantamountofsiltandclaybecomeverydenseandcloddywhentheyaretilledrepeatedly(Fig.3.6).

Figure3.6Soilwithouthumusbecomescloddy(left),whereashumus-richsoilisgranular(right).

Weaklybondedaggregatesatthesurfaceofabaresoilareparticularlysubjecttobreakdownbytheimpactofraindrops.Astheseaggregatesfallapart,thefinerparticlesmoveintotheporesatthesoilsurface,pluggingthemandformingasurfacecrust.Inthissituation,rainwaterwillhavedifficultyenteringthesoil;insteaditbeginstoflowacrossthesoilsurface,creatingapotentialforacceleratederosionofsurfacesoilparticles.Crustedsurfacesmayalsoaffectseedlingemergence.Protectingthebaresoilwithmulch,cropresidue,orvegetationwillprotecttheaggregatesfrombreakingandmaintaingoodstructureunderthecover.Thestructureofsoilsinundisturbedforestsiswellprotectedbythecanopyaswellasalitterlayeronthesurface.

Excessiveweightduetotillagemachineryorharvestingequipmentcansqueezethesoilparticlestogether(minimizethelargepores)andcompactthesoil.Theeffectsofcompactiononstructuredestructionbecomeevenmoreobviousifthemachineryisoperatedwhenthesoiliswet.Thewaterbetweentheparticlesservesasalubricantallowingtheparticlestocomeclosertogetherduetoexternalpressures.Compactionfromtheweightofmachineryimpactsthesubsoil'sstructureandwillaffecttheair–waterrelationsinthesubsoil.

Tillagepracticesthatpulverizethesoilintheplowlayeralsodestroythestructure.Inaddition,thechurningofthesoilexposeshumusintheplowlayertoair,whichincreasestherateofbreakdownofhumus.Consequentlymedium-andfine-texturedsoilsbecomedenseandcloddybecauserepeatedtillagemayhavedepletedtheirhumuscontent.Forthisreason,undisturbedsoilssuchasforestorprairielandtendtohavemorehumusandbetterstructurethantheirtilledcounterpartsinthesameregion.

Thegoalofgoodfarming,forestrypractices,andurbansoilmanagementshouldbetoprotectsoilstructure,especiallyatthesoilsurface.Aspartofthisgoal,managersshouldavoidsoilcompactionbyminimizingtillageoperations,andavoidanytillageactivityifthesoilmoistureislikelytopromotecompaction.Goodmanagementpracticesmustincludeaplantoincreasehumuscontentofthesoilbyperiodicallyaddingorganicmatter;maintainthehumuscontentatalevelthatwillsustainstableaggregates;plantcloselyspacedvegetationthathasafibrousrootsystemtorestorethehumuscontentandthesoilstructure;andprotectthestructureatthesurfacebynothavingabaresoilexposedtoraindropimpact.Ifthesepracticesare

implemented,naturalcyclesoffreezingandthawing,orwettinganddrying,couldhelprestoredamagedsoilstructureovertime.

PorosityandDensityThevolumeoccupiedbyporesinsoiliscalledporosity.Astheparentmaterialofsoilbecomesweathered,loosened,andmixedbyavarietyofforces,porespacedevelops,providingaplaceforairandwatertobeheld.Soilsthathavegoodstructureshouldhave50%oftheirvolumeconsistingofpores.Boththeamountofporespaceandthesizeoftheporesareimportant.Smallporesretainwaterverywellwhileinlargepores,waterdrainsoutandairmovesin.Sand-texturedsoilshavemostlylargepores,andthereforetheytendtodrainwaterrapidly.Thelackofsmallporesalsomakesthesand-texturedsoilsdroughty.Clay-texturedsoilsmayhaveagreaterproportionofsmallporesandtendtoretainwaterbetterandsometimesmaybecomewaterlogged.Therefore,itisdesirabletohaveabalanceofbothlargeandsmallporessuchasfoundinmedium-texturedsoils(siltloamsandloams)thatareingoodstructure.

Densityindicatestheloosenessortightnessofasoil.Densityofsoil,calledbulkdensity,includesboththesolidparticlesandtheporespacesamongthem.Ifasoiliscompacted,theamountofporespaceisreducedandtheweightofagivenvolumeofsoilisincreased.Themeasureofdensityisacomparisontowater,whichhasadensityof1g/cm3.Themineralgrainsinthesoilhaveadensityofabout2.6g/cm3.Thetotalvolumeofthesoilisaround40–60%porespace,sobyusingameanvalueof50%forporosity,bulkdensitywouldbe1.3g/cm3.Thisisone-halfthedensityofthemineralsinsolidrock(Fig.3.7).Densitycanbeexpressedintheimperialsystem,suchaspoundspercubicfoot,butitiscustomarytoexpressdensityinmetricunits.

Figure3.7Whenrocksweather,theybecomeloosenedandlessdenseassoilisformed.

Somesoilshavenaturallycompactedlayers(pans)thatmayhaveahighbulkdensity.Suchdensitiesrestrictrootpenetrationandwatermovement.Inothercases,heavytractorsandmachinerymaycauseseriouscompaction(Fig.3.8),whichlimitsplantgrowth.Inrecentyears,therehasbeenashifttowardtheuseoftillageequipmentthatproperlyloosensthesoil,leaves

someprotectivecropresidueonthesurface,andallowsforfewertripstobemadeoverthefield.

Figure3.8Thezoneofcompactionhasahigherbulkdensityandlowerpermeability.

CompositionofSoilPoresSoilporescanbefilledtotallywithairorwater.Ifamedium-texturedsoilismoistbutfreelydrained,theairandwatercontentofitsporesareprobablyaboutequal.Normally,soilsthatseemdrystillcontainsomemoistureandtherelativehumidityintheporesremainsnear100%.Thewaterintheporesisactuallyasoilsolutionbecauseitcontainstheionsofdissolvedsalts.Someareplantnutrientsthatmaybeabsorbedbyplantroots.Thesoilsolutionmayalsocontainorganiccompounds,suchashumicandfulvicacids.Humicacid,forexample,frequentlygivesthesoilsolutionabrownishtinge.Anabundanceofdissolvedhumusmaygivealkali(sodic)soilsaverydarkbrowncolor,butthisconditionisnotwidespread.

TheEarth'satmosphereisabout78%nitrogen(N2),20.9%oxygen(O2),and0.03%carbondioxide(CO2),withtraceamountsofothergases.Ifthesurfacesoilhasfreeexchange,thesoilairandtheatmospherewillhaveaboutthesamecomposition.However,whentheplantrootsandsoilorganismsareflourishinginthegrowingseasons,CO2isbeingrespiredbythelivingcellsasoxygenisbeingabsorbed.Nitrogenisessentiallyinertforallbutafewspecializedorganisms,soitscontentremainsunchanged.O2andCO2arethemainvariables.ItiscommonintherootzoneforO2todropto10%andtheCO2toriseto10%withoutilleffectstotheplants.Even5%O2and15%CO2maynotbeharmful,sincecropsvaryintheirtolerancetoCO2.Whensoilporesfillwithwater,thelife-sustainingO2issoondepleted.Cornisverysensitivetothiscondition,butsorghumcanwithstandseveraldaysoffloodingwithoutpermanentdamage.

SoilConsistenceAdescriptionofsoilconsistencegivesanindicationofhowsoilwillreacttomechanicalmanipulationatvariousmoisturecontents.Thefieldmeasurementsaremadebetweenthe

fingers,whichgiveagoodindicationofhowthesoilwillreacttotillage,traffic,digging,orsimilaractivity.Whenthesoilisdry,itisdescribedaccordingtoafixedsetofparametersastoitsdegreeofhardnessorsoftness.Inthemoiststate,thedegreeoffriabilityorfirmnessisused.Whenwet,itisrankedbyitsstickiness.Theamountandtypeofclayisthesinglemostimportantcharacteristicindeterminingsoilconsistence.Forexample,aclayeysoilislikelytobeveryhardwhendry,veryfirmwhenmoist,andverystickywhenwet.

Forengineeringpurposes,morequantitativemeasurementsofsoilconsistencecanbemadeinalaboratoryandexpressedasapercentageofwaterbyweightremaininginthesoilwhenthesoildisplaysthefollowingcharacteristics:

Plasticlimitisthemoisturecontentwhenthesoilcrumblesasitisrolledintoa“wire”betweenthepalmofthehandandafrostedglassplate.

Liquidlimitisthemoisturecontentatthepointwhenthesoilflowsinacurved-bottomdishafter25impactsinasimplemachinethatliftsthedishashortdistanceandletsitdroponahardsurface.Aspecifictoolhasbeendesignedforthismeasurement.

Plasticityindexisthedifferencebetweenthevaluesofplasticlimitandliquidlimit.

Thesevaluesareusedtopredicttherelativeeaseordifficultyofworkingwithearthenmaterialsunderdifferingdegreesofwetness.

SoilColorInChapter2,thedifferenceintheappearanceofthesoilfromoneregiontoanotherwasconsidered.Thecolorchangesreflect,forthemostpart,differencesinthequantityofhumusandthechemicalformoftheironpresent.Itistrue,however,thatthepigmentationofagivenamountofhumusisusuallydarkeringrasslandregionsthaninforestedregions,particularlyinwarmareas.

Colorofthesubsoilgivesastrongindicationofsoilhydrologyandthemineralcompositionofthesoil.Insomecases,colorisanindicatorofiron,humus,carbonates,and/orsulfates.

Varyingshadesofred,yellow,andgrayinsoilsareusuallyduetotheconcentrationandformofironpresent.Redmeansthattheironisoxidizedandnothydrated.Yellowindicateshydrationandsometimeslessoxidation.Grayindicateschemicalreductioncausedbywetnessandlackofoxygen.AnexceptiontothisisthegrayEhorizonjustbelowthesurfaceofsomewell-drainedsoils.

Graycolorsinthesubsoiloracombinationofgrayandblotchesofyellowandredmottlesareextremelyimportantforinterpretingthenaturaldrainageconditionofthesoil.Mottlesarefoundatadepthtowhichexcesswateraccumulatesduetolackofdrainageorifthewatertablerisesperiodicallyduringthewarmseasons.Evenwhenthewatertabledrops,telltalesignsofsoilcolorsareleftbehind,andtheseareusedasabasisfordesigningsepticsystems,tiledrainage,andthelike.

Theabsorptionofsolarradiationisgreaterondarksurfacesthanonlightones.Thisis

certainlytrueforbaresoils.Colordifferenceshaveacomparativelyminoreffectonthetemperatureofthesoilbelowtheshallowsurfacelayer,buteventhiscanbeimportantforseedgermination.Solarradiationhasagreaterimpactonbaresoilsthanonsoilswithplantcoverbecausewhensoilsbecomevegetated,leavesinterceptthesolarradiationbeforeitreachesthesoilsurface.

Soilscientistsuseasetofstandardizedcolorchartstodescribesoilcolors.ThesechartsarecalledtheMunsellcolors.Theyconsiderthreepropertiesofcolor—hue,value,andchroma—incombinationtocomeupwithalargenumberofcolorchipstowhichsoilscientistscancomparethecolorofthesoilbeinginvestigated.Thissystemissuperiortousingdescriptivetermsalone,whichmaynotmeanthesamethingtoeverybody.

Chapter4SoilBiologicalPropertiesBacteria,fungi,worms,insects,smallmammals,andmanyotherorganismsinhabitthesoil.Theyparticipateinandregulatemanyphysicalandchemicalprocesses.Soilorganismscreatefavorableconditionsforthegrowthofplantsandalsodecomposeplantandanimalremains.

Animalsinthesoilmakeopeningsthroughitthatinfluencethemovementofwaterandairintoandthroughthesoil.Termites,forexample,air-conditiontheirmoundsbychannelingairthroughthem.Eventhemostdesolatelandscapesonearthhaveprimitivesoils,showingtheeffectsofwaterprovidingforlifeinthesoilandthetranslocationofsaltsandothercompounds.Thereisnosoilwithoutlifeandnohigherformsofterrestriallifewithoutsoil.

Plantrootscanextenddownthroughthesoilforseveralfeet(meters).Abovegroundpartsofplantsinsomeforestsextendmorethan100–200ft.(30–60m)high.Shadefromthevegetationlowerstheamountofsoilsurfaceexposedtofullsunlight.Rootsabsorbalargeamountofwaterandthiswaterisconductedthroughthestemstotheleaveswhereitiseitherutilizedbytheplantoritpassesintotheairaswatervapor(seeFig.6.12).Themanytonsofplanttissueperacrethatdieeachyear—includingroots,leaves,fallenbranches,andbark—becomeapartofthesoilagainthroughdecompositionbysoilorganisms.

Onwell-draineduplands,leavesthatfallontheforestfloorattheendofthegrowingseasoninhumidtemperateregionsarenearlyalldecomposedbytheendofthenextgrowingseason.Inlakesandwetlands,decompositionofplantremainsisslowedbecausethecoverofwaterexcludesoxygen.Plantmaterialmayaccumulateinwetlandsaspeat(whichismadeupofidentifiableplantparts)andmuck(whichisasoilcomposedofhighlyrotted,darkorganicmatter).Inuplandmineralsoilsthisdarkmaterialiscalledhumus(Fig.4.1).

Figure4.1Atopographicsequenceofsoilsinahumidtemperateclimaticzone.

Soilsareclassifiedasmineralsoilsandorganicsoils.Thedifferenceisintheamountoforganicmatterpresent.Arbitrarily,about25%organicmatterbyweightisthedividingpoint

betweenmineralandorganicsoils.Soilswithmoreorganicmatterarecalledorganicsoils(peatormuck).Soilswithlessorganicmatterarecalledmineralsoilsbecausetheyarecomposedmostlyofinorganicsand,silt,andclaythathavebeenderivedfrommineralsandrocks.Agivenvolumeoforganicmatterismuchlighterthananequalvolumeofmineralsoil.Thus,asoilwith5%organicmatterbyweighthasabout10%organicmatterbyvolume.

OrganicMatterandHumusOrganicmatterisageneraltermthatincludeslivinganddeadorganisms,plantandanimalresiduesinvariousstagesofdecay,andhumus.Soilorganicmatter(SOM)isusuallycomposedof50%carbon,5%nitrogen,0.5%phosphorus,0.5%sulfur,39%oxygen,and5%hydrogen,butthesevaluescanvaryfromsoiltosoil.Uplandsoilsconsistlargelyofmineralparticles;however,thesurfacesoil,orplowlayer,maycontainconsiderableorganicmatter,whichisthepartiallydecomposedresidueofplantsandanimalsthatliveinthesoil.Humusgivessoilthedarkcolorwidelyassociatedwithhighfertility,althoughthisassumptionisnotnecessarilytrueforsoilsthathavebeenheavilycroppedorfornaturallyinfertilesoils.Inmostsurfacesoilsoftemperatehumidregions,thehumuscontentisbetween1and4%byweight(ortwicethatbyvolume);butthissmallquantityhasagreatinfluenceonthephysical,chemical,andbiologicalprocessesthattakeplaceinthesoil.Figure4.2showspatchesofthehumusinporesbetweenrootsandparticlesofmineralsoil.Inaridregions,thesurfacesoiltypicallyhaslessthan1%humusbyweightbecausetemperaturesarefavorablefororganicmatterdecompositionandvegetativegrowthislimitedbylowrainfall.

Figure4.2Thesurfacesoilcontainsmineralparticlesandorganicmatter.

Humusmakesupabout60–80%ofSOManditisderivedmainlyfromplants(flora),withasignificantportioncomingfromtheroots(Fig.4.3),andaverysmallfractioncomesfromsoilanimals(fauna).Itisformedbydegradationandsynthesisprocesses.Inalkalineandneutralsoils,therapiddecompositionofplantresiduesbysoilfaunaandmicroorganismsresultsintheorganicfractionofthesoilbeingdominatedbyhumus.Inacidicsoils,decompositionisslow

andplantfragmentsmakeasignificantcontributiontotheorganicfraction.Soilsformedunderprairiegrasslandsgenerallyhavegreateramountsofhumusthandothoseformedunderforestvegetationbecauseofthehighdensityofgrasslandvegetationandthefibrousrootsystemofgrasses.Intheforestthevegetationatgroundlevelisnotnearlysodense,andmostoftheorganicresiduefromlivingplantsaccumulatesonthesurfaceofthegroundasleavesintheautumn.Muchofthehumusinthesurface,5or6in.(12or15cm),offorestsoilsresultsfromincorporationofthisplantresidueintothesoilbyinsects,worms,andothersoilfauna.Inagriculturaluse,theincorporationofplantresidueandmanurecontributestotheformationofhumus,butthedecompositionofplantrootshasbeenfoundtobemoreimportant.

Figure4.3Humus,shownasadarklayer,canbederivedfromleaflitterontheforestfloororfromrootsinsurfacesoil.

Humuscanbedividedintonon-humicandhumicsubstances.Non-humicsubstancesconsistofcarbohydrates,proteins,aminoacids,fats,waxes,andlowmolecularweightorganicacids.Theyarereadilyattackedbymicroorganismsandtheyarerapidlydecomposed.Humicsubstances,ontheotherhand,arechemicallycomplexorganiccompoundswithlargemolecularweightsandtheyarethereforerelativelyresistanttomicrobialattack.Theyaremostlyresponsibleforcationexchangeandinteractionswithsoil-appliedpesticides.

Basedonsolubilityinacidandalkali,humicsubstancescanbefurthersubdividedinto:

1.Fulvicacid,whichislowinmolecularweight,lightincolor,solubleinbothacidandalkali,andmostsusceptibletomicrobialattack(15–50years)

2.Humicacid,whichismediuminmolecularweightandcolor,solubleinalkalibutinsolubleinacid,andintermediateinsusceptibilitytodegradationbymicrobes(100+years)

3.Humin,whichishighinmolecularweight,darkincolor,insolubleinbothacidandalkali,andmostresistanttomicrobialattack.

Thesoilisteemingwithmanyformsoflife,eachoccupyinganichethatisvitaltotheentireschemeoflife.Formicroorganismsandsmallanimals,thesoilprovidesenvironmentswhereconditionsoffeastorfaminemayoccursidebysideorfolloweachotherrapidly.Asamoistgrowingseasonissucceededbyadryorcoldseason,vastnumbersoforganismsdie.Withinthesoil,smallchambersfullofrichhumusanddebrismaybeseparatedbyvolumesofsoilthat,likeundergrounddeserts,arenearlydevoidofdecomposableorganicmatter.Tosurvive,therefore,mostsoilorganismsmustfindsomethingtoeatwithinafewmillimeters.Earthworms,ontheotherhand,arestrongenoughtomakechannelsandmoveseveralfeetinsearchoffallenleavesorotherplantdebris.

TheCarbonCycleLifeisessentialtotheexistenceofatruesoil.Ofthecountlessmicroorganismsthatliveinthesoil,allbutafewderivetheirenergyfromtheoxidationofcarboncompounds.Soilorganicmatter,mostofwhichishumus,servesasanenergysourcefortheseorganisms.Manyfunctionsinthereleaseofnutrientstoplantsarecarriedoutbysoilmicroorganisms,butonlythecarboncyclewillbediscussedatthispoint.

Duringphotosynthesis,plantstakecarbondioxide(CO2fromtheatmosphereandcombineitwithwatertoproducesugarandsubsequentlyallplanttissue.Theplantsdieorareeatenbyanimalsandtheresidueisreturnedtothesoil.Someofthisresiduedecomposesonthesurfacewhilesomeisincorporatedintothesoil.Ultimately,mostorganicmaterialisdecomposedbysoilorganismsandreturnedtotheatmosphereasCO2,whereitcanagainbeusedbyplants(Fig.4.4).

Figure4.4Carbonentersthebiospherethroughphotosynthesisandiscycledbackintotheatmospherebydecomposersandbyburning.

Humusiscontinuouslybeingdecomposedandnewhumusisbeingformedtoreplacetheold,exceptwherehumanmismanagementinterruptsthecycleofreturningplantoranimalresiduestothesoil,andthusaltering(orreducing)thebeneficialeffectsoforganicmatter.Thesoil—whichsupportslivingplants,animals,andhumans—iseverreadytotakeintoitselfanythingthathasdied.Anygreattreeisdestinedsomedaytofallandbeincorporatedintothesoilagain.

FactorsAffectingSoilOrganicMatterLevelsOrganicmatterlevelsofmineralsoilscanvaryfromlessthan1%incoarse-textured,sandysoilstomorethan5%infertile,prairiegrasslandsoils.Theamountisinfluencedbythefivesoilformingfactors,discussedinChapter2.Thesefactors,arrangedintheorderofimportance,areclimate>vegetation>topography=parentmaterial>age.

ThefollowinggeneralizationshavebeenmaderegardingSOMlevelsinvirginsoils:

1.Soilsformedundergrasslandsgenerallyhavegreateramountsofhumusthansoilsformedunderforestvegetation.

2.TheamountofSOMincreaseswithincreasingprecipitationanddecreaseswithincreasingtemperature.

3.Fine-textured(clay)soilshavehigherSOMlevelsthancoarse-textured(sandy)soils.

4.PoorlydrainedsoilshavehigherSOMlevelsthanwell-drainedsoils.

5.SoilsinlowlandshavehigherSOMcontentsthansoilsonuplandtopographicpositions.

TheDecompositionProcessDuringdecomposition,complexorganicmoleculesarebrokendownintosmallerandmoresolubleinorganicmoleculessuchasammonium(NH4

+)andnitrate(NO3−),amongothers.The

processoftransformingorganicformsofnutrientsintoinorganicformsiscalledmineralization.Soilanimals(fauna)performmuchoftheinitialmechanicalbreakdownofplantresidues,afterwhichsoilmicroorganisms(microflora)secreteenzymes(extracellularenzymes)ontotheremainingmaterialsandcarryoutdecomposition.Dependingonthecarbontonitrogenratioofthedecomposingmaterial,thesenutrientsmaybereleasedintothesoil(netmineralization)ortheymaybeusedbythesoilorganismstobuildtheirowncelltissues(immobilization).Soilfaunaandmicrofloraalsohaverelativelyshortlife-spansandtheyaredecomposedbyothermicrobeswhentheydie.Bybreakingdowncarbonstructuresandrebuildingnewones,soilorganismsplayanessentialroleinnutrientcyclingprocessesand,thus,intheabilityofasoiltoprovideplantswithsufficientnutrients.

Mineralizationisaccomplishedbybothaerobicandanaerobicorganismsusingenergyderivedfromcarboncontainedinthedecomposingorganicmatter.Underaerobicconditions,themainproductsofdecompositionarecarbondioxide(CO2),water,inorganicnutrients,microbialbiomassandhumus.Inenvironmentswhereoxygenisinlimitedsupply,themainproductsof

decompositionaremethane,somecarbondioxide,hydrogensulfide,ammonium,organicacidsandalcohols.ByconvertingthecarboninorganicmaterialstoCO2,microorganismscompletethebiologicalcarboncyclethatwasinitiatedduringphotosynthesis.Successivedecompositionofdeadmaterialandmodifiedorganicmatterresultsintheformationofamorecomplexorganicmattercalledhumus.

FactorsAffectingtheRateofDecompositionSincesoilfaunaandmicrofloraarelivingorganisms,theyaregreatlyaffectedbyphysicalandchemicalenvironmentalfactorssuchasmoisture,temperatureandsoilpH.Theidealconditionsfordecompositioninclude:moisturecontentnearthesoils'water-holdingcapacity,temperatureof90°to100°F,soilswithoxygencontentabove5%,andsoilpHnear7.0.Otherfactorsaffectingtherateofdecompositionarethesizeoftheresidue,chemicalnatureoftheorganicmaterialasdictatedbyitsnitrogenandlignincontentsandC:Nratio,andtypeandamountofclaymineralspresentinthesoil.Generally,solubleorganicmaterialswithsimplemolecularstructures,youngleguminousplants,andresidueswithlowC:Nratiostendtodecomposemostrapidly.Largeamountsofclaytendtolowertherateofdecomposition.

ImportanceofSoilOrganicMatterSoilorganicmatterregulatesseveralattributesthatenhanceplantproductivityandenvironmentalquality.

Soilorganicmatterisahugereserveofseveralnutrients.Mineralizationoforganicmatterbymicroorganismsreleasesnutrients(N,P,S,andmanyminornutrients)ininorganicformsthatcanbetakenupbyplants.Soilorganicmatterimpartsadarkcoloronthesoil,andthismayaltersoilthermalpropertiesasdiscussedinChapter7.Soilorganicmatterhastheabilitytoabsorbupto20timesitsmassofwater,therebygreatlyincreasingthecapacityofsoilstostorewater.Theactivitiesofmicroandmacroorganismspromoteformationofmacroporesandaggregates,resultinginimprovedsoiltilth,infiltration/drainage,andreducederosion.OrganicmattercomplexeswithAl3+andmetallicions,particularlyFe3+,Cu2+,Zn2+,andMn2+,makingthesemicronutrientsmoreavailableforplantuptakewhilereducingpotentialtoxicitiesaswellasenhancingtheavailabilityofphosphorusinlowpHsoils.Thecationexchangecapacity(seediscussiononCECinChapter5)ofsoilhumusenhancestheretentionofcations(e.g.,Al3+,Fe3+,Ca2+,Mg2+,NH4

+),therebypreventingthemfromleachingtodeepersoillayers.Soilorganicmatteralsoaffectstheefficacyofsoil-appliedherbicides.OtherbenefitsofsoilorganicmatterincludeincreasedbufferingofsoilpHandcarbonsequestration.

CarbonSequestrationOneenvironmentallyimportantfunctionofsoilisthesequestrationofcarbonthroughplantgrowth.SequestrationisthetakingofgaseousCO2fromtheatmosphereandstoringitinstable

solid(organiccompoundsandcarbonates)form.ItoccursthroughchemicalreactionsthatconvertCO2intoinorganiccarbonatesandasplantsphotosynthesizeatmosphericCO2intoorganiccompoundsingrowingplants.Aportionofthecarboninplantbiomasseventuallybecomessoilorganiccarbonduringthedecompositionprocess.

Theorganicmattercontentofsoilusuallydecreasesbybetween40and60%whengrasslandsandforestsareconvertedtocropland.ThisresultsinthereleaseofCO2intotheatmosphere.EmissionofCO2bytheburningoffossilfuelsandfromothersourceshasbeenasourceofCO2increaseintheatmosphere.TheneteffectofincreasedatmosphericCO2isnotknown,butithasbeensuggestedthatreductionofCO2intheatmospherewouldbe“environmentallyfriendly.”

Onreasonablyfertilesoilswithreliablewatersupply,yieldsinlong-termarableagriculturalsystemshavebeenmaintainedatveryhighlevelsbyapplyingsubstantialamountsoffertilizerandothersoilamendments.Intropicallow-inputagriculturalsystems,yieldsgenerallydeclinerapidlyasnutrientandsoilsorganicmatterlevelsdecline.

Thefactthatclearingofvirginlandsforagriculturalusehasresultedinlossesofalargeproportionofsoilorganiccarbonmeansthatthereispotentialtobuildsoilcarbontoimprovetheproductivityofsoils.MostSOMisfoundinthezoneofmaximumbiologicalactivity,thatisthetopsoilorplowlayer.Therefore,anythingdonetothislayerwillinfluencethelong-termbuildupordepletionofSOM.SinceSOMlevelsareabalancebetweentheratesatwhichcarbonisaddedandlostfromthesoil,wecanincreaseSOMlevelsbyincreasingcarboninputratesand/ordecreasinglossratesresultingfromdecompositionandsoilerosion.

Adoptionofwidespreadsoilconservationpracticeshasbeenhelpful.Treesareespeciallybeneficialbecausetheysequestercarboninwoodforlongperiods.

Managementpracticessuggestedtoincreasesoilorganicmatterlevelsinclude:

1.conservationtillage,

2.propermanagementofcropresidue,suchasminimumtillageandstubblemulching,maximizingeconomicplantpopulations,

3.applicationoforganicamendmentssuchasmanures,composts,biosolids,andBiochar,

4.rotationstoincludeforageorhigh-residuecrops(suchassorghum),

5.precisionagriculture,includingvariablerateapplicationoffertilizer,

6.covercrops,

7.agroforestryinwhichcropsorforagearegrownbetweentherowsoftrees,

8.pastureestablishmentusingplantswithahighproportionofbelow-groundbiomass,

9.irrigation,and

10.terracing.

Insummary,anypracticeisdesirableifitdecreasesdecompositionratesandincreasesyieldsand/orincreasestheamountofcarbonsequesteredfromatmosphericCO2.

PlantRootsandtheRhizosphereTherhizosphereisthevolumeofsoil,water,andairplusassociatedorganismsimmediatelyaroundtherootofaplant.Figure4.5showsthatthesurfaceofarootiscommonlysurroundedbygelatinousmaterialinwhichclay,organicdebris,andmicroorganismsareabundant.Plantrootsabsorbwaterandnutrientsfromtherhizosphere.TherootsmayreleaseCO2andoxygen.TheCO2makesthesoilsolutionslightlyacidsothatplantnutrientsmaybemorereadilyavailableforuptake.Theoxygenmayfavorprecipitationofirontoformafilminthesoilneartheroot.Outerlayersoftherootmaysloughoff,enrichingthesoilwithorganicmatter.

Figure4.5Therhizosphereisthevolumeofthesoil,water,andairimmediatelyaroundtheplantroot.

MicroorganismsSoilmicroorganismsdecomposeanddisposeofplantandanimalremains.Intheprocess,theseorganismsformhumus(Fig.4.6),whichisamoreactivecomponentofsoilthanmineralclays.Microorganismsalsoperformimportantstepsinvariousnutrientcyclesandinsolid,liquid,andgaseousphasesofthesoil–plantrootsystem.Withouttheseorganicprocessesthecycles

wouldloselifesupport.Thereisabiologicalrulestatingthatthesmallertheorganism,thegreateritsnumberandinfluence.Thus,theactionofmicroorganismsinthesoilisfarmorewidespreadandofgreaterimportancethanthatofinsectsandrodents.

Figure4.6Microorganismsinthesoilareinstrumentalindecomposingplantmaterial,resultingintheformationofhumus.

Somemembersofeachgroupoforganismsperformspecializedfunctionsinthesoil.Itisbeyondthescopeofthisbooktodiscussallofthese,butsomeareconsideredinthefollowingparagraphs.Table4.1summarizestheessentialfunctionsperformedbysoilorganisms.

Table4.1Essentialfunctionsperformedbysoilorganisms

Functions OrganismsinvolvedMaintenanceofsoilstructure

Invertebratesandplantroots,mycorrhizaeandothermicroorganismsthatstirormixthesoil

Regulationofwatermovementinthesoil

Plantrootsandinvertebratesthatburroworboreintothesoil

Carbonsequestrationandgasexchange

Mostlymicroorganismsandplantroots,somecarbonisprotectedinlargecompactaggregatesproducedbyinvertebrates

Soildetoxification MostlymicroorganismsNutrientcycling Mostlymicroorganismsandplantroots,somesoil-andlitter-

feedinginvertebratesDecompositionoforganicmatter

Varioussaprophyticandlitter-feedinginvertebrates(detritivores),fungi,bacteria,actinomycetesandothermicroorganisms

Suppressionofpests,parasitesanddiseases

Plants,mycorrhizaeandotherfungi,nematodes,bacteriaandvariousothermicroorganisms,earthworms,variouspredators

Sourcesoffoodandmedicines

Plantroots,variousinsects(crickets,beetlelarvae,ants,termites),earthworms,vertebrates,microorganisms(mostlyactinomycetes)andtheirby-products

Symbioticandasymbioticrelationshipswithplantsandtheirroots

Rhizobia,mycorrhizae,actinomycetes,andvariousotherrhizospheremicroorganisms,ants

Plantgrowthcontrol(positiveandnegative)

Plantroots,rhizobia,mycorrhizae,actinomycetes,pathogens,parasiticnematodes,insects,plant-growthpromotingrhizospheremicroorganisms,biocontrolagents

Biologicaldecomposition/weatheringofrocks

Cyanobacteria,nitrogen-andcarbon-fixingbacteria,fungi,andlichens

Livingorganismsareseparatedintotheprokaryotes,whicharenotclearlyeitherplantsoranimals,andtheeukaryotes,whichincludetheplantsandanimals.Bacteriaandactinomycetesareintheformergroup;mostfungiandalltheprotozoansareinthelatter.

Estimatesofthenumbersofsoilmicroorganismsinagramofsoil(aboutthevolumeofalimabean)rangefromseveralhundredmilliontoafewbillion.Mostarebeneficialtoagriculture,butallgroupscontainthosethatcancausecropdiseases.

TypesofMicroorganismsBacteriaareone-celledorganismsthatarethemostabundantformsoflifeinmostsoils.Theycanoccursinglyorjointogetheringroups.Incroplandtheyareprimarilyresponsibleforthe

decayofresidue.Theysecreteextracellularenzymesthatbreakdownorganiccompoundssuchassugars,starches,cellulose,andsoon,intobasicchemicalcomponentslikecarbonandnitrogen,whichthebacteriacanuseforenergyandgrowth.Anynutrientsnotneededbythebacteria(orotherdegradingorganisms)arereleasedintothesoilandbecomeavailableforplantuptake.Bacteriaalsoperformamultitudeofotherfunctions.Thoseinvolvedwithnitrogenarecoveredinmoredetailinthediscussionofnitrogenfixation.

Actinomycetesaremycelialbacteriathathavethreadlikeextensionsthatareallpartofthesinglecell.Theremaybeseveralmillionactinomycetesinagramofprairiesoil.Theypreferwarm,moistsoil,andtheirnumbersdonotdiminishasrapidlywithdepthasthoseofotherbacteria.Theearthyormustyodorofsoilcomesfromtheproductionofgeosminbytheactinomycetes.Actinomycetesareimportantinthedegradationofthelargerligninmoleculesinorganicresidues.Onekind,thestreptomycetes,producesantibioticsthatwedependonsoheavilyinmanymedicines.

Algaeareabundantinhabitatswithadequatemoistureandlighting.Theycanexistassinglecellsortheycanformlongchains.Likehigherplants,algaecontainchlorophyllandareabletoconvertsunlightintoATPenergyandcomplexorganiccompounds.Algaefrequentlyliveharmoniouslyorevensymbioticallywithcyanobacteria(formerlyclassifiedasblue-greenalgae)toformamicrobialcrustonbarrensoils.IntheUnitedStates,thesecrustsareparticularlywelldevelopedonariddesertsoftheSouthwest.Cyanobacteriacanfixatmosphericnitrogen(whichisdiscussedunderthenitrogencycle),protectthesoilsurfacefromerosion,andcreateafavorableenvironmentforseedgermination.Theyalsofixnitrogeninricepaddysoilsandtherebyfertilizethegrowingcrops.

Fungi,whichareofgreatimportanceindecomposingorganicresiduesinthesoil,aremulticellularorganismsranginginsizefrommicroscopictothelargemushroomsnormallyfoundonlyonmoist,untilledsoil.Fungicommoninthesoilaremadeupofamassoffiberscalledamycelium.Onehundredthousandfungimaybefoundinmycelialandsporeformsinagramofsoil.Theydobestinacidsoil(pH4.5–5.5),sotheydonotcompetewithbacteria,mostofwhichflourishinnearlyneutralsoil.Fungicandecomposeagreatervarietyoforganiccompoundsthanbacteria.Somecatchnematodesinakindofnooseandconsumethem.Somesoilfungiarealsopathogenicandcausediseases.Themyceliumalsofunctionasnetsthatsurroundandbindprimarysoilparticlesandmicro-aggregatesintomacroaggregates,therebycontributingtosoilstructuralstability.

Mycorrhizae,whichmeansfungusroot,isasymbioticrelationshipbetweencertainfungiwithrootsinthesurroundingsoil.Threads(hyphae)ofthemyceliaextendintotherootsofperhapshalfthekindsofhigherplants,whichmeansthattheseplantshave“doubleroots”ofhighefficiency.Thehyphaegrowoutintothesoilandprovidewaterandnutrients,especiallyphosphorus,fortheplants,whichinreturnprotectandinpartnourishthefunguswithsugars.Inthissymbioticrelationshipthefungusmayevenprovidesomeantibioticprotectiontotheroots.Mycorrhizaeformasheatharoundtheplantrootandeitherextendthehyphaeintothespacesbetweentherootcellsorextendthehyphaeintothecellsoftherootwheretheyarefinallydigested.Whenthehyphaepenetratetherootcells,theyformhighlybranchedstructurescalled

arbusculesthatarethesiteofnutrientexchangebetweentheplantandfungus.

Therearetwotypesofmycorrhizae:endomycorrhizae(hyphaeextendintospacesbetweenrootcells,butdonotenterintothecells)andectomycorrhizae(hyphaeextendintotherootcells).Theendogroupisassociatedprimarilywithfieldcropssuchascorn,rice,andalfalfaplusafewtreessuchasappleandcitrus.Theectogroupisassociatedmostlywithtrees,acommononebeingpine.

Anothersymbioticrelationshipdevelopsbetweenfungiandblue-greenalgaetoformlichens.Theseprimitiveplantscansurviveonbarerockbecausetheyfixatmosphericnitrogenandcanextractafewnutrientsfromthemineralsoftherock.

Protozoaareone-celledanimals.Theremaybethousandsoftheminagramofmoist,humicsoil.Theyliveinsidethefilmsofwaterthatcoversoilparticles.Ifthefilmsdryup,theprotozoachangeintoarestingforminwhichtheysurviveuntilthenextrain.Protozoaincludeamoeboid,ciliate,andflagellateforms.Theycontributetothebreakdownoforganicmatter,andsomefeedontremendousnumbersofbacteria,thushelpingtomaintainthebalanceofnature.Itisalsothoughtthatbyfeedingonothersoilmicrobes,protozoacontributegreatlytomineralizingnitrogeninagriculturalsystems.

Myxomycetesareslimemolds,whichareintermediatebetweenprotozoaandfungi.Intheprotozoanstagethecellsarefree-living.Inthefungalstagetheycometogethertoformajellylikemassthatmaybeorange,purple,orsomeotherbrightcolor.Thefungalstageproducesreproductivespores.

TheNitrogenCycleMostnutrients,suchasphosphorus,calcium,magnesium,andpotassium,arederivedfromminerals.Theyareabsorbedbyplantsandformlivingtissue.Theplantsdieandreturntothesoil,wheretheydecomposeandreleasethenutrients,whichcanbetakenupbyplantsagain.Thisisacommonnutrientcycle.

Nitrogen,however,comesfromtheatmosphere,whichconsistsof78%nitrogeningaseousform.Inthenitrogencycle,nitrogenistransformedfromgaseousnitrogenintoaformthatcanbeusedbyplants.Nitrogenundergoesseveraltransformationsinthenitrogencycle.Undercertainconditions,thenitrogenreturnstotheatmospherebeforeitisutilizedbyplants.ThevariousstepsofthenitrogencycleareshowninFigure4.7.

Figure4.7Thenitrogencycle.

NitrogenFixationNitrogenfixationisaprocessthatoccursinthenitrogencycle.Itistheprocesswherebynitrogenfromthesoilatmosphereisconvertedintoproteinintheplant.Nitrogenfixationmaybesymbioticornonsymbiotic.Inthecaseofsymbioticfixation,bacterialiveintheroottissueofplantstothemutualbenefit(symbiosis)andconvertthenitrogentoammonium(NH4

+),whichcanthenbeutilizedbythehostplant.Thebacteriasupplythemselvesandthehostplantwithnitrogen,whilethehostplantssupplythebacteriawithnutrientsandenergysources.Smallknotsoftissuecallednodulesformontherootswhenthesebacteriaarepresentandactive.Legumessuchasclover,alfalfa,peas,beans,andlocustareprimaryhostsforsymbioticnitrogen-fixingbacteria(rhizobia)(Fig.4.8).

Figure4.8Nodulesontherootsofsomeplants(legumes)containbacteriathatarecapableoftakingnitrogenfromtheairtothebenefitoftheplant.

Avigorousalfalfacropmayfix100–200lbofnitrogenperacre(110–220kg/ha)peryear,whichisonereasonforitsinclusioninacroprotation.Mostgrasses,includinggraincrops,arenotnaturalhostsfornitrogen-fixingbacteria.Scientistsaretryingtofindwaysofbreedingnewvarietiesthatcanfixnitrogen.Ifsuccessful,theresultantvarietiesmayreducetheuseofcommercialnitrogenfertilizerforsuchcropsascorn,wheat,oats,andbarley.

Symbioticnitrogenfixationisalsobroughtaboutbyactinomycetesinassociationwithseveralwoodyplants,particularlyalder,Russianolive,andsweetfern.

Somenitrogenisfixednonsymbiotically.Afree-livingsoilbacterium(Fig.4.8)ofthegenusAzotobacterfixesnitrogenthatbecomesavailabletoplantswhenthebacteriumdies.Theamountofnitrogenfixedinthismannerisseldommorethan10lbperacre(11kg/ha)peryearbutisavaluablepartofthenitrogencycle.

Nitrogenfixationalsotakesplaceintheatmosphere,particularlybylightning.Duringstorms,lightningwilloxidizeatmosphericnitrogentoformnitrousoxide(N2O).Itisthencarriedintothesoilbytherain.Thisisadifferenttypeofnitrogenfixationinthattheendproductisoneoftheoxidesofnitrogenthatneednotprogressthroughthenitrogencycletobeavailableforplantuptake.Theamountofnitrogenfixedinthiswaymayaverage10lbperacre(11kg/ha)per

year.

AmmonificationAmmonificationisthefirststepinmineralization.Itisastepinthemicrobiologicaldecompositionoforganicmaterial,suchasplantresidue,anditisbroughtaboutbythegeneralsoilpopulationofmicroorganisms.Ammonia(NH3)isaproductofthisdecompositionanditionizestoformtheammoniumion(NH4

+).Ammoniumionscanbeheldbythesoil,fixedinthestructureofclayminerals,convertedtoammoniaandlosttotheairviavolatilization,absorbedbyplants,butmostwillprogressthroughthenexttwo-partstep,nitrification,ifthesoiliswarmandmoist.Inthecaseofpaddyrice,ammoniumionsarethemainsourceofnitrogen.

NitrificationNitrificationisthereactionthatresultsintheconversionofammoniumionstonitrateions.Ifthesoiliswarm(75–85°F),hasnearneutralpH,andiswellsuppliedwithmoistureandoxygen,theammoniumionsareoxidizedfirsttothenitrite(NO2

−)formbythebacteriumNitrosomonas.Thenitriteformrarelyaccumulatesinthesoil.Thenitrate(NO3

−)formisbroughtaboutdirectlybythebacteriumNitrobacter.ThesebacteriaareabletomeettheirenergyneedsbyoxidizingNH4

+toNO3−andobtainCforbuildingtheircellstructuresfrom

carbondioxide.Nitrateisthehighestoxidationstatefornitrogen.Sincenitrateisananion,itisnotheldontheexchangessitesonsoilcolloids.This,coupledwithitshighsolubilityinwatermakesnitratesubjecttoleaching,particularlyincoarsetexturedsoils,excessivelyirrigatedfieldsandinareaswithhighrainfallintensity.Ifwaterpercolatesthroughthesoil,nitratemoveswithitandmaycontaminategroundwater.Excessivenitrateindrinkingwaterinterfereswithbloods'abilitytocarryoxygenresultinginaconditionknownasBlueBabySyndromeininfantsunder6monthsofage.Forthisreason,theUnitedStatesEnvironmentalProtectionAgencyhasset10milligramsofnitratenitrogenperliterofwaterasthemaximumconcentrationsafefordrinkingwater.Surfacerunoffofnitratecauseseutrophication,oralgalblooms,inlakesandestuaries.Eutrophicationistheslow,naturalnutrientenrichmentofstreams,lakesandreservoirs.Asthealgaegrowandthendecompose,theydepletethewateroffthedissolvedoxygen,resultinginfishkills,offensiveodors,andreducedattractivenessofthewaterforrecreationandotherpublicuses.

Duringtheprocessofnitrification,hydrogenionsarereleased.Theammoniuminfertilizersalsoundergoesthissamenitrificationprocess.Thisexplainswhysoilacidificationoccurswhenlargeamountsoforganicmaterialsorammoniumcontainingfertilizersareaddedtothesoil(seeChapter5).

ImmobilizationandMineralizationImmobilizationistheconversionofinorganic,primarilyammoniumandnitrate,nitrogenintoorganicnitrogen(aminoacidsandproteinsinmicroorganismsandplants).Mineralizationis

thereverseprocessofimmobilization,whereinorganicformsofnutrientsinorganicmaterialsareconvertedtotheinorganicformsbysoilorganismsduringdecomposition.

Theadditionofalargeamountofresidueswithinadequateamountsofnitrogen,suchaswheatstraw,tothesoilstimulatesthegrowthofalargepopulationofmicroorganisms.Afterammonificationandnitrification,theammoniumandnitrateionsmaybetakenupbytherootsofhigherplantsorbymicroorganismsdecomposingorganicresiduesinthesoil.Immobilizationistheprocessduringwhichanoverabundanceofmicroorganisms,whichalsoneednitrogentolive,mayoutcompetewithcropsfortheavailablenitrate.Asinorganicammoniumandnitrateareincorporatedintothecellsoflivingmicroorganisms,theplant-availableNlevelsinthesoilarereduced.Asaresult,cropsmaybecomenitrogendeficientanddevelopayellowcoloration.Thissituationcanbepreventedbycompositingresiduesbeforeincorporationinthesoil,blendinglowC:NratioresidueswithhighC:Nratioresiduesorbyaddinginorganicnitrogenfertilizerstothesoil.

MineralizationandimmobilizationcanhaveamajorinfluenceontheamountofavailableNinthesoil.Ashasbeendiscussed,soilmicroorganismswillbreakdownplantmaterial(andotherorganicmaterials)toobtaincarbonandenergy.Duringtheinitialstagesofdecomposition,Ncanbereleased(mineralized)fromtheorganicmaterialanditcanbetakenup(immobilized)bythebiomass.ThenetresultoftheseNtransformationswilldictatewhetheravailableNincreasesduringtheearlystagesofdecompositionordecreases.

TheamountofNmineralizedisproportionaltothequantityoftotalNinthesubstratebeingdecomposed.Ingeneral,2%Nisconsideredcritical,withnetmineralizationoccurringwhentheproportionofNinthematerialis>2%.TherelativeamountofCandN(C:Nratio)inthedecomposingmaterialalsodetermineswhethermineralizationorimmobilizationpredominates.Sincemostplantsareabout40%C,theC:NratioisprimarilyinfluencedbyNcontent.PlantswithhigherNcontentshavelowerC:Nratios,andmineralization(increaseinavailableN)isfavoredwhentheseplantmaterialsdecompose.Forexample,whenalfalfadecomposes,therelativelyhighamountofN(C:Nratio=25:1)favorsmineralizationandavailableNwillincrease.IfresidueswithlowerNcontents,andwhichhavehigherC:Nratio,areaddedtothesoil,themicrobeswillhavetoscavengethesoilfornitrogentobalancetheexcesscarbonintheresidueandimmobilization(decreaseinavailableN)willusuallyoccurwhentheseplantmaterialsdecompose.Forexample,whencorn(C:Nratio 57:1)orwheat(C:Nratio 80:1)residueareaddedtothesoil,therelativelylowamountofN(highC:Nratio)favorsimmobilizationandavailableNdecreases.ThisisbecausetheadditionalNrequiredtobalancethehighCinthestrawwillhavetocomefromthesoil.Forthisreason,itisadvisabletoadd15lbofnitrogenforeverytonofstrawupto50lbofN.

Theprocessesofmineralizationandimmobilizationoccursimultaneously.Asorganicmatterdecomposes,inorganicnitrogenisreleasedintothesoilandisutilizedbybothplantsandmicroorganisms.TheNinboththeplantsandthebodymassorganismseventuallyrevertsintoplant-availableNwhentheydieanddecomposetoreleaseinorganicnitrogenintothesoilthroughmineralization.

Insummary,everythingelsebeingequal,mineralizationandimmobilizationproceedatfairly

equalrateswhentheC:Nratioofdecomposingorganicresiduesisbetween24:1and30:1.Thisimpliesthatmicroorganismsneed1gramofNforevery24gofCinthesubstrate(food).NetmineralizationoccursatC:Nratiosbelow24:1whilenetimmobilizationoccursatC:Nratiosabove30:1.

Cropresiduesonthesoilsurfaceservetoprotectthesoilfromthedestructiveimpactofraindrops,therebyprotectingthesoilfromerosion.Thefastertherateofdecomposition,thelessthetimethoseresidueswillbeavailabletoprovidecovertothesoilsurface.Therefore,whilethedecompositionofcropresiduesisimportantfornutrientcycling,itisalsoessentialtomaintainacertainamountofresidueforsoilcover.Forthesereasons,itisimportanttopayattentiontotheC:Nratioofcropresidues,soastomaintainsoilcoverwhendesired,yetallowtheresiduetoultimatelybreakdownandberecycled.

DenitrificationDenitrificationistheprocesswherebynitratenitrogen(NO3

-)undergoeschemicalreductionintogaseousnitrogenformsincludingnitricoxide(NO),nitrousoxide(N2O)andmolecularnitrogen(N2),andisvolatilizedintotheatmosphere.Thisisaprocesswherebymicroorganismsthatflourish,underanaerobicconditions,derivetheiroxygenfromthenitrateionsorsimilaroxides.Therefore,fordenitrificationtooccurcarbonandnitratemustbeavailableandoxygenavailabilitymustberestricted.MostNO3-Nmaybelostinthismannerfromlow,wetareasofafield,especiallyduringperiodsofwarmweatherandheavyrainfallwhenthesoilstayssaturatedforprolongedperiodsoftime.Eveninaeratedsoils,small,localizedareasinthesoil(microsites)canhaveinadequatesupplyofoxygen.Thepresenceofcropresiduesandotherdecomposableorganicmatterincreasestherateofdenitrification.Forinstance,thepresenceofsurfacemulchandmoistureinno-tillsystemsgreatlyincreasesthepotentialfordenitrificationlossofsurface-appliedNandasidedressingofNfertilizermaybecomenecessarytoensureanadequatesupplyofNduringtheearlygrowthphase.Nitratelossesduetodenitrificationcanbereducedbyapplyingslowreleasefertilizersorfertilizersthatcontainnitrificationinhibitors.Nitrificationinhibitorsslowtheprocessesofnitrificationuntilperiodsofgreaterplantuptake.

Toafarmer,denitrificationcanbeacostlylossofanexpensiveplantnutrient—nitrogen.Thisisamajorreasonformaintainingadequatesoildrainageandpropertimingofnitrogenfertilizerapplication.TheN2Oreleasedintotheatmosphereduringdenitrificationisagreenhousegasthatcontributestoglobalwarming.Nevertheless,denitrificationisacrucialpartofthenitrogencycleasitistheonlypointinthecycleatwhichfixednitrogenre-enterstheatmosphereasgaseousnitrogen.Withoutit,atmosphericnitrogenwouldeventuallybedepletedbythenitrogenfixers.

VolatilizationThisistheproductionofgaseousammoniafromammoniumanditslosstotheatmosphere.

AmmoniavolatilizationincreaseswithsoilpHbecauseahighconcentrationofOH+ionspromotestheconversionofnitratetoammonium.AsdiscussedinChapter8,volatilizationlossesarehighforbroadcastunincorporatedureafertilizerormanure.Incorporationofmanureandfertilizerscanreduceammonialossesbyupto75%.Evaporationpromotesvolatilization.Thus,volatilizationisgreatestasthesoildriesafterreachingfieldcapacity.Cropresiduesthatarenotincorporatedintothesoilmayincreasetherateofvolatilization.Volatilizationlossescanbereducedbyapplyingslowreleasefertilizersorbyusingfertilizersthatcontainureaseinhibitors.Slowreleasefertilizerscontainacoatofsulfurthatmustbreakdownbeforeureaisreleasedwhileureaseinhibitorsslowtheprocessofureahydrolysis.

AmmoniumFixationThisisthetrappingofammoniumionsbetweeninterlayerspacingofsome2:1layersilicateclaymineralssuchasvermiculiteandillite.Itoccursbecausethesizeofcavityleftbyoxygeninclaysissufficientlylargetoholdpotassiumandammoniumions,buttoosmallforotherions.Whetherammoniumwillbefixedornotisdeterminedbythesourceofchargeonclays.Kaolinitedoesnotfixammonium.HydrousoxidesatlowsoilpHvaluesdonotfixammoniumorpotassiumbecausetheAl(OH)intheinterlayerspacessatisfytheclays'chargeandalsoexpandstheinterlayerspace,therebyimpedingfixation.Montmorillonitedoesnotfixammoniumunderwetsoilconditions.

ThefixationofNH4+ionsleadstoatemporaryimmobilizationoffertilizerNappliedinasoil.

TheactualamountofammoniumfixeddependsontheamountofK+inthefixedposition.ThemorethequantityofKfixed,thelessthequantityofammoniumcanbefixed.Forthisreason,NH4

+fixationcanbereducedbyKfertilizationpriortoNH4+application.

BiologicalDecompositionofRocksThreeyearsaftertheislandofKrakatoawaslargelyblownawaybyaviolentvolcaniceruptionin1883,scientistsvisiteditonlyseveralyearslaterandfoundthatthesurfaceofthefreshbedrockwasalreadybeinginvadedbycyanobacteria,oneofthemostself-supportingformsoflifeonearth.Itcanbothphotosynthesizeandfixnitrogen.Growingalongwiththecyanobacteriawerenitrogen-andcarbon-fixingbacteriaaswellasfungiandlichens.Weakacidsproducedbythesemicroorganismsweredissolvingnutrients(phosphorus,calcium,andothernutrients)fromtherocksandbuildingupahumicmatcapableofsupportingmossesandeventuallyhigherplants.TheweakacidsincludecarbonicacidformedbysolutionofCO2gasinwaterandlacticacidproducedbyfungi,andstrongeracids(nitricandsulfuric)thatwereformedbybacteria.Certainfungiandbacteriacanreleasephosphorusfrommineralparticles.Itisevidentthatmicroorganismsareinvolvedinrockweatheringfromthestart.

MacroorganismsMacroorganismsincludeworms,arthropods,andvertebrates.Inanacreofsoiltheremaybea

millionnematodes,amillionants,twohundredthousandmites,andfourthousandworms,tonamejustafew.Mostwildbeesnestinthesoilandintheprocessmakethesoilmoreporousbyexcavatingburrowsandchambers.BeforesettlementbyEuropeanimmigrants,asquirrelcouldcrossthestateofOhiowithouttouchingtheground.Manyofthetreesinthenativeforestswereplantedbysquirrels.Obviously,animallifehasgreatlyinfluencedbothplantsandsoil.

Wormsincludenematodesandearthworms.Nematodesareeel-shaped,unsegmented,colorlesswormsandaregenerallythemostabundantmulticellularorganismsinsoils.(Fig.4.9).Mostaretoosmalltobeseenwithoutamicroscope,butsomemaygrowtoacentimeterormoreinlength.Manyaresaprophytic,whichmeansthattheyfeedondeadplantresidue,butsomeareparasiticandliveontherootsofplants.Manyoftheparasiticspeciescauseimportantdiseasesofplants,animals,andhumans.Theycausegreateconomiclosstomanycrops,includingcitrus,cotton,soybeans,alfalfa,corn,andvegetablessuchaspotatoesandtomatoes.Nematodesareinvolvedinorganicmatterdecompositionandnutrientcycling,biologicalcontrolofinsectpestsandcertainplantparasiticnematodes.Theyalsoserveasfoodforothersoilorganisms.

Figure4.9Nematodesareusuallymicroscopic.Theycanbedestructivetocrops.

Earthworms(Fig.4.10)performanimportantfunctioninmixingorganicmatterwithmineralmatter.Inasense,theyaresoilfactories.Amongthemanykindsofearthworms,thenightcrawler,Lumbricusterrestris,wasbroughttotheUnitedStatesfromEuropebysettlers.

Figure4.10Earthwormsareessentialformixingorganicmaterialwithmineralsinthesoil.

Ingeneral,wormsperformanimportantaerationandmixingfunctionbyburrowing/channelingthroughthesoil,consumingorganicmatter,andbringingtheresiduetothesurfaceascastings,whichformstableaggregatesuponexcretion.Itisestimatedthatwormsbring7–18tonsofsoilperacre(16–40t/ha)annuallytothesurfaceinthisway.

Arthropodsincludespringtails,mites,andants.Springtails(Collembola)areprimitiveinsectsthatdonotgothroughstagesofmetamorphosisasdofliesandbutterflies(Fig.4.11).Theylooklikeancientfossilcreatures.Theyarenumerousindecayingleaves,andinlatewintertheyappearonsnowbanks(hencetheirnickname“snowfleas”)wheretheyfeedonscatteredpollen.

Figure4.11Springtailsandmitesplayanimportantroleinthedecompositionofdeadleavesandstems.

Mites(Acarina)(Fig.4.11)performthesamejobasspringtails,whichistoconsumedeadanddecomposingplantparts.Mitesarefoundeverywhere,eveninoceandepthsandonhighmountains.Theyconsumeorganicresiduesandfeedonnematodesandspringtails.

Inbothurbanandruralenvironments,antsareactiveintunnelingandbringingupsubsurfacesoiltoconstructmoundsofvarioussizesasshowninthefigure.Becauseantscancarryparticlesnolargerthanallowedbythegapbetweentheiropenmandibles(mouthparts),themoundscontainnostonesorgravel.Figure4.12showsacrosssectionthroughamoundnearly

1ft(30cm)highthatwasbuiltbythewesternmound-buildingant,Formicacinera.TheseinsectswereoriginallycommoninthegrasslandsoftheAmericanprairiebutarenowconfinedbycultivationtoundisturbedlandssuchasthosealongrailroadtracks,cemeteryedges,andwetlandborders.Theirmoundsarebuiltlargelyofsubsoilandarerichinorganicmaterialsthattheantsbringtothecoloniesfromnearbyvegetation.

Figure4.12Antsareactiveintunnelinginthesoilandenrichingitwithorganicmaterial.

Termitesareparticularlyactiveinsoilandplantmaterialsinsubtropicalandtropicalregions.Theyconsumelargequantitiesofdeadtrees,shrubsandplantdebris.Someoftheseinsectstendtoconcentratenutrientssuchascalciumintheirnests,which,whenabandoned,arecultivatedbyfarmersandeventuallyproducepatchesofhigh-qualitycrops.Insemiaridregions,undergroundtermitenestsmayactassinks(collectors)forirrigationwatersandtherebybecomeanuisance.Thelong-termsoil-mixingeffectsoftermitesarebeneficial,buttheimmediateeffectsmaybetroublesome.SomemoundsmaybehigherthanthoseshowninFigure4.13.

Figure4.13Intropicalregions,termitesbuildhugemoundsinwhichtheyconcentratecalciumaswellasorganicmaterialintheirnests.

Vertebratesincludemoles,mice,groundhogs,andmanyothers.Moles(Talpidae)plowsoilbyburrowingjustbelowthesurfacetowheretheycanfindearthworms,grubs,andplantrootstoeat.Thisactivityoccursbothinsodandinforesttopsoil.Itleavesthesoilloosenedandcontributestothehighporosityofnoncultivatedsoils.

Mice(Cricetidae)andshrews(Soricidae)arenumerousenoughtomakeanimpactonsoilsbytheirburrowingactivities.Whensnowmeltsinthespring,networksofrodentrunwaysareplainlyvisible.Groundsquirrels(Spermophilus),Groundhogs(Marmota),prairiedogs(Cynomys),andothermammalsmakeelaborateburrows,constructedtonotfillwithwaterreadilyduringrainyperiodsandtobeaeratedbyconvectionandupdraftaircurrents.Theserodentsbringtonsofsubsoilmaterialtothesurface.Becausetheseanimalspreferdrysites,thematerialstheyexcavatearecommonlysandyandgravellyresultinginasoilprofilethatismixedwithvarioussoilparticles,butenrichedwithvegetativedebrisandrodentexcretaintheprocess(Fig.4.14).

Figure4.14Theburrowingactivitiesofanimalscontributetotheporosityandenrichmentofsoils.

PesticideUseandSoilOrganismsBeforethedawnofagriculture,allorganismswereinbalanceandnonewereabletobuildupinnumbersbeyondthatofnaturalpopulations.Thisisnottosaythatprimitivehumanswerenotbotheredbyinsectsandthelike,buttheywerenaturalpopulations.Whenhumansbegantomanipulateplantsandanimalstoincreasetheirfoodsupply,thebalancewasalteredsothatcertainorganismsbecamedetrimentaltoagriculturalproduction.Variousformsofcontrolhavebeenused,butinrecentdecadestheemphasishasbeenuponorganiccompoundsthatareintendedfortheselectivecontrolofspecifictargetorganisms.Somepesticideskillcertainkindsofpestssuchasfungi,nematodes,insects,orrodents;someregulateplantgrowthbyspeedingituporretardingit;somedefoliateordesiccateplants;someattractinsectstodeadlytrapsorsterilizethem;andsomerepelpeststhroughprotectivecoatingssuchasarefoundonsomeseeds.

Whenproperlyhandled,pesticidescauselittleornoproblemintheenvironment.Onereason

forthisistheactionofsoilmicroorganisms.Bacteriaquicklybreakdownmostpesticidesintocomponentsthatareharmlesswhendeactivated.Componentsthatareunlikeanynaturalmolecules,however,cannotbeattackedbybacteriaandtheycanbuildupintheenvironment,suchasDDT(dichlorodiphenyltrichloroethane).

Thereareotherexampleswhereacomponentformedbymicrobialbreakdownoftheoriginalinsecticideispotentiallyveryhazardoustohumanhealth.Oneofthealdicarbsderivedfromacombinationinsecticide–nematocideusedonpotatofieldsisinthiscategory.Aldicarb(oritsderivatives)hasbeenfoundinthegroundwaterbeneathsandysoilsinWisconsin,NewYork,andFlorida.Ifthesoilisnotrapidlypermeable,thebreakdownofaldicarbintoharmlesscomponentsseemstobecomplete.

Themassiveuseofpesticidesoverlargeareashas,insomecases,beenself-defeating.Sometimesnaturalenemiesofapesthavebeeneliminated,andpesticide-resistantvarietiesofthepestshaveevolved.Well-plannedharmonizingofchemicalandnaturalcontrolmethods(integratedpestmanagement)isawiserapproach.

Limitedstrategicuseofpesticidesmaybecombinedwithecologicalpestcontrol.Thelatterincludestheencouragementofgrowthofpopulationsofnaturalenemiesofpests,releaseofmanysterileindividualsofaspecies,androtationofcropsinawaysoastointerruptpopulationexpansion.Thetobaccohornwormmoth,forexample,hasbeencontrolledbylightuseofpesticidestogetherwithavigorousencouragementofparasiticwasps(biologicalcontrol)andsomehandpicking(scouting)oflarvae.Integratedpestmanagementhasbeenwellreceivedbecauseitisaneconomicallysoundapproachaswellasbeinggoodfortheenvironment.

Therecentdevelopmentofhybridsthatareresistanttospecificinsectsandinfectionsalsooffersanopportunitytoreducetheapplicationofpesticides.Someexamplesarecornthatisresistanttocornborers,potatoesthatareresistanttopotatobeetles,andalfalfathatisresistanttoleafhoppers.Theapplicationofinsecticidestocontroltheseandotherinsectshasbeenextremelyexpensiveandcontroversial.

Chapter5SoilChemicalPropertiesThechemicallyactivefractionsofthesoilareclayandhumus.Bothclayandhumushaveelectricallychargedsitesontheirsurfaces—bothnegativeandpositive.Thesesitesattractionsoftheoppositecharge.Thetypesandrelativeamountsofionsthatareattachedwillinfluencetheplantnutrientlevelaswellasthealkalinityoracidityofthesoil.

SoilColloidalSystemThehumusandclayfractionsareoftencalledthecolloidalsystem.Acolloid,bydefinition,isanextremelysmallparticle.Clayandhumusfitthedefinition.Colloidsaretoosmalltobeseenwithalightmicroscope,butclearimagesofthemcanbemadewithelectronmicroscopes.Theupperlimitoftheirdiameteriscommonlygivenas0.0001mm,althoughparticlessomewhatlargermayreactsimilarlybuttoalesserextent.Forcomparison,itwouldtake254,000oftheseparticles,sidebyside,toextend1in.(2.54cm).Thus,thecolloidalsystemismadeupofthefinestclayparticlesandhighlydecomposedhumus.Duetotheirsmallsize,colloidshavealargespecificsurfaceareaandcarryanelectricalchargeontheirsurface.Asaresult,colloidsarethemostchemicallyactivefractionofthesoilandareintimatelyassociatedwithmanyreactionsinvolvedinplantnutrition.

Sincecolloidalclayandhumusparticleshavenegativelyandpositivelychargedsites,nutrientionsthatareessentialforplantgrowthareattractedtothecolloidalsurfacesofoppositecharge.Thepositivelychargedionsarecationsandthosewithnegativechargeareanions.Theyareheldweaklyasareservesupplyforplantsandmaybereleasedintothesoilsolutionwheretheycanbeutilizedbyplants.Withouttheattractionbetweenionsandcolloids,theleachingofcertainionsdeeperintothesoilandbeyondthereachofrootswouldbemuchgreaterinhumidregions.Indeed,itisoftenobservedthatnitrateleachesreadilyinsoilsinhumidregions.Nitrateionsarenegativelychargedandarenotattractedtothenegativelychargedsoilcolloids;therefore,nitrateionsremaininthesoilsolution.Sincethenitrateionsarequitesolubleandarenotpronetoothersorptionreactions,nitratewillreadilyleach.

Thenatureofthecolloidalsystemisnotonlydependentonthecolloidsthemselvesbutalsoonthepropertiesoftheionsattractedtothem.Theseattractedionsmaybeexchanged,partlyinaccordancewiththedominanceofspecificionsinthesoilsolution.Thisprocessiscalledionexchange.Inallsoils,exceptsomeintropicalregions,thenegativelychargedsitesoncolloidalsurfacesaremuchmorenumerousthanarethepositivesites,sotheusualprocessiscationexchange.

Tounderstandhowcolloidsinfluencesoilchemistry,itisnecessarytoknowsomethingabouttheircomposition.Claymineralcolloids,primarilysilicateclays,oxideclays,andhumuscolloids,willbediscussedseparately.

SilicateClaysMineralparticlessuchascommonfeldspargrainsfromgranitearemadeupmostlyofthreeelements:silicon,oxygen,andaluminum.Therefore,theyarecalledaluminosilicates.Smallfeldsparparticlesslowlychangetoclaymineralsbyweathering.Thesearealsoaluminosilicates,buttheyaredifferentfromfeldsparsintwoprincipalways:Theclaymineralshavesomewatermoleculesintheirstructuresotheyarecalledhydratedaluminosilicates,andtheyhaveaplatyorlayeredstructure.

Justasaplantleafismadeupofdistinctlayersofcells,theverysmall,flatclaycrystalsaremadeupofdefinitelayersofions.Mostsilicateclayparticlesaresandwich-like,withanaluminalayer(aluminumplusoxygen)sandwichedbetweentwosilicalayers(siliconplusoxygen).Theyarecalled2:1claysbecauseofthisarrangement.Smectiteandhydrousmicaareclaysofthistype.

Inclaymineralswitha1:1structure,thereisasinglesilicalayeradjacenttoasinglealuminalayer.Kaoliniteisacommon1:1clay.Platesofhalloysite,avarietyofkaolinite,tendtocurl(Fig.5.1).

Figure5.1Clayparticlesareextremelysmallandinsometypesthelayerstendtocurl.

These2:1and1:1typesofclaysarecalledlayerlatticesilicateclays.Theionsineachlayerarearrangedinlattice-likegeometricpatterns(Fig.5.2).The2:1latticeclayshavevariationswithinthegeometricpatternofionsthatgiverisetoanegativechargeonthesurface.Most2:1claysarealsoexpandinglatticeclayssotheyabsorbwaterbetween,butnotwithin,thesetsof2:1lattices.

Figure5.2Ionsinsilicateclaysformageometricpatternsuchasinthiskaolinite.

Asananalogy,clayparticlesresembleastackofsandwichesandtheexpansiontakesplace

betweenthesandwiches.Expandinglatticeclayshaveatremendoussurfaceareabecausetheinternalsurfacesareavailabletoreactwiththesoilsolution.Clayswitha1:1latticedonotexpandbecausehydrogenbondingbetweenthesetsoflatticesholdsthemtogether.

Anotherkindofclayistheoxideclaythathaslittleornoregularityinitsstructure.Inthisrespect,oxideclaysaregel-like.

SourceofNegativeChargeonSilicateClayMineralsThenegativechargeonsilicateclaysinsoilscomesfromtwosources.Atypicalsilicateclayofthe2:1typeillustratesthisprinciple.First,thesilicalayerdevelopsanegativechargefromtheoxygenionsalongtheedgeofthecrystal.Onlyoneoftheoxygen'stwonegativechargesiscombinedwithasiliconion,soattheplanewherethecrystalends,thereareoxygenionswithonenegativechargeunsatisfied.Figure5.3depictsthischargedistributionintwodimensions,withanunsatisfiedchargeateachendofthelattice.Theoxygenionsarenotshowninthisschematicdiagram,buttheirlocationissimilartotheionicarrangementinthesilicalayershowninFigure5.2.Thissourceofnegativechargeiscallededgechargeandalthoughitislow,itisthemainchargeonkaoliniteclay,whichisasilicateclaymineral.ThischargefluctuateswithsoilpH,henceitiscalledpH-dependentcharge.

Figure5.3Developmentofanegativechargeonasilicateclaylattice.

Thesecondsourceofanegativechargeariseswhenoneionissubstitutedforanotherduringtheformationofthesilicateclaycrystal,withoutanychangeinitsform.Duringsubstitution,someatomsinthecrystalarereplacedbyotheratomsofsimilarsizebutdifferentvalence.Thisiscalledisomorphous(Iso=similar,morphous=sizeandshape)substitution,anditcanoccurindifferentways.Insomeclaysanaluminumion(Al3+)substitutesforasiliconion(Si4+)intheouter(silica)layers,whereasinotherclaysamagnesiumion(Mg2+)maysubstituteforAl3+inthealuminalayer.Eitherway,onenegativechargeresultsinthecrystalandthechargeispermanentsinceitdoesnotvarywithsoilpH.Inessence,thesubstitutionsresultinadeficitofpositivecharges,andthisresultsinanoverallnetnegativechargeontheclay.

GroupsofSilicateClays

Severalgroupsoflayersilicateclaymineralshavebeenidentifiedandwithineachgrouptherearemanyspecificclayminerals.Inthisbook,onlythreeofthesegroupsarediscussedtoillustratethenatureandimportanceofclay.

SmectiteGroupMontmorilloniteisacommonmemberofthesmectitegroup.Itisa2:1-typeclaywithahighcapacitytoholdplantnutrientsandtoswellandshrinkonwettinganddrying(Fig.5.4).Variationswithinthisgroupareduemainlytotheamountofsubstitutionofmagnesiumandferrousironforaluminuminthealuminalayer.Soilsthathavehighamountsofmontmorilloniteclaycanbeverytroublesome,particularlywhenwet.Theyareexpandinglatticeclayswhereintheirstrongaffinityforwatercausestheclayparticlestospreadapartandreadilyslippastoneanother.Thisresultsinwhatiscalledlowbearingstrength,whichmeansthatfoundationsofbuildingsandroadsbuiltontheseclaysarelikelytofail(slip)andcausecrackinginthesuperstructure,particularlyonslopingground.Whenmontmorilloniticsoilsdry,cracksofnearly2in.(5cm)ormoremayopen.Debrismayfallintothesecracksandcausethesoiltobucklewhenitiswetted.

Figure5.4Layerlatticecrystalsofmontmorilloniteclayhaveahighcapacitytoholdplantnutrients,absorbwater,andswell.

Montmorilloniticsoilsbecomeverystickyanddifficulttotillwhenwetandveryhardwhendry.Asaresult,farmerscanworkthemonlyatjusttherightmoisturecontent.Unimprovedroadsonmontmorilloniticsoilsbecomeimpassableinrainyseasons.

Theinner(alumina)layerofthemontmorilloniteclaylatticeismadeupofaluminum,hydrogen,andoxygenions.Allthenegativeandpositivechargesbalanceandneutralizeeachotherwithinthislayeronlyifthethreenamedionsarepresent.Inmontmorilloniteclay,aboutone-fourthofthealuminumions(Al3+)havebeenreplacedbyionsofmagnesium(Mg2+)oriron(Fe2+);ionswithtwopositivechargeshavebeensubstitutedforionswiththreepositivecharges.Thisproducesadeficiencyinpositivecharges,whichresultsinanexcessofnegativechargesatthesurfaceofthecrystallattice.Thesearepermanentnegativechargesthatdevelopedwhenthecrystalswereformed.

SmectiteclaystendtobeassociatedwiththesubhumidtoaridclimaticregionsthathaveproducedgrasslandsintheUnitedStates.Whenfoundinthemorehumidregions,theyaregenerallyinsoilformedfromshaleorintheresiduefrombasicrocks.

HydrousMicaGroupHydrousmica(Fig.5.5)hasaslightstructuraldifferencefromtheprimarymineral(mica)thatisfoundingranite.Hydrousmicaisprobablyderivedbyweatheringofmica.Itisassociatedwithregionswhereweatheringhasnotbeensevereandwherethesoilisneitherveryacidnorverybasic.AmemberofthisgroupiscalledilliteafteralocationinIllinoiswhereitwasfirstidentified.Hydrousmicaislikemontmorilloniteinthatithasa2:1latticestructure,butthelatticelayersareheldtogetherbyamutualbondwithpotassiumionsbetweenthem.Thisbondingminimizestheswellingandshrinkingandresultsingoodbearingstrengthforthisclayandinreducedstickinesswhenwet.Illitehasalowercapacitythandoesmontmorillonitetoholdplantnutrients.

Figure5.5Layerlatticecrystalsofhydrousmicaclayshavealowercapacitytoholdplantnutrientsandtoabsorbwater.

Thepresenceofhydrousmicainasoildoesnotmakethesoilunstableinthewaythatmontmorillonitedoes.Apredominanceofhydrousmicaclayinasoilindicatesalackofsevereweathering.Suchclaysarelikelytobefoundinthecoolclimaticzoneswhereprecipitationishighenoughtoremovesolublesaltsfromthesoil.

Whentheinterlayerpotassiumiscompletelyremovedbyweathering,anexpandinglattice2:1claycalledvermiculiteisformed.Itdoesnotshrinkandswellasmuchasmontmorillonitedoes.InvermiculitethenegativechargeisderivedfromtheisomorphoussubstitutionofAl3+forSi4+intheouterlayer.Asaresult,vermiculitehasahighernegativechargethandoesmontmorillonite.

KaoliniteGroupThelatticeofkaoliniteclaysisa1:1typemadeupofonesilicaandonealuminalayer(Fig.5.6).Itcanbeseenthatkaolinitehastheleastsilicaofanyofthesilicateclays.Thisistheresultoftheintenseweatheringthatischaracteristicofwarmregionsoftheworld.

Figure5.6Layerlatticecrystalsofkaoliniteclayhaveaverylowcapacitytoholdplantnutrientsandtoabsorbwater.

Oneimportantpropertyofkaoliniteisthefixedspacingbetweenthelatticelayers.Thisisduetotheattractionofhydrogenofthehydroxylionsinanaluminalayerfortheoxygenintheadjacentsilicalayer.Thebondbetweentheselatticelayersiscalledahydrogenbond,anditisofgreatimportancebecauseitrenderskaolinitelessstickyandgivesthesoilagreaterbearingstrengththanwithothertypesofsilicateclays.Kaolinitehasaverylowcapacitytoholdplantnutrients,anditabsorbslesswaterthan2:1clays.

Kaolinite,afavoriteclayamongpotters,ismostabundantintropicalandsubtropicalregions.Nearlypuredepositsofkaolinitearevaluableassourcesforindustrialmaterials.Largeamountsareminedforuseinthemanufactureofbathroomfixtures.

AmmoniumFixationbyClaysSome2:1layersilicateclaymineralssuchasvermiculiteandillitetrapNH4

+andK+ionsbetweeninterlayerspaces,resultinginatemporaryimmobilizationoffertilizerNappliedinasoil.Fixationoccursbecausethesizeofcavityleftbyoxygeninclaysissufficientlylargetoholdpotassiumandammoniumions,buttoosmallforotherions.

IdentificationofLayerSilicateClayMineralsDifferentclaymineralshavecontrastingproperties;hence,itisimportanttoidentifythemsothatasoil'scapabilitiesandlimitationscanbeaccuratelypredicted.ThemostcommonlaboratoryinstrumentusedtoidentifysilicateclaysistheX-raymachine(Fig.5.7).

Figure5.7SilicateclayscanbeidentifiedbyuseofX-rays.

Noncrystalline(Amorphous)SilicateClays

Whenvolcanicashweathersinarelativelyshorttime,somenearlyamorphous(withoutstructure)silicateclaysform.Twooftheclaymineralsareallophane,whichisspherical,andimogolite,whichisthreadlike.Exceptfortheirsmallsize,theysharefewofthepropertiesofthelayersilicateclayslistedpreviously.Theirpresenceisgermanetotheclassificationofsoilsofvolcanicorigin.

OxideClaysTothispoint,considerationhasbeengiventoonlysilicateclays,butoxideandhydratedoxideclaymineralsarealsopresentinsoils(Fig.5.8).Normally,theseareoxidesofironandaluminum,areamorphous,andarefoundmostabundantlyinsoilsformedfromparentmaterialsrichinironandaluminumintropicalandsubtropicalregionswhereweatheringhasremovedmuchofthesilicafromtheclayfraction.Oxideclayshavelittleornocrystallinityandverylowcapacitytoholdplantnutrients.Ifironoxidesarenotveryhydrated,theygivethesoiladeepredcolor.Ferrihydriteisoneoftheironoxideclaysthatisveryhydrated;itisimportantintheclassificationofsomevolcanicregionsoils.

Figure5.8Particleofoxideclayhaslittleornocrystallinityandaverylowcapacitytoholdplantnutrients.

CationExchangeThecharacteristicofclayandhumustoattract,hold,andreleasecationsiscalledcationexchange.

Althoughmostsoilcolloidshaveanetnegativecharge,noelectricalchargeinthesoilgoesunbalancedforverylong.Electricalneutralityismaintained.

Asoilcolloidalsystem(primarilyveryfineclayandhumusparticles)hasadoublelayerofcharges.Theinnerlayerisverycloselyassociatedwiththesurfaceofthecolloidalparticlediscussedpreviously.Theouterlayerisformedbycationsinthesoilsolution,whichareattractedtothecolloidalsurfacesinproportiontothenegativechargesavailable.Thismeansthatadivalentcationsuchascalcium(Ca2+)ormagnesium(Mg2+)canneutralizetwonegativechargesofthecolloidalparticle,whereasmonovalentionssuchaspotassium(K+),sodium(Na+),orhydrogen(H+)canneutralizeonenegativechargeeach(Fig.5.9).

Figure5.9Soilcolloidalparticlesattractionswiththeoppositeelectroncharge.

Inacidsoils,aluminumions(Al3+),whichmaybecombinedwithoneortwohydroxyls(OH−),canbeattractedtocolloidalsurfaces.Theremaybemanyothercationsattractedtothecolloidsinsmallamounts.Someofthesearetraceelementsthatareofgreatsignificancetogrowingplants.

Cationsintheouterlayeraresometimescalled“swarmions”becausetheyresembleaswarmofbeesaroundahive,withthegreatestconcentrationofbeesclosetothehive(Fig.5.10).Inasoilcolloidalsystem,thesecationsbecomehydratedsotheireffectiveradiusincludesthewatermolecules.

Figure5.10A“swarm”ofpositivelychargedionsaroundanegativelychargedsoilparticleresemblesbeesaroundahive.

Theforceofattractionofcationstothenegativelychargedcolloidalsurfacediffersamongcations,dependingonthenumberofpositivechargesandtheeffectivediametersofthecations.Thegreaterthenumberofpositivecharges(valence),thegreatertheforceofattraction.Forinstance,Ca2+hastwopositivechargesandisattractedclosertothecolloidthanNa+,whichhasonepositivechargeandtendstomigratefartherfromthesurfaceofthecolloid.Theforceofattractionincreasesintheorder:Na+<Mg2+=Ca2+<Al3+.Forcationswiththesamenumberofcharges,theforceofattractionisdictatedbytheeffectivediameterofthecation.Cationswithsmallereffectivediametersareattractedmorestronglyandtheyareheldclosertothecolloidthanthosewithlargereffectivediameters.Theforceofattractionincreasesintheorder:Li+<Na+<K+.Alltheattractedcationsareinconstantmotion,butattractionholdsthemtightlyenoughsotheyarenotreadilylosttowaterthatismovingthroughthesoil.Theyareadsorbedionsbecausetheyareheldtothesurfacesofthecolloids.Thisactionisveryimportanttoplantlifebecauseitkeepsmanynutrientswithintherootzoneofthecrops.Additionofcationstothesoil,throughacidification,liming,orfertilization,enhancesthereleaseofadsorbedcationsintothesoilsolutionasthenewcationsswapplacesontheexchangesites.

Whenthereissufficientmoistureinthesoil,cationsinthesoilsolutioncanbelostfromthesoilprofilebyleaching.Thenutrientsthatareeasilyleachedarethosethatarelessstronglyheldbysoilparticles.Forinstance,nitrate(ananion)willleachmuchmorereadilythancalcium(acation).Inaddition,monovalentcations,suchaspotassium,willleachmorereadilythandivalentcations,suchascalcium,sincedivalentcationsareheldclosertotheexchangesitesthanmonovalentcations.

Insummary,cationsareelectrostaticallyattractedtothesurfaceofthenegativelychargedcolloids,butwilldiffuseawayfromthesurfaceandtowardthebulksoilsolutionbasedonconcentrationgradients.Anions,ontheotherhand,willbeelectrostaticallyrepelledfromthenegativelychargedcolloidsurface,butwilldiffusetowardthesurfaceandawayfromthebulksolutionbasedonconcentrationgradients.Theexactnatureoftheseprocessesisbeyondthescopeofthisbook,buttheyareveryimportantinexplainingmanyimportantsoilproperties.

Cations(e.g.,Ca2+)inamineralfragmentarereleasedbyweatheringintothesoilsolutionwheretheyareattractedtoparticlesofclay,aroundwhichthey“swarm.”Byexchangewithhydrogenionscomingfromaroundroots,thenutrientionsfinallyreachtheroots.Thereisanareacalledtheoscillationzoneinwhichionsaremovingaroundrootsandclayparticles.Thisistheplaceofexchange,whereonecationisreplacedbyanotherwithanequivalentamountofcharge(Fig.5.11).Forexample,onedivalention(suchasCa2+)mayreplaceanotherdivalention(suchasMg2+),oritmayreplacetwomonovalentions(K+andK+).Whenaplanttakescationsfromthesoilsolution(Fig.5.12),itreleaseshydrogenions(H+)inexchange.Forexample,whenonecalciumionistakenintotheplant,twohydrogenionsaregivenoffintothesoilsolution.Thus,electricalneutralityismaintained.

Figure5.11Acalciumion(Ca2+)(left)migratesinsolutiontowardanegativelychargedsoilparticletowhichtwopotassiumions(K+)havebeenpreviouslyattracted.TheCa2+ion(right)changesplaceswiththetwoK+ions,whichmoveonintothesoilsolution.Aninstanceofcationexchangehasoccurred.

Figure5.12Cationsmovefromamineral,intosolution,tothecolloidsurface,andonintotherootletbyionexchange.

CationExchangeCapacityCationexchangecapacity(CEC)isdefinedasthesumtotalofexchangeablecationsthatasoilcanadsorb.Toquantifythenegativechargesonthesoilcolloidsandthereforealsotheamountofcationsattractedtothosecharges,itisessentialtoexpresstheamountinstandardunits.Theunitsarecentimolesofchargeperkilogramofsoilmaterial(cmolc/kg).The“c”subscriptbeforetheslashindicates“charge.”Thequantitiesdeterminedaredesignatedasthecationexchangecapacity(CEC).Typically,thismeasurementisdeterminedonsoilsamples,butit

maybemadeonotherearthydepositssuchaslakebottomsediments.

TherearemanyvariationsinthelaboratorydeterminationofCEC,butthebasicprinciplesbehindthemethodsaresimilar:(1)Aknownweightofsoilisplacedinabeakerandreactedwithasolutioncontainingonlyonetypeofcation,suchasammonium(NH4

+).(2)Whenithasbeenestablishedthatallthenegativesitesonthecolloidsaresatisfiedwithammoniumions,theammoniumionsarereplacedwithanotherion,andtheammoniumionsreplacedaremeasured.(3)Thecmolc/kgofNH4

+determinedrepresentstheCECofthesoilsample.

Frequentlythekindsofcationsheldonthecolloidalsystemneedtobedetermined.Thiscanbedoneinasimilarmannerwhereintheexchangeablecationsinthesoilsamplearereplacedwithanothercationandthoseremovedareanalyzedindividually.Thekindsofcationsfoundonthecolloidalsystemofmostsoilsarequitepredictable.Theyarecalcium(Ca2+),magnesium(Mg2+),potassium(K+),sodium(Na+),andhydrogen(H+).Insomesoils,aluminum(Al3+)mayalsobeverysignificant.Whennitrogenisaddedtothesoilintheformofammonia(NH3)ortheammoniumion(NH4

+),theadsorptionofNH4+becomesimportant.TherangesintheCEC

forpuresamplesoftheclaysdiscussedinthischapterareshowninTable5.1.

Table5.1Therangeincationexchangecapacityofsomecommonclayminerals

Typeofclay CECincmolc/kg

Kaolinite 3–15Illite 10–40Montmorillonite 80–100Vermiculite 100–150

Formostagriculturalsoils,theCECrangesbetween3and20cmolc/kg.Theverysandysoilsareatthelowendofthescale,andveryclayeysoilsororganicsoilsmayhaveaCECmuchhigherthan20cmolc/kg.SoilCECisinfluencedbytheamountofclayinthesoil(texture),thetypeofclaypresent(mineralogy),theamountoforganicmatterpresent,andthesoilpH.TheCECofthesoilgivesastrongindicationoftheabilityofasoiltoretainandreleasenutrients,butitdoesnotreplaceasoiltestforplantnutrientsthatarediscussedlater.

AhighCECsignifiesagreatercapacitytoretaincationssuchasK+,Ca2+,Mg2+,andNH4+

amongothers.Thismeansthatmorenutrientsareheldonthesoilcolloidsandtheconcentrationofnutrientsinthesoilsolutionislow.Thisimpliesthatwhilethereareplentyofnutrientsinthesoil,theplantsmaynotbeabletotakeadvantageofthem,especiallyunderconditionsoflowsoilmoisture,sincenutrientsheldonthesoilcolloidshavedecreasedmobility.Thesecationsarealsolesslikelytoleach.AlowCECimpliesthatfewernutrientscanbeheldbythesoilandindicatesaneedformorefrequentnutrientapplications.

HumusasaColloidalSubstance

Humusisalsopartofthesoil'scolloidalsystem,anditreleasesvaluableplantnutrientsasitdecomposes.Likeclay,thesemicroscopicparticlescarrynegativechargestowhichcationsareattracted.HumushasvariouschemicalgroupsthatcanundergolossofhydrogenathighsoilpHtogenerateexchangesitesforcationexchange.Forthisreason,thenegativechargeofcolloidalhumusparticlescandevelopinseveralways.Oneisfromthemigrationofhydrogenions(H+)awayfromcarboxylgroupsthatconsistofcarbon(C),oxygen(O),andhydroxyl(OH−).Theyexistalongthesidesofthehumuscolloids(Fig.5.13).TheabundanceofhydrogenionsinsoilswithalowpHrestrictsthemigrationofH+fromthesurfaceofhumuscolloidsandreducestheirCEC.ForthisreasonhumushasapH-dependentchargeratherthanapermanentcharge.

Figure5.13Developmentofanegativechargeonahumuscolloidparticle.

TheCECofhumus( 300cmolc/kg)isatleasttwiceormoretimesashighasthatofsilicateclays,soitsvaluetothesoilforcropproductionisenormous.Further,organicmatteradditionstosandyCoastalPlainsoilswhoseclayfractionisdominatedbykaoliniteorhydrousoxidescangreatlyincreasetheCECofthesesoils.ThishigherCECmayhaveanimpactonhowsoil-testresultsforK,Mg,Ca,andothercationsareinterpreted.Forinstance,manystatesoiltestingprogramsrecognizethiseffectbyseparatingstatesoilsintogroupsforinterpretationbasedontheirCEC.

AnionExchangeUptothispoint,onlythosecolloidsthathaveanetnegativechargehavebeendiscussed.Thenegativechargeisthedominantcondition.However,thewordnetinnetnegativechargeimpliesthatpositivelychargedsitesmayalsobepresent,thoughinlessernumbers,andthisisindeedthecase.Infact,certainacidtropicalsoilscontaincolloidswithanetpositivecharge,andthesecolloidsattractandexchangesolubleanionsjustasnegativelychargedcolloidsattractandexchangesolublecations.Thus,thesesoilsexhibitanionexchangecapacity(AEC)

insteadofCEC.Solubleanionssuchasnitrate(NO3−),chloride(Cl−),andsulfate(SO4

−)areheldandexchangedonthepositivelychargedsurfaces,whereasthecationsarerepelledandremaininsolution.Phosphate(H2PO4

−)isalsoattractedtothesesurfacesbutisheldmuchmoretightlyonsurfacesofiron-,aluminum-,andcalcium-bearingmineralsbyaspecificadsorptionmechanismthatoperatesineitherpositivelyornegativelychargedsoils.

Surfacechargebecomesmorepositive(orlessnegative)asthesoilacidityincreases.Soilsthatshowpositivesurfacecharges,therefore,arecharacteristicallyacid,andtheircolloidcomponentishighinkaolinite,ironandaluminumoxides,andhydroxidesbutlowinhumusandexpanding-layersilicate(smectite)clay.

Ingeneral,weatheringremovesthe2:1claysandleaves1:1clays,suchaskaoliniteaswellasironandaluminumoxides.Undertheseconditions,theveryacidicsoilscandeveloppositivecharges.Theyareusuallyundesirableforcropproduction,notsomuchbecauseoftheircharge,butbecausetheyoftencontainenoughactivealuminumand/ormanganesetobetoxictoplants.Manyofthesesoilsalsohaveaveryhighcapacityforfixingappliedphosphorusinaformthatisrelativelyunavailabletoplants.Soilshighinhumusandsmectiteclaynevergopositive,butsometropicalsoilslowinthesecomponentscanbepositiveatpH6andbelow.

Theusualproblemswithaluminumandmanganesetoxicityandphosphorusfixationinthesesoilsmakethemgenerallyundesirable.Also,thesesoilsaresohighlyweatheredthatveryfewnutrientsarereleasedbyfurtherweathering.TheAECofmostagriculturalsoilsissmallcomparedtotheirCEC.Therefore,anionssuchasNO3

−,SO4−,andCl−arerepelledbythenet

negativechargeonsoilcolloidsandtheyremainmobileinthesoilsolution.Theyaresusceptibletoleaching.

SoilReaction(pH)Soilreactionreferstotheconcentrationofhydrogenions(H+)andhydroxylions(OH−)inthesoilsolution,whichareexpressedinmolesperliter.ThetermpHisameasureoftheconcentrationandactivityofhydrogenions(H+)inasystem.Itisdefinedasthenegativelogofthehydrogenionconcentration,or–log([H+])=log1/H+.ThemoreH+thereis,thelowerthepHandthegreatertheacidity.ApH7.0=−log0.0000001=log1/10−7,pH8.0=−log0.00000001=log1/10−8.ThepHscaleisthelogarithmtothebase10ofthereciprocalofthehydrogenionconcentration.Thus,aunitdecreaseinpHresultsina10-foldincreaseinhydrogenions,andacomparabledecreaseinhydroxylions.Thus,apHof6.0is10timesasacidicasapHof7.0and100timesasacidicasapHof8.0(seeFig.5.14).

Figure5.14HydrogenionconcentrationisexpressedaspH.

Soilaciditycanbeinthesoilsolution(activeacidity)anditcanalsobeassociatedwiththesolidphases(reserveacidity)ofthesoil(ironandaluminumoxides,CEC,etc.).Reserveacidityisthemainsourceofactiveacidity.ActiveaciditydetermineswhethersoilpHneedstoberaisedwhilereserveaciditydetermineshowmuchlimemustbeappliedtoraiseit.Soilswithhighclayand/ororganicmattercontentsgenerallyrequiremorelimeforasimilarpHchangeastheytendtohavehigherreserveaciditythansandysoilswithloworganicmatter.

ThepHscaleextendsfrom1to14,withpH7beingpreciselyneutral.ThismeansthatatpH7,theconcentrationofhydrogenandhydroxylionsisequal.Ashydrogenionsincreaseinconcentrationandhydroxylionsdecrease,thepHdropsbelow7andviceversa.ThepHrangesthatmightbeencounteredundernaturalsoilconditionsareillustratedinFigure5.15.SoilsareconsideredacidicbelowapHof5andveryacidicbelowapHof4.Conversely,soilsareconsideredalkalineaboveapHof7.5andveryalkalineaboveapHof8.ItisraretofindsoilsclosetoeitheroftheextremeendsofthepHscaleunlesstheyhavebeencontaminatedbyhumanactivity.

Figure5.15SoilreactionisusuallylessthantwopHunitsoneithersideofneutral.

SoilpHisanimportantchemicalpropertythatismostindicativeofthegeneralchemicalstatusofasoil.Forthisreason,soilpHisthemostcommonlymeasuredsoilchemicalpropertybyfarmersandurbanhomeownersalike.Itsmeasurementcanhelpindeterminationof:theamountoflimeorsulfurtoaddtothesoil,whichfertilizerstouse(somefertilizerswillacidifythesoilandothersmayraisethepH),andwhetheraparticularpesticidecanbetank-mixedwithaparticularfertilizer.

AsdiscussedinChapter8,theuseofmanureandammoniumbasednitrogenfertilizers,suchas

anhydrousammonia,ammoniumsulfateandurea,willgraduallylowerthesoilpH.Forthisreason,itisusefultocomparecurrentsoiltestpHvaluestopreviousvaluestodetermineifthereisatrendofsoilpHchange.Regular(every2–3years)monitoringofpHvaluesinafieldcanhelponeconsiderifactionisneededtoremediatethepH.

Inthelaboratory,soilpHvaluesaretypicallymeasuredbymixing10gofair-drysoilwith20mlofdouble-distilledwateror20mlof0.01MCaCl2solution,andmeasuringthepHusinganappropriateelectrodeconnectedtoapHmeter.SoilpHmeasurementisaregularpartofmost,ifnotallsoiltestprotocols.

ThegrowthofcropsisinfluencedbysoilpH.Mostfieldcropssuchascorn,smallgrains,cotton,andpasturegrasseswillgrowsatisfactorilyoverarelativelywidepHrange—from5.5to8.3—butthepreferredrangeforbestproductionisfrompH6.5to7.8.AtthispHrange,mostplantnutrientsareoptimallyavailabletoplants.Furthermore,thisrangeofpHisgenerallyverycompatibletoplantrootgrowth.Cranberriesandblueberries,however,growbestinacidsoilswithpH4.0–5.0,whilealfalfaandsweetcloverrequireapHof6.5oraboveforbestgrowth.

PlantsareusuallynotdirectlyaffectedbysoilsthatrangeinpHbelow5.0orabove9.0;however,theindirecteffectmaybedrastic.Itissomewhatlikehavingafeverthatdoesnotharmthebodyunlessitgoestoanextreme,butitcertainlyindicatesthatthereissomesortofaninfectionthatisatleasttemporarilyharmful.Suchsoilsarelikelytoproducepoorcropsforoneormoreofseveralpossiblereasonssuchas:

1.Alackofoneormoreplantnutrients.

2.Presenceofplantnutrientsinformsunavailabletoplants.LowpHreducestheavailabilityofphosphorus,calcium,magnesium,andMo.HighpHreducestheavailabilityphosphorusandmostmicronutrients(Fe,Cu,Mn,Zn).

3.Diminishedactivityofbeneficialsoilmicrobes.Forinstance,theoptimumsoilpHfornitrification(thebiologicaltransformationofammonium(NH4

+)tonitrate(NO3−)is6.6–

8.0,andismarkedlyreducedatpHbelow6.0.Rhizobia,thebacteriaresponsiblefornitrogen(N)fixationinlegumes,donotnodulateandfixNeffectivelyunderpHvalueslessthan5.5.SincesoilpHinfluencestheactivityofpathogenicmicrobesaswell,farmerscanadjustsoilpHtomanagesomeplantdiseases.Ingeneral,themostabundantanddiversepopulationsofsoilorganismsarefoundinnear-neutralpH.

4.Abundanceofionstoxictoplants.LowpHgreatlyincreasesthesolubilityofAl,Mn,Zn,andFe,whicharetoxictoplantsinexcess.AhighconcentrationofsolubleAlalsoaffectsplantgrowthnegativelythroughinhibitionofCauptakeandprecipitationofP.Inveryacidicsoils,thehighconcentrationofH+ionscausesirreversibledamagetotheuptakemechanismsofplantroots.MolybdenumisavailableathighpHandcanbetoxictoplants.

Anunfavorablesoilreactioncanberemediated(seeChapter9).ThepHofalkalinesoilscanbeloweredbyaddingelementalsulfurorsulfur-containingmaterialstoalkalinesoils.Acidic

soilsmaybeneutralizedovertimewiththeapplicationoflimingmaterialssuchasagriculturallime(CaCO3)ordolomiticlimestone(CaMg(CO3)2)amongothers.

Whenlimeisappliedtothesoil,itdissolvestoformcarbonicacidandcalciumhydroxide.Sincecarbonicacidisweak/unstable,itdissociatestocarbondioxidegasandwater.Theremainingcalciumhydroxidedissociatesintocalciumandhydroxideions.Thecalciumionsreplacetwohydrogenionsontheexchangesites,whilethehydroxideionreactswiththedisplacedhydrogenionstoformwater,therebyresultinginanincreaseinsoilpH.

TheamountoflimerequiredtoadjustthepHofasoiltoadesiredvalueistermed“limerequirement.”ThisamountisdependentuponthebufferingcapacityofthesoilandhowmuchthepHneedstobeadjusted.Thehighertheamountofclayandorganicmatter,thehigherthebufferingcapacityandmorelimeneeded.

RemediationofsoilpHhasseveralbenefitsincludingpreventionofthetoxiceffectsofaluminumandmicronutrients,provisionofnutrientssuchasCaandMg,increasedavailabilityofessentialnutrients,improvementinsoilconditionsformicrobes,andimprovedsoilstructure.

BaseSaturationBasesaturationreferstothepercentageofbase-formingions(Ca2+,Mg2+,K+,Na+)thatoccupythecolloidalsurfaces,ortheCEC.Mostcationsinthesoilareassociatedwiththecationexchangecomplexofcolloidalclayandhumusdiscussedearlier.Underacidicconditions,comparativelyfewbasicions(calcium,magnesium,potassium)arepresentonthecolloidalsystem,buttherearemanyhydrogenions.Aluminumionsonthecolloidalsystemalsogiveanacidreaction.Forinstance,inhighlyweatheredsoilsoftheSoutheasternUSAandthetropics,highrainfallintheseareasleachesmonovalentanddivalentcations,leavinganabundanceofAl3+ionsonthesoilcolloid.Thisresultsinlowbasesaturation.Incontrast,moderatelyweatheredsoilsthatformedfrombasicigneousrocks,suchasthebasalts,haverelativelyhighbasesaturation.

Inalaboratory,whenthenaturallyoccurringcationsinasoilsamplearereplacedwithanotherkind,theoriginalcationscanbecollectedandidentified.Theamountofbasicexchangeablecationsfoundmaythenbecomparedtothetotalamountofexchangeablecations.Thisisthemeansforcalculatingthepercentbasesaturation.Forexample,onthecolloidalclayparticleinFigure5.16thereare25cations,15ofwhicharebasic.Therefore,thepercentbasesaturationis(15/25)×100=60%.Ahighpercentagebasesaturationisusuallydesirableforcrops.

Figure5.16Acolloidalclayparticlehasexchangeablecationsaroundit.Each•(acid)or□(base)representsbillionsofions.

KeepinmindthatnumbersgivenhereandinFigure5.16areforillustrationonly.Theactualnumbersarebeyondcomprehension.OnemoleofchargecorrespondstoAvogadro'snumber(6.02×1023)so,forexample,1cmolc/kgofH+wouldbe1/100ofAvogadro'snumber(6.02×1021).Figure5.17givesacomparisonofthisquantitytoaspoonfulofsoil.

Figure5.17Aspoonfulofsoilweighing10g(dry)containsabout1.2quintillion(1.2×1021exchangesitestowhichplantnutrients(Ca,K,etc.)canbeheldavailableforplantroots.

BufferingCapacityInchemistry,bufferingreferstoaresistancetochangeinpH;consequently,asoilthatiswellbufferedisonewhosepHisnoteasilyalteredsignificantly.Thecationexchangecapacityofsoilsgivesthemmostoftheirbufferingcapacity,asthevastmajorityofthebasicandacidicionsareheldtothesurfaceoftheclayandhumuscolloidsareexchangeable.Thus,whenbasicionsaredissolvedinthesolutionofacolloid-richsoil,thereisverylittleincreaseinpHbecausehydrogenionsarereleasedfromthecolloidalsurfacestoneutralizethebasethatwasadded,thatisthehydrogenandhydroxideionsreacttoformwater(H++OH−→H2O).Well-bufferedsoilsneedmorelimetoraisetheirpHthanthosethatlackanabundanceofcolloids.Thesituationisanalogoustothestrengthofanarmy—itisnotsomuchrelatedtothenumberoftroopsithasonthefrontlineasitistotheforcesitholdsinreserve.

ReasonsforBasicorAcidicSoilNativesoilpHisinfluencedbysoilmineralsandamountofprecipitation.Inaridandsemiaridregionsoftheworld,mostsoilsarebasic(alkaline)ornearlyneutralfortworeasons:Theionsderivedfromweatheringofmineralsarepredominantlybase-formingions,andthereisnotenoughprecipitationtoleachthemfromthesoil.

Inhumidregionsoftheworld,leachingbyprecipitationcausesthebasestobetranslocateddeeperintothesoil,andultimatelytheyreturntothesea(Fig.5.18).Theeffectofthisprocess

overthelongspanofgeologictimeisevidentindepositsoflimestoneandotherbasicsedimentaryrockslaiddownontheseabottom.Thelimestonedeposits,whichmaybeseveralhundredfeetthick,haveresultedfromtheconcentrationofcalciumandmagnesiumcarbonatesbylivingorganismsorbychemicalprecipitationinancientseas.Otherbasicionshavebeenconcentratedassaltbedswhenseasdriedup.Inmanyplaces,basicsedimentarydepositsarecoveredwiththicksandstoneformationsthatmaybesomewhatacidorneutralinreaction.OthernaturalprocessesthatcontributetosoilacidificationincludethereleaseofH+ionsbyplantroots,nitrogenfixation,andacidrainfall.

Figure5.18Leachingofthesoilultimatelyreturnsbasestothesea.

Althoughsoilacidificationisanaturalprocess,someagriculturalpracticespromotetheacidificationofsoils.Thesepracticesincludeharvestingofcrops,whichremovescationssuchasCaandMg,theadditionofmanures,composts,andotherorganicresidues,whichreleasehydrogenionsduringmineralization,andthegenerationofacid(H+)fromthenitrificationofureaandammonium-basedfertilizers.Asimplifiedequationshowinghowtheconversionofammoniumtonitrate(nitrification)increasessoilacidityisshownhere:

Agriculturalpractices,suchastillage,alsoenhancesoilacidificationbecausetheyacceleratemineralizationoforganicmatteraswellasleaching.Theseeffects,however,arenotseriousifstepsaretakentocounteractthembytheadditionoflime.

SoilAggregationThecolloidalsystemofthesoilnotonlyisthecenterofchemicalreactionsbutalsohasmuchtodowiththephysicalstructureofthesoil.Mostofthecommonsoilcations(particularlyCa2+

andMg2+)attractedtocolloidscausethemtoclusterintowhatiscalledaflocculatedcondition.Theoppositeofflocculationisdispersion,whichmeansthatthesoilparticlesdonottendtoclustertogetherwhenwet.Sodiumions(Na+dispersesoilveryeffectivelyandcausethesoiltoflowtogethersothatitbecomesalmostimpermeabletowater(Fig.5.19).ThereasonisthattheadsorbedNa+migratesfarenoughawayfromthecolloidstoleavethenegativechargesofthecolloidsunsaturated.Likechargesrepel,sothenegative(−)particlesdisperse.Thesoillosesstabilityasaresult.Entranceofsodium-richwaterintoearthendamsmaycausethemtofail.

Figure5.19Soiliswellaggregatedbyactionofcolloidsrichincalciumions(left).Soilrunstogetherinadensemassbyactionofcolloidscontainingabundantsodiumions(right).

Thedispersedsoilconditioncausedbysodiumisveryadversetocropproduction.Itismostcommonlyassociatedwithslightdepressionsinotherwiselevelgrasslandsofsemiaridregionsaswellaswithcroplandirrigatedwithwaterhighinsodium.Sodiumalsobringsaboutastrongalkaliconditionthatcandissolvehumus,producingadarkcrustonthesoilsurfacewhenthewaterevaporates.Farmerscallthese“blackalkalispots”andtheyarerelativelyunproductive(Fig.5.20).

Figure5.20Corngrowthispooreronsodium-richsoilthanoncalcium-richsoil.

Chapter6SoilWaterManyancientcivilizationsleftevidencethatitwaswellunderstoodhowvitalwaterwastothesurvivaloftheircultures.Nomadictribesfollowedtheseasonalrainfallpatternsthataffectedthegrowthofforagesfortheirgrazinganimalsandofedibleplantsfortheirownconsumption.Someoftheearliestpublicworksprojectsinvolveddrainageandirrigationoflandstoenhancecropproduction.Theeventualcollapseofsomeoftheseancientcivilizationshasbeenattributedtopoormanagementofwaterresources.Humanrelianceonasufficientandtimelysupplyofwaterforfoodandfiberproductionisnolesscriticaltoday.

Waterstoredinthesoildoesseveralthings.First,itisessentialtoplantgrowth.Nutrientsmovewithinthesoilsolutionandareabsorbed(takenup)fromitbyplantsthroughtheroots(seeChapter8).Second,itisessentialtothemicroorganismsthatliveinthesoilanddecomposeorganicmatterandrecycleplantnutrients(seeChapter4).Third,itisimportantintheweatheringprocessandsoilformationbyacceleratingthebreakdownofrocksandmineralstoformsoilandreleaseplantnutrients(seeChapter2).Fourth,wateralsoplaysaroleinmoderationofsoiltemperatures(Chapter7).Fifth,waterservesasanactivefactorinsoilformationbytranslocatingfineparticlesdownwardanddissolvedsubstancesbothdownwardandupward.

Waterinthesoilinfluencesthetimingofmanyfarmingoperations,suchaswhentotill,whentoplant,andwhenandhowtoapplyherbicidesand/orfertilizers.Soilwaterinfluencesthechoiceofcropstobegrown.Inareaswhererainfallandsoilwateraresufficient,cornmaybegrown.Inareaswithlessrainfalland/ormoreevaporation,thereislesssoilwateravailable,andacerealcroporgrainsorghum(milo)ismorelikelytobeselectedbyfarmers.Toeffectivelymanageavailablewaterresources,itisimportanttounderstandtheprocessesofwatermovementinsoilsanduptakebyplants.

HydrologicCycleHydrologyisthestudyofthemovementofwaterontheearth.Thehydrologiccycle(Fig.6.1)isusedtosummarizealltheprocessesinvolvingwaterintheenvironment.Whenthehydrologiccycleisconsideredonaglobalscale,itiscommontobeginwithevaporationofwaterfromtheoceans.Evaporationalsooccursfromtheland,andasmallamountofwatervaporcomesdirectlyfromsnowandiceinalpineandPolarRegionsthroughsublimation(transformationoficedirectlytowatervapor).Watervaporintheatmosphereformscloudsandthewaterfallstoearthinprecipitation(rain,snow,sleet,andhail).Precipitationthatfallsontheoceancanbeevaporatedagain.Snowthatfallsinpolarormountainousregionsmaybestoredfordecadesorcenturiesbeforeitmelts.

Figure6.1Thehydrologiccycledescribestheflowofwaterintheenvironment.

Someoftheprecipitationthatfallsonlandisinterceptedbyvegetationandevaporatesbacktotheatmospherebutmostofitreachesthesoilsurface.

Precipitationthatreachesthesoilcaneitherenterthesoilorrunoffthelandtoasurfacewaterbody(stream,marsh,orlake).Surfacewatereventuallyevaporates,seepsfartherintotheearth,orflowsbacktotheoceans,whereitcanevaporateandstartthecycleagain.Waterthatentersthesoilisofmostimportancetoplantgrowth.Thiswatercanevaporatefromthesoilsurface,beabsorbedbyplantrootstobeutilizedbytheplantortranspired(evaporatefromleaves),orpassthroughandoutoftherootzonetobecomepartofthegroundwater.

Theglobalhydrologiccycleisverycomplexandinvolvesprocessesthatoccuronlargescales(precipitation)andoverlongperiodsoftime(meltingofglaciers).Nonetheless,partsofthehydrologiccyclehavestrongimplicationsforfoodandfiberproduction.Forinstance,inseveralareasoftheworld,waterfrommeltingsnowandiceiscapturedinreservoirsandusedtoirrigatecropssometimeshundredsofmiles(kilometers)away.

SoilWaterBudgetAlthoughsoilwaterisjustonecomponentofthehydrologiccycle,itrepresentsthecrucialreservoirofwaterforthegrowthofmostplants.Aneasywaytomonitorwaterinsoilsisto

considerthesoil'swaterbudget.Justasapersonmayhaveafinancialbudgetwithinputs(income,investments,etc.)andoutputs(food,clothing,shelter,etc.),thesoilhasawaterbudget.Inagriculturalsettingstherearetwoinputs:precipitationandirrigation.Waterfromprecipitationand/orirrigationcaneithermoveintothesoil(infiltration),oritcanrunacrossthesoilsurfacetoastream,marsh,orlake(runoff).Whathappenstowaterafteritentersthesoil?Itcanbestoredintherootzoneforfutureusebyplantsormovethroughthesoilandoutoftherootzone(percolation)andeventuallytogroundwater.Itmayalsoevaporate,eitherdirectlyfromthesoilorfromplantleaves(transpiration)afterbeingabsorbedbyroots.Thus,thesoil'sstoredwateristhedifferencebetweenthesumofallinputsandthesumofalloutputs.Thewaterbudgethastwoinputs(precipitationandirrigation)andthreeoutputs(runoff,evaporation,andpercolation)(Fig.6.2).

Figure6.2Waterthatentersthesoilmaypercolateorevaporateoritmaybetranspiredorstored.

Thewaterbudgetcanbeputinequationform:

whereSistheamountofstoredwater,Pisprecipitation,Iisirrigation,Risrunoff,Eisevaporationandtranspiration,andDispercolation.Eachterminthewaterbudgetequationwouldhavelinearunitsofmeasurementsuchasinches(millimeters).Typically,waterbudgetanalysesarecompletedformonthsoryearssomanagerscananalyzetrendsineachtermandconsideroptionstooptimizewateruse.

Theamountofwaterthatendsupineachtermispartlydeterminedbyclimateandpartlybypropertiesofthesoilandtherequirementsoftheplantsgrowinginthesoil.Ofcourse,humansalsohavetheopportunitytomanagethemovementofwaterbychoosingwhichcropstoplant,whenandhowmuchtoirrigate,typesoftillageandresiduemanagementpracticestofollow,ortoprovidedrainageofexcesswater.

InfiltrationandRunoffPrecipitationorirrigationthatreachesthesoilsurfaceispartitionedbetweeninfiltrationand

runoff.Therateofinfiltrationvarieswiththetextureandphysicalcondition(structureandporosity)atthesoilsurface.Sandysoil,becauseofitsrelativelylargeporesize,hasahigherinfiltrationratethanclaysoilwithitssmallerporesize(Fig.6.3).Ifthephysicalconditionofthesoilispoor,theinfiltrationrateisreduced.Asandysoilmayhaveaninfiltrationrategreaterthan1in.(2.5cm)perh,whereassomeclayeysoilsrequiremorethan12hfor1in.(2.5cm)ofwatertoinfiltrate.

Figure6.3Soilswithlargeporespaces,suchassandysoilsandwell-granulatedtypes,usuallyhavehighinfiltrationandpercolationrates,whereasthosethathavesmallporespacesorareinpoorphysicalconditionhavelowinfiltrationandpercolationrates.Runoffoccursiftherateofrainfallexceedsthewaterinfiltrationrate.

Figure6.4givesanindicationoftheratesofrunoffandinfiltrationforahypotheticalrainstorm.Therateofinfiltrationneedstobeknownwhendesigningadrainageorirrigationsystem.Onewaytoestimateinfiltrationwouldbetoobservehowlongittakesbeforewaterstartstorunoff(ifiteverdoes)duringarainstorm.Iftherateofwaterinfiltrationintosoilislessthantherateatwhichrainfalls,wateraccumulatesonthesoilsurface.Ifenoughwateraccumulatestofillthesmalldepressionsatthesoilsurface,runoffbegins.Iftheamountofrainfallandthedurationofthestormareknown,thentheinfiltrationratecanbeestimated.

Figure6.4Runoffandinfiltrationfora1.5-in.(38-mm)rainfallin1h.Theinfiltrationratedecreasesasthesoilwetsuntilrunoffbeginsafter10min.Lateinthestorm,therunoffandinfiltrationratesaresteady.Runoffwouldhavebegunlaterandbeenlessifthesoilhadahigherinfiltrationrate.

Asrunoffwaterflowsdownslopeacrossthelandsurface,itgathersmomentumandpicksupsoilparticles,whichresultsinsoilerosion.Itisgenerallydesirabletoholdasmuchoftherainaspossiblewhereitfallstoprovidewaterforcropsandtoprotectthesoilfromerosion.Onsomesoilsinhumidregions,however,itisnecessarytoencouragerunoffthroughasurfacedrainagesystemthatpreventsthesoilfrombecomingwaterlogged.Itisimpossibletoavoidallerosion,butitisimportantforittobeminimized.SoilerosionwillbediscussedfurtherinChapter10.

Afarmercaninfluencetheinfiltrationrateofasoilbykeepingaprotectivevegetativecoveronthesurfaceandbymaintaininggoodsoilstructure,bothofwhichhelpconservewaterandsoil.Bykeepingthesoilingoodphysicalcondition,thetopsoilbecomesfullof“crumbs,”whicharestable,spongyaggregates.Withthisconditionofthesoil,watermoveseasilyintoandthroughthesoil.Suchaggregatesformwhenadequateorganicmatterispresentandexcessivetillageisavoided.Thefarmerwhodepletesthesoiloforganicmatterbyremovingcropswithoutreturningplantresiduesormanureislikelytodecreasetheinfiltrationrate.Whenraindropsstrikeexposedsoil,especiallywithoutthespongyaggregates,thesoilisbeatentoapasteandasealformsonthesurface.Thesoilsurfacethentendstoshedwaterlikearoof.

Whenasealed-offsurfacebecomesdry,itformsabrittlecrustthatcaninhibitemergenceofseedlings.Seedlingssuchasthoseofbeansandpotatoesarestrongenoughtobreakthrough,liftingpiecesofthecrustlikesomanytrapdoorsopenedfrombelow.Butseedlingsofsmall-seedcropssuchasoatsandeventhelargerseedlingssuchascornmayperishwithoutever

emerging(Fig.6.5).Sometimesfarmershavetobreakthiscrustwithlighttillageafterplantingthecrop,butthismaybeonlypartiallysuccessful.

Figure6.5Ifaplantseedlingisnotstrongenoughtoliftthesoilcrust,itdies.

EvaporationEvaporationisthetransformationofwaterfromliquidtovaporregulatedbysolarenergy,windmovement,andhumidity.Soilwatercanevaporatedirectlyfromthesoiloritcanbeabsorbedbyrootsandevaporatefromstomatesontheleavesofplants.Theprocessofevaporationfromstomatesiscalledtranspiration.Evaporationfromsoilandtranspirationbyplantsmaybecombinedandcalledevapotranspiration(Fig.6.6).

Figure6.6Soilwaterreturnstotheatmospherebyevaporationfromthesoilsurfaceandbytranspirationfromplantleaves.

Inmanytemperateanddrierregions,atleasthalfthewaterlostfromfarmlandisbyevaporation.Therefore,farmingpracticesareoftendesignedtoreducethislossandconserve

moisture.Oneeffectivepracticeistoleavetheplantresiduefromthepreviouscroponthesoilsurface.Thesecropresiduesreduceevaporationbyshadingthesoilandblockingwatervapormovement.

Insmallplotsofhigh-valuecrops,mulchesareoftenusedtoholdthesoilwaterforplantsandthusreduceevaporation(Fig.6.7).Manykindsofmulcheshavebeenused:straw,corncobs,gravel,andplastic.Allcanbequiteeffective.Theselectionofonetypeofmulchoveranotherdependsonthespecificuseandavailabilityofthematerial.Organicmaterialssuchasstrawarepreferableinsituationswherethemulchcanbeincorporatedintothesoilaftereachcrop.Sandandgravelhavetheadvantageofallowingahigherpercentageofsmallrainstoinfiltrateintothesoilratherthanbeingabsorbedbythemulch.Thiscanbeanadvantagearoundfruittreesandornamentalplantings.

Figure6.7Amulchhelpspreventevaporationofwaterfromthesoil.

Theuseofblackplasticasamulchispopularinvegetableproductionbecauseiteffectivelycontrolsbothweedsandevaporation.Avariationinthispracticeistoformplasticortarpaperintoashallowconearoundthebaseofatreeorshrubandcoveritwithafewinchesofgravel.Thisallowsrainwatertoenterthesoilnearthetrunkandleavesnoplaceforweedsorgrasstogrowinhard-to-mowplaces(Fig.6.8).

Figure6.8Blackplasticortarpapercontrolsweedsandevaporation.

Farmersindryareaswherewheatisaleadingcroputilizetheprincipleofmulchingbyleavingmuchoftheplantresidueonthesoilsurfacewhentillingafterharvest.Creatingadustmulchbyfrequenttillageoffallowlandisnowoftendiscouragedbecauseithasbeenfoundthatlittlewaterisconservedbythispracticeandthatsoilmaybeleftsusceptibletoseverewinderosion.

PercolationIn1856,anengineernamedHenryDarcywasthefirsttodescribehowwatermovesthroughasaturatedsoil(allporesfilledwithwater).HedevelopedhistheorybyobservingtheflowthroughasandfilterusedtopurifywaterintheFrenchcityofDijon,famousfortheproductionofmustard.

Theforceofgravitycauseswatertomovedownwardthroughthesoil,particularlyinlargerpores.Gravitationalwaterpercolatesuntilitisadsorbedbydriersoilbeloworitreachesthewatertable.Thewatertableisthelevelinporoussubsurfacematerialsbelowwhichallporesarefilledwithwater.Thismaybewithinthesurfacesoil,inburiedsediments,orinthedeepbedrock.Groundwateristhewaterbelowthewatertable.Theamountofwaterthatpercolatesthroughtherootzonetogroundwaterisreferredtoasgroundwaterrecharge.Thewatertablesurfacehasaslightslope(muchlessundulatingcomparedtolandslopes)allowingthewatertoflowlaterallybelowground.Poroussoilorrocklayersthataresaturatedcanbeimportantsourcesofwellwater.Suchlayersarecalledaquifers.

Aswatermovesdownwardthroughthesoiltoreachthewatertableandthegroundwater,itmayencounterasoillayerthatrestrictsdownwardmovementcausingwatertobuildupandformaperchedwatertable.Iftheperchedwatertableiswithintherootzone,plantrootsgrowinginthiszonemaybedeprivedofoxygen,thusimpactingplantgrowth.

Inaflatarea,waterflowinauniformsoilwillbeprimarilydownward.Onsteepslopesorwhenthereisagentleslopewitharestrictingormoreconductivelayerinthesoil(likealayerofclayorgravel),watermovementmaystillbedownwardbutsomewatermayalsomovedownslopeorlaterally.Lateralwaterflowinmountainousareasmaycometothesurfaceagainasdischargefromflowingsprings.Inallcases,plantuptakedrawswateroutofthesoil,alteringwatermovementinthesoilneartheroots.

Wetlands,lakes,andstreamsinhumidregionsareoftenplaceswherethewatertablecomestothesurfaceandgroundwaterdischargetakesplace.Wetlandsindrierregions,however,areoftengroundwaterrechargeareas,wheresurfacerunoffcollectsandinfiltrationoccurs.Thus,wetlandsintheseareasserveanimportantroleinreplenishingaquifers.

SoilWaterStorageandMovementTwoforcesimpactsoilwaterstorageandmovement.Theyare;(1)gravityand(2)attractionofsoilparticlesforwater.Whensoilissaturatedwithmoisture,gravityisthedominantforceinmovingwaterinlargeporesdeeperintothesoil.Atthesametimethatgravityispullingwaterdownward,thesoilparticlesareattractingwaterinalldirectionsbytheforcesofadhesionand

cohesion.Adhesionistheattractionofasurfaceforwater(e.g.,thesurfaceofthesoilparticle),andthisforceisquitestrong.Cohesionistheattractionofonewatermoleculeforanother.Thetwoforcescombinesothatwaterisheldwithinsmallporesbetweensoilparticles.Becauseoneoftheforcesistheattractionasoilparticlesurfacehasforwater,itfollowsthatasoilwithverysmallparticlesandmoresurfaceareasuchasaclayattractsandholdsmorewaterthanasoilwithlargeparticlesandlesssurfaceareasuchassand.

Whenasoildries,adhesiveforcesbegintodominate,andthewaterremaininginfilmsisheldveryclosetothesoilparticleswithgreaterforceduetothesurfaceattractionforwater.Consequently,watermovementisveryslow.Watermovementinunsaturatedsoilsinvolvesacomplexcombinationofgravity,relativeamountofwater,andadhesive/cohesiveforces.Itwasnotuntil1907thatanAmerican,EdgarBuckingham,wasabletoaccuratelydescribewaterflowinunsaturatedsoils.

Asaplantrootabsorbswater,ittakessomefromthefilmsurroundingtheadjacentsoilparticle.Duetocohesionandadhesion,watermovesfromparticleswiththickerfilmstoparticleswiththinnerfilmsthatarenexttotheroots.Thisiscalledcapillarymovement(Fig.6.9).Capillarywatermaymovetotherootsfromanydirection—up,down,orlaterally.Animportantfactisthatwatermovesinthesoiltowardrootstoprovideplantswithwater.Someessentialnutrientscanalsomovewiththewater.Capillarymovement,however,isnormallyveryslowinsoil,soplantsmustcontinuallyextendtheirrootsintomoistporestoabsorbwater.Thesoilthushasacriticalroleinagricultureasitactsasastorehouseofwaterforplantstouseuntilthenextrainorirrigation.

Figure6.9Watermovesfromsoilparticleswiththethickestwaterfilmstosoilwiththethinnest.Astheplantrootabsorbsmoisture,watertendstomovetowardit(capillarymovement).Plantrootsalsogrowandextendintozoneswithmoremoisture.

WaterUsebyPlantsWaterisessentialtoallformsoflife—bothplantsandanimals.Someplantshavelowwaterrequirementsandarecalledxerophytes(xeromeanslittleornoneand“phytes”fromthewordphyto,whichmeansplant).Somehavehighwaterrequirementsandarecalledhydrophytes(hydromeanswater).Plantswithmoderatewaterneedsarecalledmesophytes(mesomeans

intermediate).

Plantsneedwatertoformcertaincompounds.Forexample,sixpartsofwaterarerequiredforeachsimplesugarproduced.Theprocessofformingsimplesugars,calledphotosynthesis,involvesthesplittingofwater(H2O)intohydrogenandoxygen.Thehydrogencombineswithcarbondioxide(CO2)toformsugars,andtheoxygenisdischargedintotheatmospherethroughopeningscalledstomatesintheleavesofplants.

Muchofthewaterstoredinthesoilisusedbyaplant.Theamountofwaterstoredinthesoilandtheamountavailabletoplantsvarywiththetextureandstructureofthesoil.Whensoilwatercontentisnearsaturationandgravitationalforcesaredominant,mostwaterinthesoilisreadilyavailabletotheplant.

Themaximumamountofwaterinasoilheldagainsttheforceofgravityiscalledthefieldcapacity.Aswaterisusedbyplantsorevaporates,thewaterfilmaroundsoilparticlesbecomesthinner,ismoretightlyheldbytheparticles,andismoredifficultfortheplanttoabsorb.Eventually,theattractionbetweenthesoilandthewaterisgreaterthantheplant'sabilitytoabsorbit.Thisamountofwaterinasoiliscalledthewiltingpointbecausetheplantcannolongerabsorbenoughwatertomaintaintranspirationandsustainlife(Fig.6.10).

Figure6.10ThewaterfilmsinAarethickestandthesoilisnearlysaturated;atBitisaboutatfieldcapacity;andthethinfilmsinCrepresentthewiltingpoint.

Betweenthesetwopoints,thefieldcapacityandthewiltingpoint,waterisavailabletotheplant(Fig.6.11).Theamountthatisavailablevarieswithsoiltexture(andphysicalcondition).Forexample,asandysoil(whichhaslargeparticlesandlowsurfacearea)maystoreabout1in.ofwaterperfootofsoildepth(83mm/m)andmostofthewaterwouldbeavailabletoplants.Aclaysoil(whichhassmallparticlesandhighsurfacearea)mayhold4in.ofwaterperfootofsoildepth(333mm/m),butbecauseofthestrongattractionofclayparticlesforwater,only1in.ofthese4in.maybeavailableforplantuse.Soilswiththegreatestamountofavailablewaterareusuallythosewithaloamytextureandgoodstructure.

Figure6.11Soilwaterbetweenfieldcapacityandthewiltingpointisavailabletotheplant.

Onlyaverysmallpercentageofthewaterabsorbedandutilizedbyaplantisforphotosynthesis.Water'sprincipalfunctionistotransportnutrientsandplantcompoundsinsolution,eitherupwardfromtheplantrootstotheupperleavesordownwardintolowerleavesortherootsystem.Mostofthewatertakenupbyaplanteventuallyevaporatesatthestomates(Fig.6.12).Waterlostfromtheplantduetoevaporationfromthestomatesiscalledtranspirationand,especiallyinhotweather,transpirationhelpscoolaplant.Lessthan1%ofthewaterabsorbedbyaplantisusedinformingplantcompounds;therestislostviatranspiration.

Figure6.12Watermovesintotherootsandthroughtheplantprimarilybycapillaryaction.

Aplant'swateruseefficiencyisdeterminedbymeasuringtheamountofwaterrequiredtoproduceacertainweightofdryplanttissue.Ittakesapproximately500lb(225kg)ofwatertoproduce1lb(0.45kg)ofwheat(foliageplusgrain).Only5lb(2.25kg),or1%ofthisamountactuallybecomespartoftheplant.Alfalfausesmorewater,requiringabout850lb(385kg)ofwaterperpound(0.45kg)ofdrymatter;whilegrainsorghum,anefficientwateruser,mayrequirelessthan300lb(135kg)ofwaterperpound(0.45kg)ofdrymatter.

DrainageItisacommonoccurrenceinmanyregionsoftheworldforthesoiltocontaintoomuchwaterduringrainyseasonsoftheyearorduringwinterwhenevaporationislow.Ifthesoiliswaterloggedtoolongduringthegrowingseason,rootsdiefromlackofoxygenorfromaccumulationoftoxiccompounds.Toridthesoilofexcesswater,drainagesystemshavebeeninstalledonmillionsofacres(hectares)ofland.Drainagesystemscaninvolvesubsurfaceorsurfacepracticesoracombinationofboth.Remarkableincreasesincropyieldscanoccurwhennaturallywetsoilsaredrained.

Subsurfacedrainageisthepracticeofburyinganetworkofperforatedpipeshorizontallyinthesoil.Thepipesinterceptpercolatingwater,orcapturethewaterinaperchedortruewatertable.Thewaterenteringthepipesmoveslaterallytoasurfaceoutletsuchasadrainageditch.Drainpipesortileswereoriginallymadeofshortsectionsofconcreteorclay,butlonglengthsofflexibleplastictubingarenowmorepopular.Thetubingisinstalledatadepthof2–6ft.(0.6–1.8m)andhasaslightdownwardgradienttothesurfaceoutlet.Subsurfacedrainage

functionsonlywhenthesoilissaturated,sothatwatercanflowfromthelargeporesinthesoilintothegapsbetweensectionsofclayorconcretetileorthroughholesintheplastictubingandonalongthetileortubingtotheoutlet(Fig.6.13).

Figure6.13Thewatertablecanbeloweredtothelevelofthesubsurfacedrainagenetwork.

Insomecases,averticaltubeisinstalledfromthesubsurfacedraintothesoilsurfacetoallowwaterpondedonthesoiltoenterthedrainwithoutpercolatingthroughthesoil.Thesesurfaceinletsorintakesarecommonlyfoundinareaswithsmalldepressionsthatfillwithwaterduringheavyrainsorspringsnowmelt.

Subsurfacedrainagecanbeinstalledindifferentpatterns,depths,andspacingdependingonlandslopeandlocationoftheoutlet(Fig.6.14A–D).Therandomdesign(Fig.6.14A)isusedwherethereareisolatedwetareas.Drainlinesarerunundereachareawithperhapsasurfaceinletinthelargerdepressions.Pattern(Fig.6.14B)andherringbone(Fig.6.14C)drainagepatternsinvolveuniformdistancesbetweenmultipledrainlines.Thepatternuseddependsonslopeofthelandanddesireddepthofunsaturatedsoil.Aninterceptordrain(Fig.6.14D)isusedtointerceptlateralflowdownaslopethatmaybecreatingawetspot.

Figure6.14Differenttypesofsubsurfacedrainagesystems.

Surfacedrainageinvolvesdiggingchannelsinthesoilandsometimesalsoshapingthelandsurfacesowaterwillrunoverthesurfaceintothechannels.SurfacedrainageisusedonsoilsthathavelayerswithlowpermeabilityorinveryflatareasliketheRedRiverValleyofNorthDakotaandMinnesota.Intheseareas,watereithercannotmovethroughthesoilfastenoughortheslopeofthelandisnearlyflatforsubsurfacedrainstofloweffectively.Combinationsurface/subsurfacedrainagesystemsinvolvesubsurfacedrainsusingsurfacedrainagechannelsfortheiroutlets.

Tohaveanoutletforsubsurfacedrains,somenaturalstreamchannelshavebeenstraightenedand/ordeepened(Fig.6.15).Thispracticeiscalledchannelization,andisoftencriticizedforimpactingwaterqualityandwildlifeconservation.Drainingwetlandsthatshouldbepreservedforwildlifehabitatorforwaterqualityprotectionisoftenundesirable.Inthesecases,channelizationisusuallyundesirable.However,onagriculturallandthatisincropsandpasture,channelscanbebeneficialtobothagricultureandwildlife.Whenintermittentwaterwaysaredeepened,apermanentstreammaybeformed.Asaresult,fishthrive,andbirds,mammals,andreptilesfindanimprovedenvironmentalongthechannelbanks.Cropsalsoflourish,andtheytooprovidefoodandshelterforwildlife.

Figure6.15Channelizationisneededtocarrywaterfromsubsurfacedrains.

Tohelpoffsetthelossofwetlandsduetofarmlanddrainageandchannelization,artificialwetlandscanbecreated.Shallowdepressionsaremadeinthesoilinlow-lyingareasandwetlandplantspeciesareseededortransplantedintothedepression.Waterfromrunofforsubsurfacedrainsisdirectedintothewetland.Thewetlandmaybedesignedtobefloodedtheentireyearoronlyafterspringsnowmeltandlargerainstorms.Theseconstructedwetlands;(1)providewildlifehabitat,(2)mayhelpreducechemicalandsedimenttransporttostreams,and(3)increasegroundwaterrecharge.

IrrigationSinceancienttimes,variouscivilizationshaveutilizedwaterfromriversandwellstoensuremorereliablecropproduction.Today,about17%oftheWorld'scroplandisirrigated.Becauseyieldsonirrigatedlandareusuallyhigh,irrigatedcroplandaccountsforadisproportionatelylargepartofcropproduction.Incountrieswherericeisthestaplediet,thepeoplerelyonirrigatedagricultureformostoftheirfood.Manyofthesecountrieshaveamonsoonclimatewheretheannualrainfallishighbutmostrainfallsduringafewmonthsandisfollowedbyanextendedperiodoflimitedrainfall.Eveninthetemperateregions,timelyrainscanbequiteunpredictablefromyeartoyearsoirrigationagricultureiscommon.

Differentirrigationsystemsareusedbasedonthecropsbeinggrown,thesourceandqualityofwater,andtheleveloftechnologyavailable.Floodirrigationinvolvesfloodingtheentirefieldwithwaterfromcanalsorpipes(seeFig.6.16).Paddyriceproductionisanexampleoffloodirrigation.Abermaroundthefieldcontainsthewaterandsometimestheplantsaregrowninrowsonridgeswiththewaterflowingbetweentherows.Withsprinklerirrigationwaterissprayedontothecropfromnozzlessuspendedabovethecanopy.Oftenthewatersourceisawellandelectricityoranengineisneededtopumpthewaterand/ormovethesprinklers.Sub-irrigationcanbeaccomplishedwithsmalltubesburiedinthesoilthroughwhichwaterispumped.Waterseepsintothesoilthroughemittersspacedalongthetubing.Anothertypeofsub-irrigationinvolvesputtingwaterbackintosubsurfacedraintubestore-wetsoilintheroot

zone.Dripirrigationutilizessmalltubesonthesoilsurfacewithdrippersormicro-sprinklersplacedneartheplants.Waterispumpedthroughthetubesandtheamountofwaterapplieddependsonthelengthoftimeandthenumberandsizeofthedrippersormicro-sprinklers.Dripirrigationismostoftenusedwithhighvaluecropsandwherethereisaverylimitedavailabilityofwaterforirrigation(seeFig.6.16).

Figure6.16Examplesoftypesofirrigationsystems:(A)surfaceorflood,(B)sprinkler,(C)sub-irrigation,and(D)drip.PhotoscourtesyofUSDANRCS.

Thefirsttwoconditionstoconsiderforirrigationfeasibilityareasourceofwaterandadequatesoildrainage.Thequalityofirrigationwaterisimportantbecauseitmayhavelong-termimpactsonsoilproperties.Thetwocommonqualitiesofirrigationwaterthatareofconcernarehightotalsaltcontentandahigh-concentrationofsodium.Insomecases,otherionssuchasboron,lithium,andseleniummaybepresentattoxiclevelsintheproposedwater

source.Sometimesspecialwater-utilizationtechniquescanbeemployedtoenabletheuseofwaterofmarginalquality.Insomeinstances,irrigationwatermaycontainbeneficialions.Nitratenitrogen(NO3

−)mightbepresentinirrigationwaterathighconcentrations,suchasinthePlatteRivervalleyofNebraska.Intheseinstances,acreditshouldbegivenfortheamountofnitrogenappliedduringirrigation.

Allirrigationwatercontainssalt,andwhenirrigationwaterevaporates,thesaltstendtoaccumulateandmightnegativelyimpactplantgrowth.Thisiswhentheinternaldrainageofthesoilbecomesimportant.Ifsaltsaccumulate,theonlywaytoremovethemisbyapplyingmorewater.Whenthiswatermovesdownward,thesaltsdissolveandmovewiththewater.Thisiscalledleaching.Waterusedforleachingcanbeeitherfromrainfallorirrigation.Theremovalofsaltfromirrigatedsoilmaycreateotherproblems.Ifinternaldrainageofthesoilisgood,saltscanbeleacheddownthroughtherootzoneandeitheraccumulatethereorcontinuetomovedownward.Ifsoildrainageispoor,saltscanmoveoffthesoilsurfaceaswatermovesacrossit(asinriceproduction).Insomesoilswithpoorinternaldrainage,subsurfacedrainagesystemsmaybeinstalled.Aswatermovesthroughthesoilintothedrainagesystem,saltsmovewithit(suchasintheImperialValleyofCalifornia).Regardless,thesaltconcentrationinthewaterincreasesasitmovesacrossorthroughthesoil.Thewatercomingoutmaycontainthreetofourtimesthesaltcontentcomparedtothewatergoingintothefield.

Dischargingwaterwithhighsaltcontentintothedrainagesystemofanareacancreateproblemsforthosedownstreamfromthepointofdischarge.Ifsaltconcentrationistoohigh,itcanbedetrimentaltohumanandlivestockconsumptionorforreuseasirrigationwaterforcropproduction.

Ifsodiumconcentrationishighintheirrigationwater,thesoilwilldevelopahighexchangeablesodiumlevel.SodiumisattractedtoalesserextenttocolloidalsurfacesthanisCa2+,butittakesonlyabout15%Na+ontheexchangecomplextocauseadispersalofclayparticles.Soilswithhighexchangeablesodiumwillnormallydevelopsurfacecrustsandsoilaggregateswilldisperse.Theseconditionsdecreasetherateatwhichwatermovesthroughthesoil,whichinterfereswithdrainageandsalinitycontrol.ScientistsattheU.S.SalinityLaboratorydevelopedthesodiumadsorptionratio(SAR)tocharacterizethesodiumstatusofirrigationwaterandsoilsolutions:

wheretheconcentrationsofNa+,Ca2+,andMg2+areexpressedasmolesofcharge/L.SARvaluesarenotusedalonebutratherwithothermeasuresofsalinityorsodicity.ASARbelow13–15forasoilextractisgenerallyconsideredacceptable.

Aboutthree-quartersoftheirrigationwaterintheUnitedStatescomesfromsurfacewaterssuchasriversandreservoirswhererainwaterand/orsnowmelthavebeenimpounded.Theremainingquartercomesfromundergroundaquifers.Somewellstapshallowaquifersthatarereplenishedbyprecipitationannually,suchasintheCentralSandsofWisconsin,butothers“mine”deepwaterthatwastrappedinaquifersthousandsofyearsagoandisnotbeing

replenishedtoanappreciabledegree.Thissituation,referredtoasoverdraft,isoccurringinpartsoftheHighPlainsofTexas,wherethewatertableisdropping.ThispotentialproblemexistswheretheOgallalaaquifer,whichreachesfromSouthDakotatoTexas,hasbeensupplyingirrigationwatersincethe1930s.ThesolutionforextendingthelifeoftheOgallalaaquifer,whilemaintainingagriculturalproduction,isimprovedefficiencyintheuseofirrigationwater.

WaterConservationTherecognitionofhigh-qualitywaterasavaluableresourcehasledtoextensiveresearchonimprovingirrigationefficiency.Floodirrigationhasalowerwateruseefficiencyassignificantwatercanbelosttoevaporation,seepagebelowtherootzone,andastailwater(waterthatflowsoutofthefield).Sprinklerirrigationtypicallyhasahigherwateruseefficiencyalthoughtherecanstillbelargeevaporationlosses.Sub-irrigationanddripirrigationarethemostefficienttypesofirrigation.Withthesesystemsthewaterisappliedslowlywhererootscanreachitsoverylittlewateriswasted.However,thecostofthesesystemsishighandtheirinstallationandoperationrequiregreatermanagementskill.

Other,generalwater-conservingtechniquesnowincommonuseinclude:

1.improvedtimingofwaterapplicationbasedonmeasuredsoilmoistureintherootzone(oftencontinuouslymonitoredwithsensorsburiedinthesoil),

2.plasticliningofsupplyditches,

3.selectionofcropsandvarietieswithhigherwateruseefficiency,

4.optimizedplantpopulationdensity,and

5.attentiontoplantnutritionandhealthmaintenance.

Tominimizewaterlossbyevaporation,plantresiduesormulchcanbeespeciallyeffective.

Chapter7SoilTemperatureAllplantsneedsunlighttogrow.Lightfromthesunsuppliestheenergyneededforphotosynthesisandalsowarmsthesoilandairinwhichcropsgrow.Soiltemperatureaffectsalmosteveryphysical,chemical,andbiologicalactivitythatoccursinthesoil.Managementdecisions,suchaswhentoplant,areoftenbasedonsoiltemperature.Knowledgeoftheflowofenergyinthesoil–plant–atmospheresystemhelpstounderstandhowplantsrespondtoclimateconditions.

ImportanceofSoilTemperaturePlantsSoiltemperatureisagreaterinfluencingfactorinplantgrowthratesthanabovegroundairtemperatures.Soiltemperatureinfluencesdateofplanting,timetogermination,andnumberofdaysforacroptomature.Idealsoiltemperaturesforgerminationvarydependingonthecropandtheseedcharacteristics.Mostseeds(providedotherconditionsareideal,suchasadequatesoilmoisture)requireminimumsoiltemperaturesof40–60°F(4.5–15.5°C)togerminate.Insomecasesextremesoiltemperaturemayrestrictgerminationandplantgrowthandextremelyhightemperaturecancausesevereheatstresstoyoungseedlings.Coolersoiltemperaturesmaydiminishtheabilityoftherootstoabsorbwaterandnutrients.Innormalsituations,withincreasedtemperatures,germinationandgrowthofseedlingsareenhanced,androotdevelopmentisfaster.

Optimumsoiltemperaturesforplantgrowtharegenerallyhigherforplantsthathaveevolvedinwarmclimatesthanthoseevolvedincoolerclimates.Cropslikecottongrowwellinwarmsoilconditionswhilepotatoes,rye,andoatsprefercoolersoilconditions.Similarly,differenttreeshavepreferencesforcoolerorwarmersoilconditions.InforestedareasoftheMidwesternUnitedStates,adenseoak–hickory–mapleforestisfoundgrowinginthecoolersoilsofthenorth-facingslope,whilesouth-facingslopes(warmersoils)oftenhaveasparsestandofredcedarandburroakwithgrassbetweenthetrees.

MicroorganismsMicroorganismsarevitaltothebreakdownofplantandanimalresidues(organicmatter)withtheresultantreleaseofessentialplantnutrientsincludingnitrogen,phosphorus,andsulfur.Soiltemperatureinfluencesthegrowthandactivityofmicroorganismsinthesoil.Eachorganismhasanoptimumtemperatureatwhichitsmetabolicactivityisatitshighestlevel(mostrapidbreakdownoforganicmatter).Normalsoiltemperaturesaregenerallycoolerthantheoptimumformostorganisms.Asaresultanyincreasesintemperature(approachingtheoptimumtemperature)couldresultinincreasedmicrobialactivityandfasterreleaseofplantnutrients.

Whilewarmersoiltemperaturescanhelpthebeneficialsoilorganisms,thesameistrueforthesoilorganismsresponsibleforplantdiseases.Microbesarealsoresponsibleforbreakdownofmanyoftheorganicwastesandpesticides.Warmertemperaturescouldhelpthemicrobesdetoxifyasoilfaster.Extremesinsoiltemperaturecanhaveaslowingeffectonmicroorganismandplantgrowth.Extremelyhightemperaturescandestroypathogenicorganismsandweedseedsasisdoneincompostingoperations.

SolubilityofMineralsSoiltemperaturehasaninfluenceonthedissolvingofmineralsbywater.Warmwaterhastensthedissolutionofminerals,makingnutrientsfromthesemineralsavailableforplantuse.Highsoiltemperatureandadequatesoilmoisturemean,inmostcases,morenutrientsinsolution.Soilsinclimateswhererainfallisexcessiveandsoiltemperaturesarehighgenerallyhavelownutrientlevelsbecausethenutrientsfromdissolvedmineralsmayhaveleachedoutoftherootzone.

SoilMoistureSoiltemperatureimpactsthephysicalformofwaterinthesoil(ice,liquid,orvapor).Thebehaviorofliquidwaterisalsoaffectedbysoiltemperature.Temperaturehasanimpactonthedensityandviscosityofthewater,bothofwhichareimportantindeterminingtherateofwatermovementinsoil.Warmertemperaturesallowfasterwatermovementinasoil.

Extremesoiltemperaturescouldresultinfrozenwaterorexcessiveevaporationlosses.Asoilwithafrozenlayeroficecouldimpededownwardmovementofwater,resultinginpondingofwaterabovetheicelayer.Waterfromspringrainsonfrozengroundcouldresultinfloodingofsurroundinglowlandsduetodecreasedinfiltrationandincreasedsurfacerunoff.Inthissituation,thesurfacesoilislikelytoerodeaswell.

Waterexpandsuponfreezing.Soilscontaininga(significant)amountoffineparticlesthatcanretainwaterwillexpandmorethancoarsetexturedsoils.Thisexpansioncouldresultininternalpressurescausingthesoilparticlestomoveawayfromtheicelensesformedduringfreezingofthesoilwater.Insomeareaswhereperennialcropsareplanted,pressurecausedbyfreezingcouldpushorlifttheplantoutofthesoil,aphenomenoncalledfrostheave.Asaresulttheplantispushedupwardwhiletherootsremainanchored,causingtherootstobreakawayfromtheplant,andiftheactionissevereenoughcouldresultineconomiclossestoafarmer.Similarly,frozensoilconditionscanpushrocksburiedinthesoilupwardtothesoilsurface.

Frozenwaterpocketscanputunevenpressuresonstructuressuchasroadsandshallowfoundations,causingthestructuretoshift/settleinanunevenmanner.Thisisoneofthereasonsfootingsthatsupportfoundationsofbuildingsareplacedatadepthbelowthefrostzoneincoldclimates.Inthehigherlatitudes,soilsmaybefrozenforlongerperiodsoftheyear.Infarnorthernregions,thesubsoilisalwaysfrozen.Thiscondition,whichiscalledpermafrost,existsinmuchofnorthernCanada,Alaska,andSiberia.Inrecentyearsthepermafrosthasshownrapidmeltingoftheicelayerwithseriousimplicationstoplantsandpotentially

increaseddecompositionratesoforganicmaterial.

Warmersoilconditionscanpromoteevaporation.Asthewatervaporescapesthesoilsurface(driersoil),theheatstoredinthewaterislostfromthesoilandthereforeithasacoolingeffectonthesoil.However,inputsofenergycouldquicklymakeuptheenergylostviavaporwithanincreaseinthetemperatureofthedriersoil.Lossofwaterduetotranspirationcanalsohavethesameimpactofdryingthesoilandheatingthesoilmuchfaster.

FiresSometimesextremelyhighsoiltemperaturesarecreatedbyfires.Thedurationandintensityofthefirewilldeterminesoiltemperaturesandthedepthtowhichtheextremeheatisconducted.Extremelyhightemperaturehastheabilitytoburntheorganicmatterinthesoil,resultinginaninstantreleaseofplantnutrients.Whilenutrientreleasemaybeatemporarybenefit,thelossoforganicmatterandthedestructionofmicroorganisminthesoilmayhavelong-termconsequences.Hightemperaturefromoccasionalburningcanalsoresultindestructionofweedseedsandpathogenicorganisms.However,someseedsthathavehardcoatingsonlygerminateafterbeingexposedtohightemperatures.Sometimessoiltemperaturesareintentionallyraisedbytheadditionofcertainchemicalsthatproduceaheat-generatingreactioninordertocleanuplandscontaminatedwithorganicpollutants.Thehightemperaturemaycausetheorganicpollutanttovaporize.

SoilFormation/ClassificationTemperatureandwaterplayanimportantroleasactivesoil-formingfactors.Solubilityofminerals,decompositionoforganicmatter,moistureconditions,andcompetitivenatureofdominantplantsarerelatedtosoiltemperature.Thetranslocationofsolublematerialsandtheirsubsequentaccumulationintheprofileleadstosoilhorizondifferentiation.Soilsonsouth-facingslopesintheNorthernHemispherewilltendtobeshallowerandcontainlessorganicmatterduetothemoreextreme(warmanddry)climate.

Meanannualsoiltemperatureisgenerallyusedtogroupsoilsintovariousthermalregimes.Thethermalregimeisusedforsoilclassificationpurposes.Thethermalregimeofasoilcharacterizesthetypeofvegetationthatcanadapttothetemperatureconditions.Forexample,soilsin/onwetlandscomparedtodrylandsandofnorth-facingslopesversussouth-facingslopescouldhavedistinctlydifferentthermalregimes,resultingindistinctlydifferentvegetationandsoilproperties.

FactorsAffectingEnergyInputsInputsofenergyhaveawarmingeffectonsoils.Themostsignificantsourceofthermalenergygainisfromsolarradiation.Thesurfaceofthesunwithatemperatureofabout10,300°F(5,704°C)radiatesenergytotheearth.Incidentalandsporadicinputsofenergymaycomefromfires,warmrains,warmairmassesblowingacrossthelandsurface,condensationofdew,biologicalactivity,andsimilarevents.

AtmosphericConditionsTheamountofsolarenergyreachingtheearth'ssurfacewilldependontherelativepositiononearth,seasonoftheyear,atmosphericconditions,soilcover(vegetation,snow,ormulch),andotheraspects.Asenergyfromthesunradiatestowardtheearth,itmustpassthroughtheearth'satmosphere.Onitsjourneythroughtheatmosphere,solarradiationmaynotreachtheearth'ssurfacebecauseitmaybeintercepted,absorbed,orreflectedbytheclouds,variousgases,orparticulatematterintheatmosphere.Notallofthesolarradiationfromthesunthatreachesthetopoftheearth'satmospherereachesthesoilsurface.

LandCoverAportionofthesolarenergythatpassesthroughtheearth'satmospherecouldbeintercepted,absorbed,orreflectedbylandcoversuchasvegetation,snow,ormulch.Theportionofthesolarenergythatfiltersthroughthelandcoverandmakesitthroughtothesoilsurfacewarmsthesoil.Butonlyasmallportionofthesolarradiationactuallyreachesthelandsurfaceandhelpstowarmthesoil.

Thetypeanddensityofvegetativecoverinfluenceshowmuchsolarenergyreachesthesoilsurface.Athickcrop/forestcanopy/turfgrassorlayerofsnow/mulchshadesthesoilsurfacefromincomingsolarradiationandkeepsthesoiltemperaturescool.Inthespring,soilswithlargeamountsofcropresidueonthesurfacewarmmoreslowlythanbaresoils.Baresoilswarmandcoolfasterthansoilsthatarecoveredwithvegetationorsnow.Cropresiduesorothertypesofmulchalsoreduceevaporation.Thus,notonlydoresiduesshadethesoil,butbylimitingevaporation,theyalsokeepthesoilmoist.Amoistsoilwarmsmoreslowlybecauseittakesmoreenergytowarmwaterascomparedtoairinthesoilporespaces.Theseeffectsshouldbeconsideredwhenmodifyingmanagementpracticesaftertheadoptionofminimumtillagepractices.

ColorofSurfaceSoilSolarradiationthatreachesthesoilsurfaceiseitherabsorbedbythesoilorreflectedbacktotheatmospheredependingonsoilsurfaceconditions.Thetermalbedoisusedtodescribetheamountofincomingsolarradiationthatisreflectedbyasurface.Mostsoilsreflectfrom10to30%(albedo=0.1–0.3)oftheincomingsolarradiationthatreachesthesoilsurface.Alight-coloredsurfacewillreflectmuchofthisradiation(higheralbedo),allowinglesseramountsofenergytobeabsorbedbythesoil.Therefore,dark-coloredsoilsurfacesgainalotmoreenergythanlight-coloredsurfaces.Thesamesoilwillhaveahigheralbedowhenitisdrythanwhenitiswet.Bycomparison,mostvegetationreflectsmoresolarradiationbacktotheatmospherethansoils(albedo=0.2–0.3).

SlopeAspectWhenthesunisdirectlyoverhead,itsraysstrikethesoilsurfaceatrightanglesandmoreheatisabsorbedthanwhenthesunisatalowerangle.Thesunismorenearlyoverheadinthe

summer,resultinginahighlevelofenergy(heat)absorption.Inthefall,winter,andspringmonths,thesunappearslowerintheskyanditsraysstrikethesoilsurfaceatalowerangle,resultinginlessheatabsorption.Thesedifferencesareactuallyduetotheabsolutesurfaceareaoverwhichagivenamountofsolarradiationisdistributed.Obviously,soilsthatslopetowardthesuncaninterceptmoreenergyandthusbewarmerthansoilsslopingawayfromthesun.Asoilslopingtothesouth(intheNorthernHemisphere)warmsmorerapidlyinthespringthanoneslopingtothenorth.Somespecialtycropsmaybeplantedonthesouthsideofaneast–westrowbecauseoftheexposuretomoredirectsunlight.Thesouthsideheatsrelativelyrapidly,whichspeedsgerminationofseedsbyasmuchas3–5days.

IncidentalSourcesofEnergyHeatedairmassesmaybeblowninfromoneareatoanother.Whenthewarmairmassisblowingoveracoolersoil,therewillbeenergygainstothesoil.Rainorirrigationwaterthatiswarmerthanthesoilcanalsowarmthesoil.Biologicalactivityintheupperlayersofthesoiladdssmallamountsofheattothesoil.Changesintemperatureduetotheincidentalsourcesareoftennotassignificantcomparedtodirectsolarradiationorfire.

EnergyInputsandTemperatureChangeThetemperatureincreasefollowinganinputofenergyisdependentonthenetamountofenergyinput,theheatcapacityofthesoil,theamountofheattransferredtothesubsoil,andtheamountofheatlosttotheatmosphere.Energyinputswerediscussedearlierinthischapter.

HeatCapacityHeatcapacityistheamountofenergyittakestochangethetemperatureofamaterialby1°C.Thus,amaterialthathasahighheatcapacityexhibitsasmallerchangeintemperatureforthesameamountofenergyinputorenergylost.Eachcomponentofthesoil(air,water,solids)reactsdifferently(resultanttemperaturechange)toenergyinputs.Amongsoilcomponents,waterhasahigherheatcapacitythansoilsolids,andsoilsolidshaveahigherheatcapacitythansoilair.Ittakesabout3,000timesmoreenergytowarmanequalvolumeofwaterascomparedtoair.

Asoilwhenwetwillhaveamuchhigherheatcapacitythanthesamesoilwhendry.Thus,ittakesmoreenergytoheatawetsoilthanadrysoil.Likewise,wetsoilscomparedtodrysoilswillshowaslowerdecreaseintemperatureduetoheatlossgoingfromsummertofalltowinterseasons.Forthisreason,wetsoilsareconsideredcoldsoilsandtheytakelongertowarminthespringthandrysoils.Thus,plantingseedsinawetsoilmustbedelayedduringthespringuntilthesoilwarms.Germinationisdelayedincold/wetsoilsaswell.

Heatcapacitiesofsoilsvarydependingontherelativeproportionofsolids,water,andairpresent.Theairandwatercontentofasoilareconstantlychanging,resultinginadynamicheatcapacityofthesoil.Sincewaterhasanoverwhelmingeffectontheheatcapacity,evaporation,transpiration,irrigation,precipitationevents,anddrainagecanbringaboutsignificantchanges

intheheatcapacityofasoil.Therefore,soilmoistureisamajorfactorincontrollingtherateoftemperaturechange.Withaconstantchangeinthesoilmoisturecontent,itmakesstudyingtheamountandrateofheatmovementcomplexanddifficulttopredict.

Thebulkdensityofasoilplaysaminorroleinalteringtheheatcapacityofasoil.Compactedsoils(highdensity)willhavefewerlargeporesthannoncompactedsoils(lowerdensity).Drycompactedsoilswillhaveahigherheatcapacitythannoncompactedsoils(note:Fig.7.1isforsoilthermalconductivity).However,forawetsoil,densityandporosityplayaminorroleinalteringtheheatcapacityofthesoil.

Figure7.1Thethermalconductivityofasoildependsonitsporosityandwetness.

HeatTransferinSoilsThermalenergyistransferredasaresultofatemperaturedifferencewithinorbetweenobjects.Heatalwaysflowsfromawarmobjecttoacoolerone.Onlythesoilsurfaceissubjecttoinputsofsolarenergy.Oncetheenergyisabsorbedbythesoilatthesurface,thesurfacesoilisattemptingtoreachequilibriumwiththeatmosphereaboveaswellasthesubsoilbelow.Asaresult,energyintheformofheatisconstantlymovingatalltimesinthesoil.Subsoiltemperaturechangesareafunctionofheattransferintwodirections:fromthesurfacesoiltothesubsoil,andfromthesubsoiltotheatmosphereviathesurfacesoil.Heatcanbetransferredbyradiation,conduction,orconvection.Allthreetypesofheattransfertakeplaceinthesoil–plant–atmospheresystem.

RadiationAllobjectsaroundusradiateenergyintheformofinvisibleelectromagneticwaves(short,intermediate,andlongwavelengths).Solarradiationwasdiscussedpreviouslyastheprimarysourceofenergyinputintothesoil.Thesoilsurfaceifwarmerthantheairabovewillradiateenergytotheair.

ConductionHeatconductionoccurswhenenergyistransferredfromonemoleculetoanadjacent,coolermolecule.Theabilityofamaterialtoconductheatiscalleditsthermalconductivity.Metalssuchascopperandironhavehighthermalconductivitieswhereasmaterialssuchaswoodandplastichavelowthermalconductivities.Thelatterarecalledinsulators.Thethermalconductivityofasoildependsontheproportionofthesoilvolumeoccupiedbythesolid,liquid,andgaseousphases.Whilethermalconductanceisameasureoftherateofenergy

movement,theresultantchangeintemperatureismoreafunctionoftheheatcapacityofthesoil.

Mostsoilmineralshaveathermalconductivityabout5timesgreaterthanwater,10timesgreaterthanorganicmatter,andover100timesgreaterthanair.Asolidrockisabletoconductheatfasterthanasoilbecauseithasneitherairnorwaterwithinit.Similarly,acompactedsoilorasoilwithfewporescanconductheatfasterthananoncompactedsoilbecausethecompactedsoilhasmorecontactbetweensoilparticles(Fig.7.1).Whenasoiliswet,ithasamuchhigherthermalconductivitythanwhenitisdrybecausetheairintheporespacesisapoorconductorofheatandthusactsasaninsulator.Forexample,whencomparingawetsoiltoadrysoil,ittakesalotmoreheattoraisethetemperatureofwaterinawetsoil,althoughawetsoilisabletorapidlyconductheat.

ConvectionHeatistransferredbyconvectionwhenthemovementofaheatedfluidsuchasairorwaterisinvolved.Furnacesthatheatbuildingsbyblowingwarmairthroughasystemofductsaresometimescalledconvectionfurnaces.Becauseheatalwaysflowsfromwarmertocoolerobjects,heatistransferredfromawarmsoiltoairmoleculesinacoolwind.Warmairislighterthancoldair,andheatcanalsobetransferredbyconvectionwhenwarmairrises.Onspringmornings,sunshineonadark,baresoilheatsthesoilsurfaceandtheairaboveit,causingthewarmairtoriseintotheatmospherebyconvection(Fig.7.2).

Figure7.2Heatcanbetransferredfromwarmsoiltocoolairbyforcedorfreeconvection.

Warmspringrainsoncoldsoilscouldcarryenoughheattoinstantlyincreasethetemperatureofacoldsoil,butthenewlygainedsoilwaterhasatendencytoslowtheincreaseofsoiltemperaturefromfutureenergyinputs.Summerrainscanalsocoolawarmsoil.Aswaterisdrainedoutoftheshallowerpartsofthesoilprofile,theheatstoredinthewatermoveswiththewatertodeeperpartsofthesubsoil.Thiskindofsubsoildrainagewhereexcesswaterfromtherootzoneisremoved(lowerheatcapacity)helpsthesoilwarmupfasterwithrenewedsolarenergyinputs.

SensibleandLatentHeat

Sensibleheatisheatenergythatistransportedbyabodythathasatemperaturehigherthanitssurroundingsviaconduction,convection,orboth.Heatstoredinthesoilmaybetransferredtotheairjustabovethesoilsurfaceandthisheatlossfromthesoilisconsideredsensibleheat.Thissituationistypicallynoticedwhengoingfromsummerintothefallseasonaswellasaftersunset.

Latentheatdescribestheamountofenergyintheformofheatthatisrequiredforamaterialtoundergoachangeofphase(alsoknownas“changeofstate”).Inthesoil,energyusedtochangeicetowateristhelatentheatoffusion,andtheenergyusedtoconvertliquidwatertovaporiscalledthelatentheatofvaporization(Fig.7.3).Theenergyintheformofheatisutilizedwhengoingfromsolidtoliquidtogas,butheatisreleasedwhengoingintheoppositedirection.

Figure7.3Thesurfaceenergybudgetsummarizesheatflowinthesoil–plant–atmospheresystem.Incomingsolarradiationevaporateswater,warmstheair,andwarmsthesoilthatemitslong-waveradiation.

Latentheatissimplyashiftofenergyfromonephaseofwatertoanother.Forexample,convertingliquidwatertovaporinvolvesthetransferofenergyfromtheliquidtovaporphase,

resultinginmorestoredenergyinthevaporphase.Thestoredenergyislostfromthesoilonlywhenthewatervaporisphysicallyremovedfromthesoilbyevaporation.Thelossesofwatervapor(driersoil)resultsinalossofenergy,asaresultonewouldexpectcoolingofthesoil.Instead,adriersoilbecauseofitslowerheatcapacitycanrapidlyincreaseintemperaturewithanygaininsolarradiation.

Condensationofwatervaporintheformofdewonthesoilsurfacewillreleaseenergystoredinthevaporbackintothesoilasthevaporconvertstowater.Duringcoolnights,whentheleafsurfaceshavecooledenoughtohavewatervaporintheaircondenseonleafsurfaces,thecondensedwaterreleasesheattotheplant'smicroenvironment.Prolongedpresenceofdewonleafsurfacescausesplantpathogenstoproliferate.

Intemperateregions,thechangeofseasonsfromfalltowintercouldresultinfreezingofthesoilmoisture,thefreezingprocessreleasesheatstoredinthewater.Thisheatislosttotheatmosphere,resultinginfurthercoolingofthesoil.Thisconceptisutilizedbymanagersofcitrusorchardswheretheyspraytheirtreeswithwaterbeforeananticipatedfreeze;thesubsequentreleaseofheatkeepstheleavesandfruitfromfreezing.

Thereisaninteractionbetweenthelatentandsensibleheattransfer.Whenasoilismoist,thelatentheatdominatesenergytransferfromincomingsolarradiation(keepingtheairabovethesoilcool).Butwhenthesoildries,theamountofenergyusedaslatentheatdecreasesandmoreenergyisusedtowarmthesoilsolidswitharesultantincreaseinsensibleheat,therebywarmingtheair.Ifnorainfallsforalongperiod,thecropsgrowingonthesedrysoilsmaysuffernotonlyfromalackofmoisturebutalsofromexcessiveheatduetoincreasedsensibleheat.Understandingsensibleheatflowisimportantbecausemanycropshaveanairtemperatureatwhichtheygrowbest.

SoilTemperatureFluctuationsThetemperatureofasoilatagiventimeisafunctionofthecombinedeffectofthermalpropertiesofsoils,atmosphericconditions,heattransfer,andnetstoredenergy(gainsandlosses).Thedynamicnatureofenergyfluctuationsarereflectedindailyandseasonalvariationsofsoiltemperature.

Duringthedayduetoincomingsolarradiation,thesurfacesoilwarmsandbeginstocooloffasthesunsets.Thetemperatureofthesoilatthesurfaceisgenerallyslightlywarmerthantheairaboveit.Atmidsummerasoilwithoutvegetativecovermayexhibittemperaturefluctuationsatthesurfaceasmuchas40°F(22.2°C)inthecourseofaday.Astheenergymustbeconductedtothesubsoil,thefluctuationindailytemperatureisprogressivelylessindeepersectionsoftheprofile.Forexample,atadepthof6in.inthesamesoil,thevariationintemperatureinadaymaybeonly10°F(5.6°C),andatadepthof24in.(60cm)thechangeinonedaywouldbealmostnegligible(Fig.7.4).

Figure7.4Variationsofsurfaceandsubsoiltemperaturesthroughouttheday—warmingduringtheday,coolingatnight.

Depthisalsoafactorinthevariationofsoiltemperatureoverayear.Onanannualbasis,thehighesttemperaturesintheupper1ft(30cm)ofsoilarenormallyreachedinlatesummer,whereasthelowesttemperaturescomeinlatewinter.Atlowerdepthsof2–4ft(0.6–1.2m),highandlowtemperatureslagbehindthesurfacetemperaturesby2–3months.

Depth-relatedsoiltemperaturescanrangewidelyduringtheyeardependingonwhichpartoftheworldtheyarelocated.Manysoilsinthetemperateregionshowarangeofasmuchas60°F(33.3°C).Inthespring,moreheatgoesintothesoilduringthedaytimethangoesoutatnighttime,andthesoilslowlywarmsfromdaytoday.Inthefall,theoppositehappensandlessheatisconductedintothesoilasthedaysgrowshorterandcoolerwhereasagreaterproportionofheatradiatestotheatmosphereduringthelongnights.

ManagingSoilTemperatureSincetemperaturehassuchaprofoundeffectontherateofbiologicalprocessesintheupperfewinchesofasoil,considerableefforthasbeenputintodevelopingwaystomodifythesurfaceenergybalanceinhorticulturalandagriculturalproduction.Inmanagingsoilsfortemperature,attemptsshouldbemadetocreatenearlyoptimumconditionsforenhancingplantgrowthandproductivity.Sinceenergyislostorgainedatthesoilsurface,amajorityofthetemperaturecontrolmethodsfocusonmodifyingconditionsatthesoilsurfaceinhopesofrestrictingorenhancingmovementofheatinthedesireddirection.Anotherwaytomanagesoiltemperatureisthroughthealterationofsoilmoistureconditions.

SurfaceConditionsTillingthesoilandcreatingaroughsurfaceislikelytolowerthealbedoandimproveheatgain,resultinginincreaseddaytimehightemperatures.Becauseofincreasedmacroporesinfreshlytilledsoils,transmissionofheattothesubsoilisslower,andasaresultloose,tilledsoilswilllosemuchoftheheatgainedduringthedaytotheatmosphereatnight.

Manipulationofthesurfacealbedohasbeenusedtoincreaseanddecreasesoiltemperature.Asdiscussedearlier,plantingofvegetationhasanabilitytoshadethesoilwithfoliageincreasingthesurfacealbedoandthereforeprovidingacoolingeffectonthesurfacesoil.Inhotandaridregions,increasingthesurfacealbedobywhiteningthesurfacewithawhitepowdercanhaveacoolingeffectonthesoil.Conversely,insubarcticregions,soiltemperatureshavebeenincreasedbyblackeningcropresiduetolowerthesurfacealbedo.Thesetypesofpracticesareonlytemporaryandrelativelyexpensivesotheyareonlyfeasibleinextremecasesorwithhigh-valuecrops.

Surfaceslopeaspectcanalsobemodifiedonasmallscaletoincreasesoiltemperature(Fig.7.5).Thisisespeciallyimportantatplantingtimeathigherlatitudeswhencold,wetsoilsdelayemergenceandearlyseedlinggrowth.Tillageoperationscanbeusedtocreatearidgeandfurrowgeometryineast–westrows.Soilintheridgewillwarmfasterbecauseitdriesmorequicklyanditabsorbsmoresunlightonitssouth-facingslope(intheNorthernHemisphere).Seedsplantedintotheridgewillgerminateandgrowfasterthaniftheyareplantedinflatsoil.

Figure7.5IntheNorthernHemisphere,solarradiationatmiddayproducesthehighesttemperatureondarksoil,butsoiltemperatureisalsoinfluencedbyseveralotherfactorsshownhere.

MulchesMulchesappliedtothesoilsurfacewiththeintentofenhancingorrestrictingheatflowincludeplasticorpapersheeting;organicby-products(cropresidues,leaflitter,woodchips,etc.),andgravel.Allthesemulchescreateawaterand/orheatbarrieratthesoilsurface.Porousmulcheslikecropresiduesandwoodchipsreduceevaporationbyprovidingabarriertowatervapor.Mulchesmadeofcropresiduealsohaveahigheralbedothantheunderlyingsoil.Soilsunderaporousmulchcoverwillbecoolerandmoisterthanbaresoilsduringthegrowingseason.Organicmulcheswithlargeparticlestendtohavealargevolumeofairbetweentheparticles.Sinceairisapoorconductorofheat,itservesasaninsulatorandkeepstheheatfromflowingintooroutofthesoil.

Clearplasticfilmmulchesnotonlyreduceevaporation,butalsoincreasesoiltemperaturebytrappinglong-waveradiationundertheplasticand/orbydecreasingthesurfacealbedo.Adarkplasticmulchwillabsorbmoreheatfromthesun.Plasticmulchesarecommoninhorticulturalapplicationswhereyoungplantsrequireawarm,moistenvironment.

SoilMoistureControlWaterregulationmaybethemostsignificanttemperaturecontrolmethodforsoils.Presenceofexcesswatercanresultinlowerthanoptimumsoiltemperature.Excesswateriscommoninlow-lyingareasorpoorlydrainedsoils.Sinceadriersoilwillheatfaster,attemptstoavoidordecreaseexcesswatercontentofthesoilcouldbehelpfulinregulatingsoiltemperature.Tocontrolexcesswaterinasoil,wecanavoidtheentryofexcesswaterintothesoil(surfacedrainage)orremovetheexcesswaterthathasalreadyinfiltratedintothesoilprofilebydrainingit(subsurfacedrainage).

Surfacedrainageinvolvesthemodificationoflandsurfaceslopessoastocreateabalancebetweenoverlandflow(excesswater)andtheamountofwaterinfiltratingthesoilsurface.Subsoildrainagerequirestheplacementofdrainagetiles/perforatedtubinginthesubsoilorditchesthatwillallowtheexcesswaterinthesoilporestofreelyflowintothetile/tubingorditch.Thetubingorditchisslopedsoastocarryawaytheexcesswaterintoalargerreceivingbodysuchasastream.Theidealconditionforsoilwarmingistohaveenoughwatertoproviderapidheatmovementtothesubsoil,butnotsomuchwaterastoslowtheincreaseinsoiltemperature(duetothehighheatcapacityofwater).

Innature,soiltemperaturesareinfluencedbytheconstantlychangingmeteorologicalconditionsatthesoil–atmosphereinterface.Externalinfluencessuchasdayornight,summerorwinter,cloudyorsunnydays,rainevents,warmorcoldwaveeventsareconstantlyaffectingthetemperatureofasoil.Whenfactorssuchasgeographiclocation,vegetativecover,andmostimportantlysoilmoistureconditionsareadded,predictingsoiltemperaturebecomesevenmorecomplicatedanddifficult.

Chapter8SoilFertilityandPlantNutritionWater,carbondioxide,andcertainchemicalelementscalledplantnutrientsareessentialforplantgrowth.Waterissuppliedbyeitherrainfallorirrigation,carbondioxidefromtheatmosphere,andtheessentialplantnutrientsfromthesoil,fertilizers,orothersoilamendments.

SoilFertilitySoilfertilityincludestheabilityofasoiltoholdplantnutrients,thelevelofplantnutrientspresent,andtheavailabilityofthenutrientsforuptakebyplants.Asoilthathasahighlevelofessentialnutrientsavailableforusebyplantsisusuallyaproductivesoilifitalsohassufficientsoilwaterandifthecropsarewellmanaged.Plantnutrientsexistinthesoilinseveraldifferentforms.Theyincludethefollowing:

Minerals.Examplesincludethefeldspargroup,whichisthemostabundantgroupofmineralsintherocksoftheearth.Somearehighinpotassiumandothersincalcium.Nutrientsarereleasedfromthemineralsbyweathering.

Cationsoranions.Theseareplantnutrientsthatexistonthesurfaceofclayorhumus.Thesesurfacesarecalledtheexchangecomplexandarepositiveornegative.Theyattract,hold,andexchangethecationsoranions(seeChapter5).

Chemicalcompounds.Therearemanychemicalcompoundsthatforminthesoil.Anexamplewouldbetheformationofphosphoruscomplexesonthesurfaceofcalciumcarbonate.

Solubleions.Numerousionsexistinthesoilsolution.Plantsabsorbalargeportionoftheiressentialnutrientsfromthissource.Thispoolofnutrientsissmall,butcanbereadilyreplenishedthroughcationexchangereactionsandotherbufferingmechanisms.

Organicmatter.Plantresiduesandorganicmattercontaintheelementsthatplantsrequireforgrowth.Asdecomposition(mineralization)occurs,thesenutrientsarereleasedandcanbeusedbyplants.

Theavailabilityofnutrientstobeabsorbedbyplantsvariesaccordingtotheforminwhichtheyexist(seeTable8.1).Asanillustration(Fig.8.1),phosphorousisreadilyavailabletoplantsasinorganicorthophosphateions(HPO42−andH2PO4

2−).Thebulkofthesoilphosphorousexistsaslowsolubilityorganicandinorganicphosphorouscompoundsthatarenotreadilyavailableforplantuptake.Nutrientsinsoilsolutionarequitereadilyavailableforusebyplants.Thoseontheexchangecomplexaregenerallyavailableforabsorptionbyplantsbutnotquiteasreadilyavailableasthoseinthesoilsolution.

Table8.1Elementsrequiredforplantgrowthandprincipalformsinwhichtheyaretakenupbyplants(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Nutrient Chemicalsymbol

Form(s)absorbed

MacronutrientsCarbon C CO2

Hydrogen H H2O

Oxygen O H2O,O2

Nitrogen N Ammonium(NH4+),nitrate(NO3

−)

Phosphorus P Orthophosphateions(HPO42−),(H2PO4

−)

Potassium K Potassiumion(K+)Calcium Ca Calciumion(Ca2+)Magnesium Mg Magnesiumion(Mg2+)Sulfur S Hydrogensulfate(HSO4

−),sulfate(SO42−)

MicronutrientsBoron B H3BO3(boricacid),H2BO3

−HBO3−2,BO3

−3,B4O7−2

Copper Cu Copper(cupric)ion(Cu2+),copperhydroxideion(Cu(OH)+)

Chlorine Cl Chlorideion(Cl−)Iron Fe Ferrousiron(Fe2+),Fe(OH)2+,Fe(OH)2+,ferriciron

(Fe3+)Manganese Mn Manganeseion(Mn2+)Molybdenum Mo MoO4

2−,HMoO4−

Zinc Zn Zincion(Zn2+)Co Cobalt Cobaltion(Co2+)Nickel Ni Nickelion(Ni2+)

Figure8.1Phosphorusexistsoriginallyasacomplexmineralwithverylowsolubility.Weatheringbreaksitdownintolesscomplexforms,someofwhichcanbeusedbyplants.

Nutrientspresentascomplexchemicalcompounds,orasprecipitatedsaltsaswellasthoseinorganicmatterarenormallyonlymoderatelyavailabledependingtoagreatextentonsoilwatercontent,soiltemperature,andsoilpH.Thosepresentasmineralsareslowlyavailable.Theyarereleasedonlyasthemineralbreaksdownduringtheprocessofweathering(seeChapter2fordetailsonweathering).

Mostnutrientscanbeclassifiedasreadilyavailable,moderatelyavailable,orslowlyavailable.Itisdesirabletohaveanadequatesupplyofnutrientsinareadilyavailableform.

ConditionsAffectingLevelandAvailabilityofPlantNutrientsCertainsoilcharacteristicsinfluencetheavailabilityofnutrients.Oneissoiltexture,therelativeamountofsand,silt,andclay.Becauseclayparticlesprovideapartoftheexchangecomplex,thepercentageofclayinthesoilinfluencesthecapabilityofasoiltoholdnutrients.Thepercentageofclaydeterminesthesizeofthe“nutrientwarehouse.”

Thetypeofclayisalsoimportant.AsexplainedinChapter5,threeofthetypesofclayinsoilsintheUnitedStatesarekaolinite,illite,andsmectite(montmorillonite).Eachtypehasadifferentcapacitytoholdnutrients(cationexchangecapacity,CEC).Ingeneral,soilswithahighCECwillbemorefertilebecausemorenutrientcationscanbeheldonthesoilcomplex.

TheCECisrelativelylowforkaolinite(3–15cmolc/kg),moderateforillite(10–40cmolc/kg),andhighforsmectite(80–100cmolc/kg).Itfollowsthatasoilwith20%clayassmectitewouldhaveamuchgreatercapacitytoholdnutrientsthanasoilwith20%clayaskaolinite.Theclaycontentandthetypeofclayarebothimportantinsoilfertility.

Soiltexturecanalsoinfluencewaterretentionanddrainage.Sandiersoilstendtodrainmorequicklyandretainlesswaterthandosoilswithhigherclaycontents.Sandysoilsalsohavelargeporespaces,allowingformoreleachingofnutrients.Theeffectsofsoilwateronnutrientavailabilityarediscussedinthefollowing.

Structureisdefinedasthearrangementofsoilparticlesintoaggregates.Agoodsoilstructureisessentialforwaterandnutrientmovementandretention,aswellasrootgrowth.Largespacesbetweenaggregatesallowsoilwater(andthenutrientsdissolvedtherein)toflowmore

freely,resultinginleachinglosses.Smallornospacesbetweenaggregates,especiallyduetocompaction,preventwaterfrommovingthroughthesoilprofile,resultinginrunoff.

Organicmatterisanotherimportantsoilcharacteristicthat,ifhighenoughincontent,canfavorablyimpacttheavailabilityofnutrients.Ithasathreefoldeffectonfertility.First,thefractionoforganicmatterthatishumus(thecolloidalfraction)issimilartoclayparticlesinthatithasanexchangecapacityrangingfrom50to200cmolc/kg(dependingonthepHofthesoil)andattractsandholdsnutrientsforplantuptake.Second,asorganicmatterdecomposes(mineralizes),theessentialplantnutrientsitcontainsarereleasedandorganicacidsareformedthatincreasetheavailabilityofmostnutrients.Third,anadequateleveloforganicmatterinasoilisgenerallydesirablenotonlyfromaplantnutrientstandpointbutalsobecauseofitsfavorableeffectonsoilcharacteristicssuchasphysicalcondition,water-holdingcapacity,andinfiltrationrateasexplainedinChapter6.

Soilwatercontentalsoinfluencesnutrientavailability.Mostnutrientsutilizedbyplantsareabsorbedfromthesoilsolution.Ahigherlevelofsoilwaternormallymeansahigherlevelofmostnutrientsinsolution,leadingtoimprovednutrientuptakebydiffusionandrootinterception.Adequatemoisturealsoincreasestherateoforganicmatterdecomposition(seeChapter5),whichreleasesN,P,andS.SolubleandmobilenutrientssuchasNinthenitrateformmaybelostinaprocessknownasleachingaswaterpercolatesbelowtherootzone.Poorlydrainedorverywetsoilsincreasethesolubilityofmineralssuchasironandmanganese.Furthermore,nitratemaybelostduetodenitrification(seeChapter4)infloodedsoils.Lowmoisturecanresultinreducednutrientuptakeduetoformationofinsolublecompounds.Highersoiltemperatureusuallyleadstogreateravailabilityofmostplantnutrients,asexplainedinChapter7.

SoilpH,whichisameasureofthedegreeofsoilacidityoralkalinity,alsoinfluencestheavailabilityofnutrientsthroughitseffectsonrootgrowthandnutrientform(seeChapter5).MostnutrientsareattheirhighestlevelofavailabilitywhenthepHisslightlyacidtoneutral(pH6.0–7.0).Acidicsoilsreducerootgrowth,whichiscriticaltoPuptake.ThepHisalsoimportantinNtransformations,suchasmineralization,nitrification,andNfixation,asthebacteriainvolvedaresensitivetopH.ThedifferentformsofNhavedifferentavailabilitiesastheyhavedifferentleachingcapabilities.Asasoilbecomesmoreacid,certainnutrientsbecomelessavailable;ifasoilbecomesalkaline,theavailabilityofcertainnutrientsdecreases(seeFig.8.2).

Figure8.2TheinfluenceofsoilpHonnutrientavailability.Thewiderthebar,thegreatertheavailability.

NutrientMobilityinSoilsNutrientmobilityinthesoilaffectstheeaseofitsuptakebyplants,andthelikelihoodofitsleachingthroughthesoil.Mobilityofnutrientswithinthesoilisinfluencedbysoilphysicalpropertiessuchassoiltextureandstructure,soilchemicalpropertiessuchasCECandAECandpH,aswellassoilconditionssuchasmoisture.Calcium,potassium,andmagnesiumarepositivelychargedions(cations).Mostsoilcolloidshaveanetnegativecharge(seeChapter5).Sinceoppositechargesattract,thesecationsareattractedandheldontothecationexchangesitesandarereleasedonlywhenexcesscationsareaddedtoreplacetheirplaceontheexchangesites.Forthisreason,thesecationsareconsideredlessmobileandareslowlyavailabletoplants.Theirmovementandenrichmentinwatersisrarelyanenvironmentalissue,particularlywhereerosioniscontrolled.

Sulfuroccursastheanionsulfateform(SO4−),whichdoesnotbindtocationexchangesites

andisthusmobileinmostsoils.Nitrogenusuallyexistsinthesoilasammonium(NH4+)and

nitrate(NO3−)forms,henceitsmobilitydependsontheformitisin.TheNH4

+-Ncanbeheldoncationexchangesitesandisthereforerelativelyimmobile,andisnotsusceptibletoleaching.Onthecontrary,thenegativelychargedNO3

−ionisnotheldontheexchangesites,

henceitisverymobileinsoilwater.Inaddition,nitratesarehighlysolubleinwaterandaresubjecttolossestogroundwaterbyleaching,andtosurfacewatersbyrunoff(seeChapter4).

PhosphorusnormallyexistsinsoilswithpHvaluesbetween5and7astheortho-phosphateion(H2PO4

−,HPO42−).Unlikethenitrateanion,theortho-phosphateionformsverytightbondsto

soilparticles.Asaconsequence,phosphorusistypicallyimmobileinsoil,anditdoesnotreadilyleachoutoftherootzone.ThepotentialforP-lossismainlyassociatedwitherosionandrunoff.ThelackofPmobilityalsoreducestheavailabilityofP-fertilizertoplants.Bothnitratesandphosphoruswhentransportedtosurfacewatersstimulatealgalgrowthtothepointofcrowdingoutothermoredesirablespeciesthroughaprocesscalledeutrophication(seeChapter4).

Fornutrientstobeutilizedbyplants,theymustmovetothesurfaceoftheplantroots,whereabsorptiontakesplace.Therearethreeprocessesbywhichnutrientsmovetotherootsurface.Thesearerootinterception,massflow,anddiffusion.

Rootinterception:Rootinterception,alsoknownascontactexchange,occurswhentherootcomesintodirectphysicalcontactwithnutrientsassociatedwithsoilcolloidsasitgrowsthroughthesoil.Rootinterceptiongenerallyincreasesasthesurfaceareaandmassoftherootincreases,therebyenablingtheplanttoexploreagreatervolumeofsoil.Mycorrhizaealsoincreasethesurfaceareaexploredbyrootstherebyenhancingrootinterception.Rootinterceptionisanimportantmodeoftransportforcalciumandmagnesium.However,sincethevolumeofsoiloccupiedbyrootsisusuallylessthan1%,rootinterceptionisaminorpathwayfornutrienttransfer.

Massflow:Massflowoccurswhendissolvednutrientsinthesoilsolutionaretransportedtothesurfaceofrootsasplantstakeupwaterfortranspiration.Massflowdecreasesassoilwatercontentdecreases.Nitrate,sulfate,calcium,andmagnesiumarelargelysuppliedbymassflow.

Diffusion:Diffusionisthemovementofnutrientstotherootsurfaceinresponsetoaconcentrationgradient.Continueduptakeofanutrientsbyplantscausesitsconcentrationinthesoilsolutionadjacenttotherootsurfacetodecrease.Thiscreatesaconcentrationgradientfromthebulksoiltotherootsurfacethatcausesnutrientstomovetotherootsurface,wheretheycanbetakenup.Diffusionislargelyresponsibleforsupplyofphosphorusandpotassium.

MethodstoIncreasetheAvailabilityofAddedNutrientsThereareanumberofwaystoincreasetheavailabilityofnutrients.Forexample,potassiumfertilizationpriortoapplicationofammoniumfertilizerscanbeusedtoreduceNH4

+fixationinsoilswithvermiculiteandillitetypesofclay.Earlyseasonuptakeofphosphateionsbycroprootscanbefacilitatedbyplacingphosphorus-containingfertilizerinorclosetotheseed-rowatplanting.Inthisway,phosphateionsaretakenupbytherootsbeforetheyreactwithcationsdominatingunderacidic(e.g.,Al3+orFe3+)oralkaline(e.g.,Ca2+orMg2+)soilconditions.Underalkalinesoilconditions,thephosphatefertilizercanbeappliedinbandswithafertilizer

thatgeneratesammonium(NH4+)ions.Thisallowsslightacidificationofthesoiladjacentto

thefertilizerband.Alternatively,compoundnutrientfertilizergranulesthatcontainnitrogen(N),phosphorus(P),and/orelementalsulfurcanbeappliedtoalkalinesoils.Inthiscase,thesoiladjacenttothegranulewillbeacidifiedandPuptakewillbeenhanced.Additionoflimetoacidicsoilscanalsoenhanceavailabilityofsomenutrients.

InhighpHsoils,soilappliediron(Fe)fertilizersoftendonotsuccessfullycorrectFedeficiencies.ThisisbecausetheFe3+ionsfromtheFefertilizerreactsofastwithsoilthatthenutrientistiedupandrenderedunavailabletoplants.Inthesesoils,Feenhancedcanbecorrectedthroughfoliarapplicationofsolubleironfertilizercompounds.ByavoidingthesoilandapplyingtheFedirectlytotheleaves,thesmallamountofFerequiredbyplantsissuccessfullyintroducedintothecrop.

PlantNutritionEssentialElementsAtleast17elements,calledplantnutrients,areessentialforplantgrowth(Table8.1).Thefirstgroupincludesthreeelements—carbon,hydrogen,andoxygen—thatarethebasicbuildingblocksofallplantcompounds.Thesethreeareneededinmuchlargerquantitiesthanallotherscombined.TheinitialproductofphotosynthesisisthesimplesugarC6H12O6.Thecarbonandoxygencomefromcarbondioxide,andthehydrogencomesfromwater.Theoxygeninthewaterisgivenoffbyplantsandgoesbackintotheatmosphere.Thisprocessassuresusofacontinuingsourceofoxygen.

Thesecondgroupofessentialelements,calledmacronutrients,consistsofnitrogen,phosphorus,potassium,sulfur,calcium,andmagnesium.Theyareclassifiedasmacronutrientsbecausetheyareusedinrelativelylargequantitiesbyplants.

Anothergroupofeightelementsiscalledmicronutrientsbecausetheyarenormallyusedinsmallerquantities.Thisgroupincludesiron,zinc,manganese,copper,boron,molybdenum,nickel,andchlorine.

Somescientistscontendthatsomeotherelementsmayalsobeessentialforplantgrowth.Includedinthisgrouparesiliconandsodium.Thesetwoelementsplusvanadium,cobalt,andiodineareoftencalledbeneficialelementsbecausetheycanbeusedbyplantsassubstitutesfornutrientsthatareessential.

Approximately90%ofthedryweightofaplantismadeupofcarbon,hydrogen,andoxygen;thebalanceconsistsoftheotheressentialelements.Mostofthisremainderconsistsoftheelementsclassifiedasmacronutrients,whereaslessthanapproximatelyone-tenthofthis10%isinthemicronutrientgroup.Severalelementsnotknowntobeessentialmayalsobeincluded

inthe10%.

NaturalSourcesofPlantNutrientsPlantsobtaincarbon,hydrogen,andoxygenfromsoilwaterandatmosphericcarbondioxideandassimilatethemintoglucoseduringphotosynthesis.Theremaining14elementsarenormallyabsorbedinionicformfromthesoil(Fig.8.3,Table8.1),althoughsmallamountscanbetakeninthroughtheleavesifplacedthereinsolutionbyprecipitation,foliarapplication,orsprinklerirrigation.Nitrogencomesoriginallyfromtheatmosphere,whichisnearly79%N,andisinaformthatplantscannotuse.

Figure8.3Carbonandoxygencomefromcarbondioxideintheair,hydrogenfromwaterinthesoil,andotherelementsareabsorbedbyplantsfromthesoil.

NaturehasseveralmethodsbywhichatmosphericNisconvertedintoformsthatplantscanuse.Theyincludethefollowing:

Bacteriaandleguminousplantsjointogetherinaprocesscalledsymbioticnitrogenfixation,which,whencombinedwithotherstepsinthenitrogencycle,providesnitrogeninaformusablebyplants(seeChapter4).

Fixationofnitrogenbysoilbacteriawithoutthehelpoflegumescallednonsymbioticfixation.

Theactionoflightningdischargingintheatmospherecausesnitrogenoxidestobeformed,whicharethenbroughttoearthbyrain.Theselattertwosourcesofnitrogenproviderelativelysmallquantitiesforplantgrowth.

Withinsoils,mostNisintheformoforganicmatter.WhilethispoolofNisveryimportant,Nintheorganicformgenerallyisnotavailabletoplants.Asdiscussedinearlierchapters,theorganicNmustundergomineralizationtobemadeavailableforplants.

Theremaining13essentialelementsarenaturallyderivedfromtheweatheringofrocksandmineralsoftheearth.Phosphorus,forexample,comesprincipallyfromamineralcalledapatite,whereasmagnesiumcomesfrommineralssuchasserpentineanddolomite.Whenrocksandmineralsundergoweathering,elementsarereleasedandbecomepartofthesoilsystem.

Sulfurisoftenemittedintotheatmospherebycoal-burningfacilitiesassulfurdioxide,whichismovedbyaircurrentsandthencarriedtothesoilbyprecipitation(Fig.8.4).Thiscanalsobetrueforasmallportionofthenitrogenusedbyplants.Plantnutrientscanalsobeaddedthroughirrigationwater.

Figure8.4Thesulfurinfossilfuelssuchascoalisthesourceofsulfurdioxide(SO2)emissionintotheatmospherewhenitisburned.

RoleofEssentialPlantNutrientsEachplantnutrientplaysoneormorespecialrolesinplantgrowth.Table8.2listseachnutrient,oneofitsfunctionsinplantgrowth,andsomedeficiencysymptoms.Anutrientmaybetheessentialpartofaplantcompound,thusprovidingitwithastructuralbase.Calcium,forexample,ispartofcalciumpectate,whichisacompoundthatisapartoftheplantcellwall.

Table8.2Essentialplantnutrients,functioninplantgrowth,anddeficiencysymptoms(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Plantnutrient

Functioninplant Deficiencysymptom

Nitrogen Essentialpartofaminoacids,protein,andchlorophyll

Yellowingofmidriboflowerleaves

Phosphorus Partofenergytransfercompounds

Reddish-purplecolorofleavesofyoungplant

Potassium Regulationofosmosisandwateruseandtransportationsystem

Browningofouteredgesoflowerleaves

Calcium Formationofcalciumpectateusedincellwalls

Nodevelopmentofterminalbudsandapicaltipsofroots

Magnesium Centralatomofthechlorophyllmolecule

Interveinalchlorosisofmiddleorlowerleaves

Sulfur Essentialpartofthreeaminoacidsessentialforproteinformation

Uniformlychloroticupperleavesandslowgrowth

Iron Componentofchlorophyllandcofactorforenzymaticreactions

Interveinalchlorosisinyoungleaves

Zinc Involvementinauxinmetabolismandpartofdehydrogenaseenzyme

Spottedwhiteoryellowareasbetweenveinsofupperthirdofleaves;alsolackofterminalgrowth

Manganese Electrontransportandpartofenzymesystem

Interveinalchlorosisinyoungleaves

Copper Partofoxidaseenzymesystem YellowingandstuntingofyoungleavesNickel Involvedinenzymeconverting

ureatoammoniumInterveinalchlorosisinyoungleaves—progressingtonecrosis

Molybdenum Partofnitratereductaseenzyme YellowingofmidriboflowerleavesBoron Growthanddevelopmentofa

newmeristematiccellsPalegreenyoungleaves;leavesdieandterminalgrowthceases

Chlorine Osmoticandcationneutralization

Partialwiltingandlossofleafturgorwhenmoistureisadequate

Othernutrientsmaybeessentialformakingcompoundsinvolvedinplantgrowthprocesses,suchasphosphorusasapartofadenosinediphosphate(ADP)andadenosinetriphosphate(ATP),whicharetwocompoundsinvolvedinthetransferofenergywithinaplant.The

compoundsusedasstorageofplantfoodssuchasproteinrequirenitrogenandsulfur.

Anothergroupisinvolvedintheregulationofcertainenzymaticprocesses.Enzymesinplantssuchascatalaseandlactaseactascatalystsoractivators.Theyoftencontainmicronutrientssuchasironandcopper.

DeterminingNutrientNeedsTobeabletoproducetopyields,allessentialplantnutrientsmustbepresentinadequatequantities.Themostcommonelementtobedeficientformostcropsandlawnsisnitrogen.Phosphorusisnormallythesecondmostcommonelementtobedeficient.Potassium,calcium,andmagnesiumareoftenlackinginsoilsintheeasternhalfoftheUnitedStateswhererainfallnormallyexceeds25in.(625mm)peryear;whereasinthewesternUnitedStates,theseelementsarewellsuppliedinmostsoils.

Severalmethodsareavailabletodetermineifanutrientisdeficientandthequantityneededtocorrectthedeficiency.Thesemethodsincludechemicalanalysesofthesoilandtheplant,nutrientdeficiencysymptoms,andgrowthtests.Inorderforchemicalanalysisofthesoilandplantstobeofvalue,soilandplantsamplesneedtoberepresentativeoftheareainquestion.

SoilSamplingThemethodmostoftenusedfordeterminingnutrientneedischemicalanalysisofsoil.Todeterminenutrientneedmostaccurately,twothingsarerequired:(1)asoilsamplethattrulyrepresentsthefieldinquestion,and(2)thechemicalmethodthathasbeenadequatelyresearchedandproperlycorrelated/calibratedforthecropsandsoilsinquestion.

Forsoilsampling,therearethreecommonapproaches.Theyaresampling(1)bysoiltype,(2)onagridbasis,or(3)onamanagementzonebasis,whichdeterminessamplinglocationsbasedonyieldmaps,remotesensing,pastmanagementhistory,andsoon.Thesamebasicprinciplesapplytosamplinglawnsandgardensbutonasmallerscale.

Tosamplebysoiltype,diagramafieldbysoiltype(suchmapsareavailableinsoilsurveys)andobtainacompositesamplefromeachsoiltype(seeFig.8.5).Foreachcompositesample,systematicallyorrandomlytake10–15individualcoresofonesoiltypeusingasamplingtubeorashovel.Asoilcorecoversoneinchsquareareaandmayextendtovaryingsoildepths,dependingonthetypeofthecroptobegrown(shallowrootedvsdeeprooted),thefield(pasture,no-till,tilled),andtypeoftest(mobilenutrientsvsimmobilenutrients)tobeperformed.Usually,soilcoresaretakentoadepthof6in.(15cm)inpasture,turf,no-tillland,tilledfields,orgardens.Testingformobilenutrients,suchasnitratesandsulfatesrequiresdeepersampling.Takesampleswithstainlesssteelorchrome-platedsamplingtoolsandcleanplasticbucketstoavoidcontaminatingthesampleswithmicronutrients,particularlycopperandzinc.Donotusebrass,bronze,orgalvanized(zincplated)tools.

Figure8.5Propercollectionofsoilsamplesisextremelyimportant.Testsmadeoncarelesslytakensamplescanbemisleadingandcostly.

(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Thoroughlyhomogenizethecoresandremoveapproximately1pint(0.5l)or1lb.(0.4kg)tosubmitfortesting.Repeatthesameprocessforeachsoiltypeinthefield.Forfieldslargerthan12acres(5ha),proportionatelymorecoresshouldbetaken.Themorethecorestaken,the

morereliablethemeasureoffertilityofthefield.Onesampleshouldnotrepresentmorethan25acres(10ha).Ifonesoiltypeareainafieldistoosmalltobefertilizedseparately(e.g.,lessthan5acres,oraround2ha,insize),donotsampleanyofthearea.

Ifthesamplesaretakensystematically,theareasampledshouldbetraversedinazigzagpattern(SeeFig.8.5,step2)toprovideauniformdistributionofthesamplingsitesintheentirearea.Highvariationwithinthefieldbeingsampledwilldecreasetheaccuracyandreliabilityofasoiltestrecommendation.Forthisreason,auniformsoilshouldnotdifferinsoiltype,color,slope,drainage,texture,pastmanagement,andnaturalvegetation,andanypartsofthefieldshowingthesedifferencesshouldbesampledseparately.Areaswithrecentlime,manure,composts,andfertilizeradditionsshouldnotbesampled.Severelyerodedsections,manureorlimestockpileareas,wetspots,oldbuildingsites,fencerows,andareasadjacenttogravelroadsshouldalsonotbesampled.Soilsamplestakenfromtheselocationswouldnotbetypicalofthesoilintherestofthefield,henceincludingthemwouldproducemisleadingresults.

Tosampleonagridbasis,divideafieldinto3–5acre(1.2–2.0ha)squaresasagrid.Takeonecompositesample(of8single-samplecoresamples)fromeachsquare(seeFig.8.6.).Thegridsmaybebasedonyieldmaps,andsoon,asdiscussedpreviously.Thesesamplingareasarenotnecessarilyrectangular.

Figure8.6Gridsamplingisanalternativemethodofsamplingwheresoilsarequitevariable.(CourtesyofPlantFood,Fall1994,PotashandPhosphateInstitute,Atlanta,GA.)

Forgridsampling,variouspatternsofsamplingcanbeusedbyshiftingfromthecenterofthegridtorandomizethesites.Thisapproachtosamplingisbeingusedforthecomputerizedapplicationofnutrientsorvariable-rateapplication,oftencalledprecisionorsite-specificnutrientmanagementprograms.

Ideally,soilsamplesshouldbetakenascloseaspossibletoplantingortothetimeofcropneedforthenutrient(2–4weeksbeforeplantingorfertilizingthecrop).Thisisbecausenutrientconcentrationsinthesoilfluctuatewiththeseason.However,togivesufficienttimetoplanandimplementlandmanagementdecisionsbeforethebusyplantingseason,itismorepracticaltocollectsoilsamples3–6monthspriortoplantingtime.Donotcollectsampleswhenthesoilistoowetortoodryasitwillbedifficulttomixthecores.Asarule,ifthesoilistoowetortoodrytoplow,itistoowetordrytosample.

Toensureaccurateresultsandminimizechangesinnutrientlevelscausedbysoilorganisms,soilsamplesshouldbehandledwithgreatcare.Moistsoilsamplesshouldbestoredinacoolboxduringsamplingandinarefrigeratoraftersampling.Ifitisnotfeasibletorefrigerateorfreezethesamplessoonaftercollection,takethemtothesoiltestinglaboratoryorairdrythembyspreadingthesoilonaplasticsheet.Breakupallclodsorlumpsanddryatroomtemperature.Acirculatingfanmaybeusedtofacilitatedrying.

Takesoilsamplesatthesametimeoftheyear,onceevery2or3years.Samplingatthesametimeofyearminimizestheeffectofseasonalvariationsonsoiltestresults.

SoilTestMethodsCompositesamplesshouldalwaysbeplacedinacleancontainertoavoidcontamination.Itisbesttousecontainersprovidedbythesoil-testinglaboratory,ifavailable.Besuretoprovidetheproducer'sname,address,andfieldnumberforeachsample.Alsoprovideinformationoncroppinghistoryforatleast2years,previousfertilizeruseandmanureapplications,andyieldlevels,croptobefertilized,andanticipatedyieldpotentialforthenextcrop.Besuretochooseawell-qualifiedlaboratorywhoseagronomistisfamiliarwiththesoilsandcropsintheproducer'slocale.State-operatedsoil-testinglaboratoriesvaryintheirspecificinstructionsforsoilsamplingsoitisadvisabletocheckwiththelocalcountyagriculturalextensionagent.

Asoiltestisachemicalassessmentofthenutrient-supplyingabilityofasoilatthetimeofsampling.Manymethodsofsoiltestingareavailable.Somemeasurethetotalcontentofanutrientinthesoil,whileothertestsattempttomeasurethe“available”nutrientlevels.Mosttestingthatisdonetopredictfertilizerneedsforacropisinthesecondcategory,whichistoprovideanindex,oranestimateofthenutrient-supplyingabilityofthesoil.Duringtesting,thisfractionisseparatedfromthesoilusinganextractingsolutionthatismixedwiththesoilforaspecifiedperiodoftime.Thenutrientintheextractantisthenanalyzedafterfiltration.

Foranymethodtowork,thesoiltestshavetobeevaluatedonthebasisofactualcropresponse.Withoutlocallyapplicablecropresponsedata,asoiltestisuseless.Eachtestmustbecorrelated/calibratedwithlong-termfieldexperimentsandfertilizertrialssoastoassessthelikelihoodofayieldresponsetoadditionoffertilizer.Forthecalibrationtobecomplete,thesesoiltestcorrelationtrialsmustbeconductedforseveralyearsonaspecificcropgrowingonaspecificsoiltype.

Soilsthatcontainahighamountofavailablenutrientsasindicatedbyhighsoiltestvaluesrequirelessfertilizerinputthandosoilsthatcontainalowamountofavailablenutrients.A

lowsoil-testvalueforaparticularnutrientindicatesthatthecropwillnotobtainenoughofthatnutrientfromthesoiltoproducethehighestyieldunderaveragesoilandclimaticconditions.Supplementationthroughfertilizeradditionwillbenecessarytocorrectnutrientdeficiency.Ingeneral,thereisahighchanceofgettingaresponsetoanutrientifthesoiltestislow.

Thetestisthencalibratedtodeterminetheamountofeachnutrientneededtomaximizeprofitfromfertilizerapplication.Thesoiltestlevelabovewhichcropyieldsremainthesameeventhoughsoilfertilitycontinuestoincreaseisknownasthecriticalsoiltest.Inotherwords,noresponseisexpectedabovethecriticalsoiltestlevelandaresponseisexpectedbelowit.Nutrientguidelinesforfieldstestingbelowthecriticalsoiltestvaluearedeterminedbyconductingyieldtrialsonanumberoffields,acrossafullrangeofsoiltestlevels.Thetreatmentsoneachfieldareselectedtorepresentthefullrangeofratesofthenutrientinquestion(e.g.,0,30,60,90,and120lbor0,12,24,36,and48kgofP205/acre).Theresultsfromthesetestsindicatehowmanypoundsoffertilizerareneededatagivensoiltestleveltoreacheconomicallyoptimumyield,thatis,thefertilizationlevelthatbringsthemostprofitperacre(i.e.,thevalueofextrayieldvsthecostofextrafertilizer)atagivensoiltestlevel.Therefore,byusingthebestsoil-testingproceduresandsoundfertilizerrecommendationsbasedonadequatefieldresearch,laboratoriesareabletopredicttheoptimumeconomicrateoffertilizerthatisenvironmentallysound.

Potentialyieldsvary;hence,therelationshipbetweensoiltestvaluesandcropresponsewillvary.Thisisbecauseyieldisaffectedbyclimate,disease,andweedsaswellassoilfertility.Theinterpreterofthesoiltestresultsshouldalsoconsider(andshouldbeknowledgeableabout)potentialyieldlevelsforanygivenareaorevenspecificfarms,ifpossible.Previousyieldhistory,croppingsystems,andfertilizerpractices,ifknown,areneededforbestrecommendations.

FertilizerRecommendations:ApproachesandPhilosophiesAlthoughsoiltestingisthebasisfordeterminingtheadequacyofmanynutrients,soiltestsdonotprovidethefinalanswerofwhatfertilizerrateneedstobeappliedtoanindividualfieldinagivenyear.Therearetwoapproachescommonlyusedforgivingfertilizerrecommendationsforphosphorusandpotassium.Thesearethebuild-maintenanceandnutrientsufficiencyapproaches.

Build-MaintenanceApproachThestrategyhereistomaintainsoilfertilityforfutureyearsbyapplyingmorenutrientsthanthecropremoves,sothatyieldsarenotlimitedbynutrientlevelinthesoil.Tothisend,enoughfertilizerisappliedtobothmeetthenutrientrequirementsoftheimmediatecropandtobuildupthelevelofnutrientinthesoiltoacriticalsoiltestleveloverafewyears.Thecriticalsoiltestlevelisthesoiltestlevelatwhichnearmaximum(90–95%)yieldisobtained.Itisbasedonyieldresponsecurves,whicharetheresultofyearsofresearchandtrials,andarespecifictoaparticularsoilandclimaticconditions.

Oncethesoiltestvaluehasbeenraisedtothecriticallevel,thesoilislargelycapableof

supplyingcropnutrientrequirementsinagivenyearandsoiltestlevelismaintainedat,orabove,thecriticallevelbyapplyingfertilizerratestoreplaceonlytheamountofnutrientsexpectedtoberemovedbythecrop.Thiskeepsthesoiltestlevelfromfallingbelowoptimumbetweensoiltests.Oncethesoiltestreachesalevelwherecropremovalwillnotreducethesoiltestleveltobelowoptimum,noadditionalnutrientsareaddedexceptforthesmallamountssuppliedinstarterfertilizerapplications.

Sincenutrientavailabilityinthesoilisincreasedovertime,forfutureyears,morefertilizerisused.Whilethisreducestheriskofnutrientdeficienciesrelatedtotheiravailabilityinsoil,profitabilityinagivenyearisdecreased.Thisapproachalsoincreasestherisksofover-fertilizingandnegativeimpactsontheenvironment.

Sinceplantnutrientsrarelyworkinisolation,over-fertilizingcanalsoleadtoantagonisms,wherebyhighlevelsofonenutrientmayinfluencetheuptakeofanother.Forinstance,excesscalciumlevelscancausepotassium,boron,ormagnesiumdeficienciesinsomesoils,excessmagnesiumcanreducepotassiumuptakeandviceversa,andexcessphosphoruscanleadtoreducedzincuptake.Therefore,balancedfertilizationisimportantforincreasingcropyieldsandimprovingnutrientuseefficiency.

SufficiencyApproachThisapproachutilizesthelimitingfactorconcepttomakenutrientrecommendations.Thelimitingfactorconceptstatesthatcropyieldincreaseswillceasewhenanutrientorfactor“runsout;”thatis,itcannotpromotefurtherincreases.Whenthatfactorissupplied,yieldwillincreaseuntilanotherfactorbecomeslimiting.Fertilizersareappliedonlytomeetthenutrientrequirementsofthecrop.Thegoalistomaximizeprofitabilityinagivenyear,whileminimizingfertilizerapplicationsandcosts.Noconsiderationisgiventofuturesoiltestvalues.Whensoiltestlevelsarelow,cropsarelikelytorespondtoadditionalnutrientsandtherecommendedfertilizerratesexceednutrientremovalbythecrop.Assoiltestlevelsincreasetothecriticalsoiltestlevel,fertilizerrecommendationdecreasetoalmostzeroasthesesoilsareunlikelytodemonstrateanyyieldresponsetoaddedfertilizernutrients.Overall,thereisdecreasedfertilizerusagewiththisapproachleadingtoaslowerincreaseinsoiltestvaluescomparedtothebuild-maintenanceapproach.MostlaboratoriesanduniversitiesintheUSusethisapproachfortheirfertilizerrecommendations.

SoilTestingforNitrogenNitrogenexistsinorganicandinorganicformsinthesoil,andinterconvertsreadilyamongthoseforms,causinglargevariationsininorganicNconcentrations.Forthisreason,soiltestingfornitrogenhasnotprovenuseful,particularlyinthemorehumidclimateoftheCornBeltandSouthern/SoutheasternUnitedStates.Intheseareas,weather-inducedvariationsininorganicNconcentrationsgreatlyaffecttheabilityofasoiltesttoaccuratelypredictNavailabilitytothecropinagivengrowingseason.So,whileasoiltestisoneofthebestmethodstodeterminetherightrateofP,K,andseveralothernutrientsinthesehigher-rainfallareas,Nfertilizationprogramsaregenerallynotbasedonpre-plantsoil-testing.Rather,nitrogenmanagementdependsonknowledgeofnitrogenrequirementsofthevariouscropspecies.

PlantSamplingTheprocedureforcollectingplantsamplesforchemicalanalysisissimilartocollectingsoilsamples.First,determinewhetherthecropinafieldisrelativelyuniform.Thenselectaplantpart(orparts)fromabout15placesinthefield.Ifthegrowerisconcernedaboutaproblemarea,takeonesample(15subsamples)offreshmaterialfromtheaffectedareaandanothersamplefromtheareaofhealthy/normalgrowth.Samplesshouldbetakenassoonastheproblemappears.Thesetwoplantsampleswillprovideacomparisonofthetwoareasandanindicationofwhethernutrientsupplyisadequateforoptimumgrowth.

Plantparttobesampleddependsonmanyfactors—ageofplant,typeofplant,nutrienttobetested,andsoon.Takecarenottocontaminatethesampleswithsoilasevenasmallamountofsoilwillcausetheresultstobeinvalid.Contactyourlocalfarmadvisororlaboratoryconsultantforinformationonhowtosampleaspecificcrop.

Areportfromthetestinglaboratoryprovidesresultsonthesoiland/orplanttestsandnormallygivesrecommendationsonthetypeandquantityofnutrientsthatneedtobeadded.

NutrientDeficiencySymptomsNutrientdeficiencysymptomsinplantsmaybeseenaspoorgrowth,lackofgreencolor(chlorosis),orbrowningoftissue(necrosis).Thebest-knownnutrientdeficiencyistheonecausedbylackofnitrogen.Onacornplant,forexample,thegreentissuealongthemidribofthelowerleavesturnsyellow.Thetypeofsymptomanditslocationontheplantsuggestthenutrientthatisdeficient.SeeFigure8.7forcommondeficiencysymptomsforvariousnutrients.Table8.2alsolistscommondeficiencysymptomsforeachplantnutrient.

Figure8.7Sometypicalnutrientdeficiencysymptomscausedbylackofaspecificnutrient.

NutrientMobilityWithinthePlantCertainnutrients,whendeficientintheplanttissue,havetheabilitytomovefromolderleavestoyoungerleaveswheretheyareneededforgrowth.Nutrientswiththisabilityaresaidtobemobile,andincludenitrogen,phosphorus,potassium,magnesium,andmolybdenum.Othernutrientsdonothavetheabilitytomovefromoldtonewgrowthandaresaidtobeimmobile.Immobilenutrientsincludecalcium,sulfur,boron,copper,iron,manganese,andzinc.Knowingwhetheranutrientismobileorimmobilecanprovideuswithclueswhendiagnosingdeficiencysymptoms.Ifthedeficiencysymptomsshowupontheyounger,newgrowth,weknowthatthedeficientnutrientisimmobile.Ontheotherhand,ifdeficiencysymptomsappearinoldermatureleaves,weknowthatthedeficientnutrientismobile.

Deficiencysymptomsshouldnotbesolelyrelieduponformakingfertilizerrecommendations.Thisisbecausedeficiencysymptomsmaybecausedbyotherfactorsincludinginsectdamage,diseasesandmanyphysiologicalproblems.Inaddition,althoughthelawoftheminimumstipulatesthatthenutrientintheshortestsupplywillbethefirsttolimitgrowth,othernutrientsmaybedeficient.Collectingbothsoilandtissuesamplesfromboth“poor”and“good”areasofafieldisthebestwaytodiagnosenutrientdeficiencies.

Thereareotherdiagnostictoolsforassessingthenutrientstatusofcrops.Theseinclude

chlorophyllmeters,leafcolorchartsandon-the-gosensors.

BiologicalGrowthTestsBiologicalgrowthtestsmayalsobeusedtodeterminenutrientneeds.Asimplemethodistosplitafieldandapplyonetypeofnutrientononehalfandanothernutrientontheotherhalf.Ortryonerateofanutrientononehalfanddoubletherateontheotherhalf.

Biologicaltestscanalsobeusedingreenhousesforashortgrowthperiod.Forsuch“pot”tests,uptoagallon(4l)ofsoilisbroughtintothegreenhouseanddividedintosmall,1-pint(0.5-l)containers,whichreceivevariousratesofthenutrientsinquestion(leavingoneuntreated).Rapid-growingplants(suchassmallgrains)areplanted,harvestedinashorttime(suchas30days),andweighedtodeterminewhichrateprovidedthegreatestgrowth.

AddingPlantNutrientsIfthelevelofessentialplantnutrientsinthesoilislowortheiravailabilityisdecreasedforsomereason,nutrientsneedtobeaddedtoachievegoodcropyields.Unlessnutrientsremovedinharvestedgrainandplantbiomassarereplaced,soilfertilitywilldeteriorateandcropyieldswilldecline.

FertilizersEachessentialplantnutrient(exceptcarbon,hydrogen,andoxygen)canbeappliedasacommercialfertilizer.Therearemanydifferenttypesandforms—somany,infact,thatitwouldbedifficulttodescribeallofthemhere.Butafewarediscussednext.

Fertilizerscomeindry,liquid,andgaseousforms.Somecontainonlyoneessentialnutrient,whereasotherscontaintwoormore.Percentagesoftheessentialnutrientsinafertilizeralsovarywidely.Thepercentageofanutrientornutrientsinafertilizerisguaranteedtobeataminimumleveloraboveasrequiredbystatelaws.

Anutrientguaranteeisexpressedinthreenumbers,suchas20-10-5.Thefirstnumber(20intheexample)representsthepercentageofavailablenitrogen(asN).Itmaybeinnitrate(NO3

−),ammonium(NH4+),ororganicform.Thepercentageofthenitrogenineachformmust

appearonthelabel.Thesourceofanyorganicnitrogenalsomustbeshown.Theformofnitrogenisimportantinfertilizertimingasmostplantsusenitratespreferablyandthemicrobialconversionofotherformstonitratesrequirestime.Thesecondnumber(10intheexample)isthepercentagephosphoruscontentexpressedasphosphate(P2O5).TheactualphosphoruscontentofthematerialcanbecalculatedbymultiplyingtheP2O5by0.44.ToconvertbacktoP2O5,multiplyphosphorusby2.29.Thethirdnumberisthepercentagepotash(K2O)content(5intheexample),whichisanexpressionofthepotassiuminthematerial.Toconverttoelementalpotassium(K),multiplytheK2Onumberby0.83.ToconvertbacktoK2O,multiplythepotassiumnumberby1.20.OtherexamplesofgradesaregiveninTable8.3.If

thereareotherguaranteednutrientspresent,theyarelistedasadditionalnumberswiththesymbolsfortheelements.Forexample,ifthe20-10-5givenherealsocontains2%zincand1%manganese,thegradewouldshowas20-10-5+2%Zn+1%Mn.Thisgradeguaranteeisalwayslistedonthefertilizercontainer(whetheritisinasack,box,orbottle)andalsoontheinvoice.Themanufacturersofthefertilizermaketheguaranteeanditisenforceablebystatelaw.Thisnormallyassurestheproducerofpurchasingthecorrectproduct.

Table8.3Fertilizergrades(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Grade N P2O5 K2O

(%) (%) (%)20-10-5a 20 10 50-20-20a 0 20 2010-30-10a 10 30 100-0-60b 0 0 6032-0-0a 32 0 0

aMixedfertilizerswithtwoormorenutrients.

bStraightfertilizerswithonlyonenutrient.

Thepercentageofanutrientinafertilizerisimportantbecauseitdeterminestheamounttouseperacretoobtainagivenquantityofaneedednutrient.Forexample,afertilizerwitha20-5-10gradeis20%N,5%P205,and10%K2Obyweight.If100lb(45kg)ofthisfertilizerwereappliedevenlyoveranacre,theamountsN,P2O5,andK2Oappliedwouldbe:

TheconcentrationofnutrientsinthefertilizerismultipliedbytheamountofthefertilizermaterialappliedperacretofindtheactualamountofN,P205,andK2Oappliedperacre.IfcropPguidelinescallsfor15lb(6.75kg)P205/acre,theamount20-5-10fertilizerneededwouldbe:

Theamountof20-5-10fertilizermaterialtobeappliedperacreiscalculatedbydividingthecropP205requirementbytheconcentrationofP205inthefertilizer.

Afertilizerthatcontainsonlyonenutrientiscalledastraightfertilizer(orafertilizermaterial).Anexamplewouldbeammoniumnitrate,whichcontainsonlynitrogenasanutrient.Mixed

fertilizerscontainingtwoormorenutrientswouldbeamixtureoftwoormorestraightfertilizers.Anexamplewouldbemixingammoniumnitrate(33-0-0)withacalciumphosphate(0-46-0)toproduceagradeoffertilizerthatmightbe16-20-0.Othertermsoftenusedtodescribefertilizersarecompleteandbalanced.Acompletefertilizer,atermoflittlesignificancefromacropproductionstandpoint,isonethatcontainsallthreeoftheprimarynutrientsandwouldbeagradesuchas24-10-8(Fig.8.8).

Figure8.8AcompletecommercialfertilizerisreportedintermsofvaryingpercentagesofN,P2O5,andK2O.

AbalancedfertilizercontainsequalamountsofN-P-K(10-10-10).Abalancedfertilizerprogramisonewhichprovidesnutrientsbasedoncropneedsandnutrientdeficiencies.Itmightinclude,forexample,onlynitrogenifitistheonlynutrientdeficiency.Orifnitrogen,phosphorus,potassium,zinc,andboronareneeded,thefertilizerthatwouldprovidetherequirednutrientsforabalancedfertilizerprogrammightbea15-5-10+1%Znand0.5%B.

Nitrogen,thefertilizernutrientusedingreatestquantities,comesinallthreeformsofdry,liquid,orgas.Anhydrousammonia(NH3)istheprincipalsourceofnitrogenusedintheUnitedStates.Itisinagaseousformwhenappliedtothesoil(Fig.8.9)butisstoredasaliquidwhenunderpressureoratlowtemperatures.Itcontainsonlyonenutrientanditsnitrogencontentis82%.Thisisthehighestnutrientconcentrationinanycommonlyusedfertilizer.Ammoniaisthebaseforproducingothernitrogenfertilizers,suchastheexamplesgiveninTable8.4.

Figure8.9Nitrogenmaybeappliedasanhydrousammonia(NH3)gasfedfromapressuretankthroughhollowknivesthatcutintothesoil.

Table8.4Combinationusedtoproducenitrogenfertilizers(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Combinations Product Percentages(N-P2O5-K2O)

NH3+HNO3 NH4NO3(ammoniumnitrate) 33.5-0-0

NH3+H2SO4 (NH4)2SO4(ammoniumsulfate) 21-0-0

NH3+H3PO4 NH4H2PO4(ammoniumphosphate) 11-48-0

NH3+CO2 (NH2)2CO(urea) 45-0-0

Note:NH3=ammonia,HNO3=nitricacid,H2SO4=sulfuricacid,H3PO4=phosphoricacid,CO2=carbondioxide.

Afterammoniaismanufactured,itismixedwithvariousacidsinliquidformandtheresultingproductisthendriedtothesolidform(Fig.8.10).Whileintheliquidstate,ureaandammoniumnitrateareoftenmixedtoformasolutioncontaining28or32%nitrogen.ThisisthesecondmostwidelyusedsourceofnitrogenintheUnitedStates(Fig.8.11).

Figure8.10Mostnitrogenfertilizersstartwithammonia,whichreactswithvariousacids.Theyexistingaseous,dry,orliquidforms.

Figure8.11Liquidfertilizermaybeappliedtothesoilor,ifsufficientlydiluted,itcanbeusedasafoliarapplication.

Phosphorusfertilizersarederivedfromamineralcalledapatite,whichisacalciumphosphate

andisaforminwhichthephosphorusisnotreadilyusablebyplants.ThemineralisminedfromdepositsjustbelowthesurfaceofthesoilinFloridaandIdahointheUnitedStatesandinMoroccoandtheformerSovietUnion.Apatiteiscommonlycalledrockphosphate(Fig.8.12).Itistreatedwithanacid(eithersulfuricorphosphoric)toproduceacalciumphosphate(either0-20-0or0-46-0)inwhichthephosphorusisinamoreusableformthaninapatite.Phosphoricacidalsocanbeproducedfromtheapatiteandcanthenbetreatedwithammoniatoproduceanammoniumphosphate(Table8.4).Phosphorusfertilizersareavailableeitherintheliquidformorinthedryform.PhosphorusissecondtonitrogeninquantityusedintheUnitedStates.

Figure8.12Rockphosphatesformakingfertilizerareminedfromopenpits.

Potassiumfertilizersaremanufacturedfrommineralssuchasmuriateofpotashorlangbeinite,whichoccurindepositsintheearth.SomedepositssuchasthoseinCanadaarefairlyshallow,whereasotherssuchasthoseinNewMexicoarequitedeep(Fig.8.13).Muriateofpotashisrefinedtoproducepotassiumchloride(0-0-60);langbeiniteisusedtoproducepotassium-magnesiumsulfate(0-0-22+11%Mg).Othercommonpotassiumfertilizersarepotassiumsulfate(0-0-50),potassiumnitrate(13-0-44),andpotassiumphosphate(0-26-26).

Figure8.13Potash,apotassiumcompound,isminedfromdepositsintheearth.

Theotherfertilizerelementscomefromvarioussources.Calcium,forexample,comesmostlyfromlimestone(calcite)andgypsum.Magnesiumisusedeitheraspotassium-magnesiumsulfate,limestone(dolomite),ormagnesiumsulfate.Sulfurisusuallyappliedaselementalsulfur,athiosulfate,orasoneofthesulfateformssuchasammoniumorpotassiumsulfate.

Themicronutrientsiron,zinc,manganese,andcopperareusuallyusedinoneofthreeforms.Asulfatesaltsuchaszincsulfateiscommon.Anotheriscalledachelate,whichisanorganicformthatreactswiththemicronutrienttomakearelativelysolubleproduct.Athirdformisanoxide,suchaszincoxide.

Itisimportanttousethesourceoffertilizerbestsuitedforanygivencropandcondition.Alocalfertilizerdealer,consultant,oragriculturalagentshouldbeconsultedforspecificsonthebestonetouse.

EffectofFertilizersonSoilpHDifferentfertilizermaterialshavedifferentimpactonsoilpH.Generally,fertilizerswithhighproportionsoftotalnitrogenandarederivedfromammoniumsources(suchasurea,ammoniumsulfate,ammoniumphosphate,orammoniumnitrate)canacidifysoilswithrepeatedapplications.Mostfertilizersprovidethe“LimeEquivalent”onthebag'slabel.Thelimeequivalentistheamountoflimestone(calciumcarbonate)ittakestoneutralizetheacidifyingeffectsofusingonetonofaparticularfertilizer.

SomefertilizerscanalsoincreasesoilpH.Thesefertilizersareusuallylowinammonium,buthighinnitrate.Additionally,thesefertilizerssometimescontaincalciumfromcalciumnitrate.Thelimeequivalentforthesefertilizersisalsogiven,butitindicatestheequivalentliming

effectratherthanthelimeneededtooffsetacidity.

Thefollowinggeneralizationsmaybeusedasaguide:

Ammonium(NH4+)orammoniumformingfertilizers,suchasurea,willcauseadecreasein

soilpHovertime.

Nitrate(NO3−)sourcescarryingabasiccationshouldbelessacid-formingthanNH4

+

fertilizers.

ThepresenceofCa,Mg,K,andNainthefertilizerwillslightlyincreaseorcausenochangeinsoilpH.

Elementalsulfur,ammoniumsulfate,andcompoundssuchasironoraluminumsulfatescanreducethesoilpH.

SaltDamageWheninorganicfertilizersareappliedtothesoil,theconcentrationofsolublesaltsincreasesinthesoilsolutionsurroundingthezoneoffertilizerapplication,particularlywhentherearehighratesofevaporationandinsufficientrainfalltoleachthesalts.Ahighconcentrationofsolublesaltsinthesoilsolutioncanhaveharmfuleffectsonplantsandgerminatingseeds.Thesaltindexisameasureofthepotentialsaltdamagetotheplant.Solublesaltscanalsooriginatefrommanureandmineralsintheearth.

Problemsassociatedwithsaltdamageinclude:

1.Iftheconcentrationofsaltinthesoilsolutionisgreaterthanthesaltconcentrationinplantrootcells,moistureavailabilitywillberestrictedandwaterwillnotbeabsorbedbytheplant.

2.Thehighconcentrationofsaltswillcausewatertoleavetheplantbyosmosisandenterthesoil.Excessivewaterlosscausestheprotoplasmtoshrinkawayfromthecellwalls(plasmolysis).Asaresult,theplantwithersandexhibitssymptomssimilartothosecausedbydrought.

3.Highconcentrationsofsolublesaltsmayalsoresultinelementaltoxicitiesofsodiumandchlorine.

FertilizerSaltindexThefertilizersaltindexwasdevelopedtoclassifyfertilizersaccordingtotheirpotentialtocausesaltinjurytoplants.Itisameasureofthesaltconcentrationthatafertilizermaterialinducesinthesoilsolutionanditismeasuredbyplacingthematerialinthesoilanddeterminingtheosmoticpressureofthesoilsolution.Sodiumnitrateisthestandardandhasasaltindexof100.Otherfertilizersareassignedasaltindexvaluerelativeto100,whichdescribesthepotentialofthefertilizertocausesaltinjuryrelativetothedamagecausedbyanequalamountofsodiumnitrate.Afertilizerwithasaltindexlessthan100hasalowerpotentialtocausesaltdamageincomparisonwithafertilizerwithasaltindexgreaterthan

100.

Nitrogenandpotassiumsaltshavehighersaltindicesthanphosphorussalts,hencetheyhavemoredamagingeffectsongerminationwhenplacedclosetotheseed.SaltinjurycanbeavoidedbyapplyingNandKfertilizersonthesoilsurface(broadcastapplication)orbyplacingthemtothesideandbelowtheseed.

SoilAmendmentsAsoilamendmentisamaterialaddedprimarilytochangeorenhancethephysical,chemicalorbiologicalcharacteristicsofsoil,ratherthanasplantfood.Examplesincludelimingmaterialssuchaslime(calciumcarbonate),dolomite(calciummagnesiumcarbonate),calciumoxide,magnesiumoxide,calciumhydroxide,magnesiumhydroxidewoodash,andslags,whichareusedprimarilytoincreasethepHofasoilandmakeitlessacid.Limingmaterialsshouldnotbeaddedwithureaorammoniumfertilizers,asNwillbelosttotheatmosphere.ElementalsulfurisusedtodecreasepHandmakeitlessalkaline.Gypsum(calciumsulfate)isusedprimarilyasanamendmentonsoilswithexcesssodium(sodicsoils),whichcausesapoorphysicalcondition.Thecalciuminthegypsumreplacesthesodium,resultinginasoilwithimprovedstructure.Thesodiumcombineswiththesulfateandisleachedfromthesoilbyirrigation.

Thesesoilamendmentsalsoprovideplantnutrientsiftheyareneeded.Forexample,limestoneprovidescalciumandinsomecasesmagnesium.Gypsumprovidescalciumandsulfur.

AnimalManureandGreenManureCropsAnimalmanuresareexcellentsourcesofplantnutrients.Becausemanureisoftenderivedfromplantmaterial,itissimilartoorganicmatterandthereforecontainsessentialnutrients.Whenmanuredecomposesinthesoil,itsnutrientsarereleasedandmadeavailableforplantuptake.

Manurenotonlyservesasasourceofplantnutrientsbutalsoaddsorganicmattertothesoil,whichimprovesitsphysicalcondition,water-holdingcapacity,cationexchangecapacity,andotherdesirableproperties.Formaximumvalue,manureshouldbeinjectedorworkedintothesoilsoonafterapplication(Fig.8.14).Adisadvantageofmanureisthatitoftencontainsweedseeds.

Figure8.14Animalmanureimprovessoilstructureaswellassupplyingnutrients.

Thenutrientandmoisturecontentofmanureisquitevariable,dependingprincipallyonthetypesoffeedutilizedbytheanimalsandhowthemanureishandledbeforeitisapplied.Itshouldbeappliedasoftenasconvenienttodoso,butinmostcases,manurehastobestoredandappliedlater.Alternatewettinganddryinginapileresultsinthereleaseofammoniagasintotheatmosphereasthemanuredries.Whenrewettedbyrain,nitratesleachoutandmaypresentadangerifallowedtorunoffintoawatersystembeingusedbyhumansoranimals.Manystateshavelawsthatrestricttherunofffromfeedlotsandmanurepiles,andrequireownerstoprovideholdingpondsorlagoons(Fig.8.15).Ifamanurepiledrieswithoutsubsequentrewetting(indryareas),lossofammoniaisminimized.

Figure8.15Lagoonsprovidestorageandmaintainthenutrientvalueofmanure.

TheaveragenutrientvalueofmanurefrombeeffeedlotsinTexasisshowninTable8.5.Thenutrientcontentofmanureisalsoquitevariable;thus,onlyaveragesarepresentedinthetable.Thevaluesofmoisturecontentrangedfromalowof8%toahighof62%,withanaverageof33%.Oneton(900kg)ofmanurewithamoisturecontentof33%andaveragenutrientpercentageswouldbeequivalentto400lb(180kg)ofan8-6-11gradefertilizer.

Table8.5Averagecontentofessentialelementsinbeeffeedlotmanure(basedon30samplesfromTexasHighPlainsfeedlots,figuredat30%moisturecontent)(Eash,NealS.,CaryJ.Green,AgaRazvi,andWilliamF.Bennett,eds.SoilScienceSimplified.5thed.Ames,Iowa:Wiley-Blackwell,2008.Copyright©2008,JohnWiley&Sons,Inc.)

Nutrients Content Pounds/ton(kg/metricton)ofmanure(%)

Nitrogen(N) 1.6 32(16)Phosphorus(P2O5) 1.3 26(13)

Potassium(K2O) 2.2 44(22)

Calcium(Ca) 0.7 14(7)Magnesium(Mg) 0.2 4(2)

(ppm)a

Iron(Fe) 1525 3.00(1.50)Zinc(Zn) 100 0.20(0.10)Manganese(Mn) 105 0.21(0.105)Copper(Cu) 7 0.02(0.01)Boron(B) 15 0.03(0.015)

appm=partspermillionor1/10,000ofapercent.

Whenanimalsareconfinedinbarnsmostofthetime,theirmanureiscommonlystoredfor6monthsormoreinlagoonsorlargetanksandthemoisturecontentismorelikelytobearound90%.Itisinjectedintothesoilasaslurryfromtanksonwheels(Fig.8.16).Thisistypicalfordairyfarms.Undertheseconditions,thenutrientcontentislikelytobeaboutone-thirdofthatshowninTable8.5.

Figure8.16Atractor-poweredmobiletankandpumpunitforinjectingliquefiedmanureintothesoil.

TheN/Pratioinanimalwastesistypically1:1to2:1(N/P2O5),butmostplantsrequire3:1to5:1(N/P2O5).Therefore,animalwasteisrelativelyhighinP.Ifmanuresareappliedtomeet

theNneedsofplants,excessiveamountsofPoftenresult.SincePisrelativelyimmobileinthesoil,anyPnottakenupbyplantswillaccumulateinsoiltolevelsfarinexcessofamountsneededforoptimalcropgrowth.TheexcessPinthesoilcanbelostvialeaching,erosionandrunoffandcancausenutrientenrichmentingroundandsurfacewatersourcesandcauseeutrophication.AmoreenvironmentallyapproachistoapplymanuretomeetPneeds(basedonsoiltesting).ThiswillresultinlowerNapplication,buttheremainingNneedscanbemetwithfertilizers.Thisapproachrequiresmorelandsincesmalleramountsofmanurewillbeutilizedoneachfield.

Theeconomicvalueofmanureisalsovariable,dependingonthenutrientpercentages.Inmostcases,approximatelyone-halfofthenutrientsarereleasedandavailablethefirstyear.Therateofdecomposition,however,varieswidely.Onthisbasis,iftheanalysisisknown,avaluecanbeplacedonmanure.Becausemanurehasrelativelylownutrientpercentages,thevolumethatmustbehandledisrelativelyhighifsufficientplantnutrientsaretobeapplied.Consequently,manureisnormallyusedfairlyclosetothefarmorfeedlotwhereitisproduced.Manureapplicationratesaregenerally10–15tonsperacre(22–34Mg/ha[megagram/hectare]).Manureshouldnormallybeappliedonan“as-is”basisforcropsthathaveasignificantnitrogenrequirement.

Cropsplowedundersoil(Fig.8.17)toimprovefertilityandphysicalconditionofthesoilarecalledgreenmanure.Thebestcropsforthispurposearelegumessuchasalfalfaandcloverbecausetheyarehighinnitrogencontent;however,nonlegumecropssuchaswheatorsudangrasscanalsobeused.

Figure8.17Cropscanbeplowedunderasgreenmanuretoprovideorganicmatter.

FertilizerPlacementMethodsThemajorfunctionofcorrectfertilizerplacementistoenhancenutrientavailabilityandplantuptake.Correctfertilizerplacementencouragesmaximumcropyieldsbecauseitoftenimprovestheefficiencyofnutrientuptake.Thisleaveslessnutrientsinthesoil,thereby

protectingbothsurfaceandgroundwaterquality.

Numerousplacementmethodsareavailablebutmostgenerallyinvolvesurfacetosubsurfaceapplicationsbefore,atorafterplanting.Priortoplanting,fertilizerscanbebroadcastonthesurfaceorincorporatedintothesoil,appliedasabandonthesurfaceorappliedasasubsurfaceband.Atplanting,fertilizerscanbebandedwiththeseed,belowtheseedorbelowandtothesideoftheseed.Afterplanting,applicationisusuallylimitedtoNasatop-dressoraside-dress.

Thereisnosinglebestmethodandseveralfactorsneedtobeconsideredinmakingfertilizerplacementdecisions.Theseincludesoilconditions,soiltestlevel,soilPbufferingcapacity,croptype,salteffectofthefertilizer,conveniencetothegrower,andequipmentavailability.

BroadcastMethodsAbroadcastinvolvestheapplicationofnutrientsuniformlyonthesoilsurfaceandmayormaynotbeincorporated.AmajorityoffertilizerusedintheUSAisappliedusingbroadcastmethods.Thepopularityofbroadcastpre-plantapplicationsofsolidfertilizershasincreasedinrecentyearsduetothedesireto:reducefertilizerinjurytoplants,cutdownontheamountoftimespenthandlingfertilizersandthedevelopmentofbulkblendsthatenabletheapplicationofalargequantityofmaterialatonetime.

SurfaceBroadcastSurfacebroadcastisamethodbywhichfertilizerisapplieduniformlyoverthesurfaceofanentirefield(Fig.8.18).Itisbestsuitedforhigh-speedoperationsandheavyapplicationrates.Highcapacityfertilizerspreadersareoftenusedtospindryfertilizerorsprayliquidfertilizeronthesoilsurfaceoronagrowingcrop.Abroadcastapplicationisreferredtoasatop-dressapplicationwhenappliedatplanting,orwhenappliedtothestandingcrop(afteremergenceofcrop).Caremustbetakenintopdressingtoensurethatfertilizerisnotappliedwhentheleavesarewetoritmayburnorscorchtheleaves.BroadcastingistheonlyoptionforapplyingNandKfertilizerstoexistingstandsofperennialforagecropssuchcoolseasongrassesandalfalfa.

Figure8.18Fertilizerdistributionusinga)broadcasttopdressedandb)broadcastincorporatedmethodsofplacement.

Advantages:

1.Itisfast,easyandeconomical(saveslaborandtimeduringplanting).

2.Itresultsinrelativelyuniformdistributionoffertilizer.

3.Itrequiresrelativelyinexpensiveapplicationequipment.

4.Itcanreduceworkloadatcriticaltimesduringtheyearsincethereareseveralopportunitiesforapplication.

5.Largeamountsoffertilizercanbeappliedwithoutthedangerofinjuringtheplant.

6.Itprovidesapracticalwaytoapplymaintenancefertilizer,especiallyinforagecropsandno-tillsystems.

Disadvantages.Broadcastinghaslowefficiencyofnutrientusebecause:

1.ItenhancesNimmobilizationandlossesofNviavolatilizationinhighsurfaceresiduesystemssuchasno-tillsystems.

2.NandPlossesbyerosionaswellasNlossesbydenitrificationarehighercomparedwithplacementinthesoil.

3.Itleavesmorefertilizeravailabletoweeds.

4.ItrequiresrainfallorirrigationtomovemobilenutrientssuchasNO3-NandSO4-Sintotherootzone.Ifthesoildriesout,thesenutrientsbecomeunavailable.

5.Itleavesimmobilenutrients(P,K,andsomemicronutrients)onthesoilsurface,makingthemunavailabletotheplantrootsystem.

6.Itdoesnotincreasetheoverallfertilityofthesoilasitdoesnotstorefertilizerdeepintheprofileforlaterseasonplantnutrition.

7.Uniformityofapplicationcanbepoorforlowratesoffertilizers.

BroadcastIncorporatedAbroadcastapplicationisreferredtoasabroadcast-incorporatedwhenthebroadcastedfertilizerisimmediatelyincorporatedintothesoilbytillage,resultinginfertilizerbeingmixedthroughouttherootzone(Fig.8.18).Thisimprovesontheefficiencyofsurfaceapplication.Plowingcreatesanutrient-richzoneafewinchesbelowsoilsurfacewhereyoungplantrootscanabsorbit,therebyimprovingnutrientavailability.

Advantages:

1.Reducesvolatilizationanderosionlossescomparedtobroadcastplacement.

2.Increasesuptakeofimmobilenutrients(i.e.,PandK)byincreasingtheprobabilityofcontactbetweenplantrootsandfertilizedsoil.

3.Improvedplantuptakeleavesfertilizerlessavailableforweeds.

4.Distributionofnutrientsthroughouttheplowlayerencouragesdeeperrootingandexplorationofthesoilforwaternutrients.

5.IncorporationpromotesrapidnitrificationofNH4+toNO3

+.

6.Incorporationwithplowmold-boardandplowchiselstoresfertilizerdeepintheprofileforlaterseasonplantnutrition.

7.Reducesthechanceofsaltinjurytoseedlings.

8.Increasestheoverallfertilityofthesoil.

9.MaximizescontactbetweensoilandfertilizertherebycreatingmorepotentialsitesforadsorptionandsubsequentretentionofKandMginlowCEC(sandy)soils.

Disadvantages:

1.Requiresmoreenergytoincorporatethefertilizer.

2.ResultsinpotentiallyhigherNandSleachinglossesthanwithsurfaceplacement.

3.Stimulatessomeweedgrowth.

4.Sometillageimplements,suchasamoldboardplow,maydistributethemajorityofthefertilizerpossiblytoodeeplyfortherootsofyoungseedlings.

5.PotentialforleachinglossesofNandsulfur(S)arehigherthanwithsurfaceplacement,particularlyinwetyears.

6.Maximizescontactbetweenthesoilandfertilizer,therebyincreasingtheopportunityfortie-up/fixationandreducedavailabilityofPandK.

7.Someimplements,suchasachisel,incorporatefertilizeronly2–3in.(5–7.5cm)intothesoilsurface.Thisleavesnutrientsinaccessibletorootswhenthesurfacesoildriesout.

Nutrientsthataregenerallybroadcastincludenitrogen,sulfur,calcium,andmagnesium(aslimingmaterials),copper,manganese,zinc(butbandingismoreefficientforallthesemicronutrients),andboron.Potassiummaybeappliedbyeitherbroadcastorbandmethods.However,broadcastingispreferredinsandysoilsthathavealowCEC(lessthan3meq/100g)whilebandingispreferredinsoilscontainingalotofvermiculiteand/orillitewhereKfixationoccurs.

Nutrientsthataregenerallynotbroadcastincludeiron(generallyappliedasafoliarspray)andmolybdenum(generallyappliedasaseedtreatment).PhosphorusisbroadcastonlyinsoilswithmoderatetohighlevelsofP(atsoilpHvaluesof5.5–6.5).

Pop-UpApplicationPop-upfertilizerapplicationsrefertoplacingsmallamountsofnutrientsindirectcontactwiththeseed(Fig.8.19)toenhancetheavailabilityofnutrientstoyoungplantsandenhanceearlyseedlingvigor.Itisalsoknownasstarterfertilizer.NitrogenandPareusuallypop-upfertilizercomponents.Theupperlimitofpop-upfertilizerthatcanbeusedisdeterminedbytherelativesalttoleranceoftheplant.LargeamountsofN,K,andSfertilizerscannotbeusedinpop-upapplicationsduetotheirhighsaltcontents.Boroncannotbeusedinapop-upapplicationashighconcentrationsofBaretoxictoplants.

Figure8.19Pop-upordirectseedcontactmethodoffertilizerplacement.

Solidfertilizersusedinpop-upapplicationsshouldbehighlysolubleinwater.Theyshouldalsohavealowsaltindex,ahighnutrientcontentandminimalcontentofNmaterialsthatproduceammonia.

Advantages:

1.Bothliquidanddryfertilizerscanbeused.

2.Promotesearlyplantgrowthandenhancesseedlingvigor.Thisearlystimulationofcropgrowthisoftentermed“pop-upeffect.”

3.Decreaseslossofnutrientsbyerosioncomparedwithsurfaceplacement.

4.Positionsfertilizerwhererootsystemsofseedlingscanmorereadilyusenutrients.

5.Positionsfertilizersoitismoreavailabletothecropsthantotheweeds.

6.PlacingPwiththeseedincreasestheconcentrationofavailablePsufficientlytopartiallyoffsetthedetrimentaleffectsofextremesinsoilpH,compactionandlowsoiltemperaturesunderconservationtillagesystems.

Disadvantages:

1.Cancauseseedlingdamageiftoomuchfertilizerisapplied.

2.Retro-fittingplanterscanbeexpensive.

3.Ureaanddiammoniumphosphatecannotbeused.

Totalapplicationratemustbekeptbelow10lb(4.5kg)ofN+K2Osoastopreventsaltburn.

BandApplicationBandingreferstoplacingnutrientsbelow,above,ononeside,oronbothsidesoftheseedorseedlingssothatdevelopingrootswilleasilyreachthenutrients(Fig.8.20).Fluidorsolidfertilizerscanbeused.Allplantnutrients,withtheexceptionofboron(B),canbesuccessfullybanded.HighconcentrationsofBinsoilsaretoxictoplants.

Figure8.20Bandfertilizerplacementmethod.

Bandingcanbedonebeforeoratthesametimewithplanting/drilling,orafterthecropisplanted.Alowrateoffertilizerisplacedincloseproximity(atleast2in./(5cm)tothesideand2in.(5cm)deeperthantheseedsorplants).Thisprovidestheplantswithaconcentratedzoneofnutrientswhilepreventingsaltdamageandammoniatoxicity.

Advantages:

1.Requireslessfertilizerperacrethanbroadcasting.

2.Jump-startsearlyplantgrowthbyincreasingPavailability.

3.Thepositioningoffertilizersissuchthatnutrientsaremoreavailabletothecropsthantotheweeds.

4.DecreasesPandKfixationbylimitingsurfaceareaofcontactoffertilizerwiththesoil,therebyincreasingtheiravailabilitytoplants.

5.AbsorptionaswellasmovementofPandNtoplantrootsismuchsloweratlowersoiltemperatures.BandingapplicationsofNandPimproveplantuptakeatlowersoiltemperatures.Thisimprovesgrowth,therebypromotingwinterhardiness.

6.Fertilizerapplicationandplantingoperationscanbedonesimultaneously.

7.Reducesnutrientlossesduetosoilerosion.

8.Similartobroadcast–incorporate,bandingammonium-basedNfertilizers(e.g.,ammoniumnitrate,ammoniumsulfate,andurea)belowthesoilsurfacereducesvolatilizationlosses.

9.ItslowsNH4+conversiontoNO3

−(nitrification),reducingtheriskofleaching.

10.Enhancesseedlinggrowththerebyresultinginstrongerseedlingsthatarelesspronetosufferfrompestsanddiseases.

11.Hashighnutrientuseefficiency.

Disadvantages:

1.Thereisriskofsaltburntoplants.

2.ThereisincreasedNO3−andSO4

−leachinglossescomparedwithsurfaceplacement.

3.Slowsplantingifappliedwithadrill.

4.Itiscostlyasitrequiresexpensiveequipmentorequipmentmodification.

Side-DressApplicationAsurfaceorsubsurfacebandingtreatmentafterthecropisplantedisreferredtoasaside-dressapplication.Inthiscase,fertilizerisappliedbetweenrowsofgrowingplantstosupplyNduringperiodsofrapidgrowthasthisiswhenthecropneedsitmost.Themostcommonuseofside-dressapplicationonfarmsistheapplicationofNfertilizerbetweentherowsofagrowingcornwhentheplantsare12–24in.(30–60cm)tall.TheamountoffertilizerNtobeappliedisdeterminedbasedontheresultsofaPre-Side-dressNitrateTest(PSNT),whichassessesthelikelihoodofayieldresponsetotheadditionofside-dressN.Side-dressapplicationisnotrecommendedforimmobilenutrients(PandK)becausemostcropsneedtheseearlyinthegrowingseason.

Advantages:

1.Providesavaluableopportunitytoapplytherecommendednitrogenthroughouttheseasoninsmalleramounts(splitapplication),ratherthanapplyingallthenitrogeninasingleapplication.

2.Applicationscanbemadewhenevertheequipmentcanbeoperatedwithoutdamagetothecrop.Thisallowsagrowermoreflexibilityinapplicationtime.

3.SplitormultipleapplicationsprovidesNwhentheplantneedsitmost,resultinginhighnutrientuseefficiency.

4.Providescropswithadditionalnutrientsifappliedduringthegrowingseason.

Disadvantages:

1.Timingoftenfallsduringthewetandbusyseason;slowprocess.

2.Subsurfaceside-dressapplicationswithaknifetooclosetotheplantcancausedamagetorootsbypruningorbyfertilizertoxicity.

PointFertilizationThisconsistsofopeningaholeinthesoilwithastickorhoe,andplacingaquantityoffertilizerintothesoilnearthecrop.Thissystemiscommonlyusedintheproductionofperennialshrubortreecrops.Itisalsousedinmanydevelopingcountries,wherecompetitionfromweedsisminimizedbyplacingsmallquantitiesoffertilizerinaholedugnearahillofsorghumorcorn.

FertigationFertigationistheapplicationofdissolvedfertilizersandchemicalstothesoilthroughanirrigationsystem,whichappliesbothwaterandnutrientstoplants.TheprincipalnutrientsappliedbyfertigationareNandS.FertigationofPislesscommonduetoconcernsoverprecipitationofPinwatershighinCaandMg.

Advantages:

1.Nutrientscanbeappliedclosetothetimeofpeakcropdemand,thuspreventingluxuryconsumptionofnutrients.

2.Mayreducelossesofnitrogenduetoleachinganddenitrification.

3.Providesanopportunitytosplitnitrogenrecommendationintoseveralapplications.

4.Resultsinreducedoperationcoststhrougheliminationofoneormorefieldoperations.

5.Nutrientsmaybeappliedcontinuallythroughoutthecropgrowingperiod.Thisenablesahighdegreeofflexibilityinnutrientmanagement.

6.Canbeusedtocorrectmidseasonnutrientdeficiencies.

7.Combiningfertilizationandirrigationintoasinglefieldoperationsavesontimeandlabor.

8.Highnutrientuseefficiency.

Disadvantages:

1.Irrigationequipmentneeded(injectionpump,etc.).

2.Thereisriskofunevendistributionofnutrientswithlowratesoffertilization,inwindy

situationsandunderrowirrigation.

3.Requiresawell-managedandequippedirrigationsystemforuniform,maximumefficiency.

4.Nutrientsmaybeleachedbeyondtherootzone.

FoliarApplicationThisistheapplicationofasmallamountofsolublefertilizerormineralthroughdirectsprayingontotheaerialportionofplants.Thisisacommonwaytoapplymicronutrientssincemicronutrientsarerequiredinmuchsmallerquantitiesthanmacronutrients.

Advantages:

1.Foliarfertilizerssupplyplantcellswithnutrientsmorerapidlythanthesoil.

2.Canprovideaquickwaytocorrectnutrientdeficiencies.

3.Veryeffectiveformicronutrientsasthesearerequiredinsmallamounts.

4.Itisthemosteffectivemeansoffertilizerapplicationinsituationswheretheproblemofnutrientfixationbythesoiloccurs.

Disadvantages:

1.Itisexpensive.

2.Itislimitedtosmalland/orrepeatedapplications.

3.Haslimitedcapacitytosupplymacronutrientsbecauseofsalthazards.

4.Theresponseisusuallytemporary,hencerepeatedapplicationsarerequired.

InjectionInjectionisusedtoplaceliquidorgaseousfertilizerbelowthesoilnearplantroots.

Advantages:

1.Reducedvolatilizationlossesthroughpreciseapplicationofnutrients.

Disadvantages:

1.Slowandexpensive(requiresspecializedequipment).

2.Mobilenutrientsmaybelostvialeaching

TimingofFertilizerApplicationDeterminingtheappropriatetimetoapplyfertilizersinthesoilisjustasimportantaschoosingthecorrectamountsofplantnutrientsanddeterminingtheproperzoneinthesoilinwhichtoapplythefertilizer.Farmerssometimesapplyfertilizersoonafterthepreviousyear'sharvest.Whilethiscoincideswithavailabilityofequipmentandlabor,thismaynotbeefficient

agronomicallyasfertilizerappliedtoofarinadvanceofcropdemandmaybelost,resultinginnegativeeconomicandenvironmentalconsequences.Optimaltimingoffertilizerapplicationensuresanadequatesupplyofnutrientsduringpeakandcriticalcropdemandperiods.Thismaximizesnutrientrecoverybythecrop,therebyreducingthepotentialforlossofnutrientsfromthesystem.

Propertimingoffertilizerapplicationsinthefieldaimsto:

1.Provideasufficientamountofthenutrientwhentheplantneedsit.

2.Avoidexcessavailabilityofnutrients,especiallyofN,beforeoraftertheprincipalperiodsofplantuptake(especiallyduringenvironmentallysensitiveperiodsofgroundwaterrecharge).

3.Makenutrientsavailablewhentheywillstrengthen,notweaken,plants.

4.Enablefieldoperationstobeconductedwhenitisfeasible.

Somebasicfactorstobeconsideredinfertilizertimingdecisions.

1.Nutrientelement:Thegreaterthepotentialforlossofagivennutrientfromthesoil,thegreatertheimportanceoftimingofapplication.Propertimingiscriticalwithnitrogenfertilizer,asNissusceptibletolossfromthesoilthroughseveralpathwaysincludingleaching,denitrification,volatilization,andrunoff/erosion(seeChapter4).Forexample,fallapplicationofNforspring-plantedcrops,suchascorn,shouldnotbepracticedinhumidareasbecauseofhighriskofloss.Toslownitrificationinthefallandavoidincreasednitrateleachingand/ordenitrification,fallapplicationofNshouldonlybepracticedinlatefallafterthesoiltemperaturedropstobelow50°F(10°C)andexpectedtocontinuecooling,evenindrierareaswithlowriskofloss.TimeofapplicationislesscriticalwithPandKandfallapplicationisgenerallyconsideredareasonablepracticeastheriskofrunoffissmallinthatseason.However,phosphorusapplicationismostefficientwhenmadeatorasclosetoplantingtimeaspossible,especiallyinlowPsoiland/orsoilswithhighcapacitytoconvertsolublePintolessavailableforms.Applicationtimingisalsoanimportantconsiderationwithpotassiuminsandy,lowcationexchangecapacitysoilsinhigh-rainfallareas,becauseofthehighpotentialforleachingintheseenvironments.

2.TheNform:TheNforminafertilizerproductcanaffectthepotentialforlossandoptimaltiming.InsouthernUS,thetemperaturesfavornitrificationduringagreaterportionoftheyear,henceammoniacalNappliedbeforeplantingwouldbeconvertedtonitrateandlostvialeaching.Fertilizermaterialscontainingsignificantportionsofnitratearenotsuggestedforfallapplication,becauseofleachingandgaseouslosses.Instead,springpre-plantand/orside-dressapplicationsarepreferableastheytypicallyprovidealowerriskoflossandgreaterprofitability.And,evenwithspringapplication,materialscontainingsignificantportionsasnitratearemoresusceptibleforlosswithearlypre-plantapplication.

3.Cropandplantnutrientuptakepattern:Cropnutrientdemandisnotconsistent

throughoutthegrowingseasonbutitischaracterizedbyaninitialstageofslowuptake,followedbyaphaseofrapiduptake,followedbyaperiodofdeclininguptakeasthecropmatures.Thereforefertilizerapplications,particularlyN,canbetimedandtargetedatspecificgrowthstagestoincreaseNuptake(cropyieldand/orquality)andtoreducethepotentialforlosstotheenvironment.Timingoffertilizerapplicationcanalsobetargetedtocorrectspecificnutrientdeficienciesduringthegrowingperiod.

4.Soilcharacteristicsandenvironment:Certainsiteandsoilcharacteristicsinfluencethepotentialfornutrientloss,nutrientretention,andsupplycapacityandareimportantconsiderationsindecisionsoffertilizertiming.Thecharacteristicsincludeslope,soiltexture,temperatureanddrainage.Thegreaterthesoil'scapacitytoretainandsupplyandprovideacrop-availablenutrientthroughoutthegrowingseason,thelesstheneedforacriticaltimingemphasisforthatnutrient.ForexamplePandKfertilizerscanbeappliedonceonmostsoilsintheCornBelttosupplycropneeds.Incontrast,PfertilizerproductsneedtobebandedatornearplantingtimeinsoilswithveryhighPfixationcapacitysuchasthehighlyweatheredsoilsinthesouthernUSAandthecalcareoussoilsoftheWest.PropertimingofNfertilizersisimportantonsandy-texturedsoilsinareasreceivinghighrainfallduringwinterandspring.NitrogenfertilizerapplicationsonthesesandysoilsareusuallysplitintotwoorthreeapplicationstoincreaseNuptakeandreducethepotentialforlosstotheenvironment.Conversely,thereislittleadvantagetosplittingapplicationofNbetweenfallandspringinaridenvironmentswithlowlosspotential,henceallNmaybeappliedpre-plant.

Otherfactorsaffectingfertilizerapplicationtimingdecisionsincludefeasiblyofconductingtheoperation,farmsize,otherfieldoperations,availabilityofequipmentandlabor,andfertilizerdistributionlogistics.

PrecisionFarmingThecomputeragehasledtomanyinnovationsinagriculture.Onesuchinnovationisprecisionfarming.Becausesoilsinmostfarmfieldsvaryconsiderablyinsuchpropertiesasorganicmatter,topsoilthickness,texture,structure,andplantnutrientcontent,itisinefficientforanentirefieldtoreceivethesameamountoffertilizerwhenthecropyieldpotentialandsoilnutrientlevelvaryfromoneareaofthefieldtoanother.Itisclaimedthatfertilizercostsarereduced,theenvironmentisbetterprotected,andcropyieldsarehigheriftheproperamountoffertilizerismorepreciselyappliedtoeachofthevariousmanagementzonesinthefield.ItcanbedemonstratedthatthevariationsinsoilpropertiesareoftenmoredetailedthanthesoilmapsdiscussedinChapter11.

ThefirststepinusingprecisionfarmingtechniquesinvolvessamplingthesoilbyusinganATV(allterrainvehicle)inagridpatternwitheachsamplingpointcommonlyrepresenting2.5acres(1ha)(seeFig.8.5).Managementzonescanalsobeused.ThepositionofeachsamplesiteismonitoredandprogrammedintothecomputerviatheGPS(globalpositioningsystem),whichreliesonasetofsatellitesabout200miles(320km)abovetheearth.TheATVis

equippedwithareceiverthatcollectssignalsfromatleastthreesatellitesforaccuratepositioningbytriangulation(Fig.8.17).Withanenhancedsystem,thesamplingsitesusuallycanbepinpointedtowithin1m.Eachsamplinglocationappearsasapointonacomputerscreen.

Thesecondstepistoanalyzethesamplesfornutrientcontentandothersoilcharacteristics(suchaspHandorganicmatter).Fromtheseresults,apatternofthefertilizerrequirementsthroughoutthefieldcanbeestablishedandstoredonacomputerdisk.

Thethirdsteprequiresafertilizerapplicatorwithseparatebinsforindividualfertilizer,eachequippedwithaugerstotransfertheircontentstothespreadingapparatus.TheaugersareregulatedbyacomputerinthecaboftheapplicatorwhosepositionhasbeendeterminedbytheGPS.Inthisway,therecanbeacontinuousadjustmentintherateandmixtureoffertilizerappliedasthetruckgoesbackandforthacrossthefield.

Inthefourthstep,thecombinethatharveststhecropisequippedwithadevicethatcontinuouslymonitorstheyieldofthegrainharvestedfromallpartsofthefieldinrelationtoapreviouslyprogrammedgridsequencedescribedinthefirststep.Theyieldmapisusedtosettheyieldgoal,whichisafactorusedtodeterminetheamountofnutrienttoapply.

Farmersmaycontractfortheprecisionfarmingtechniquesdescribedpreviouslyandmanyarecurrentlydoingso.Theextenttowhichprecisionfarmingbecomesacommonpracticedependsonhowfarmersjudgeitseconomicvalue.

OrganicFarming/GardeningOrganicfarmingorgardeninginvolvesproducingcropswithoutapplyingcommercialfertilizerorchemicalpesticides.Organicfarmingusuallyincludestheslowreleaseofanaturallybalancedsupplyofnutrientsfromdecayingorganicmatter(Fig.8.18),suchascropresiduesandanimalmanure.Cropsofhighqualityandquantitymaybegrownbyorganicmethods.Inpartsoftheworldwherecommercialfertilizersarenotavailable,farmersmustrelyondecayingorganicmatterforsupplyingcropswithnutrients.Enoughfoodcouldnotbeprovidedfortheworld'spopulationiftheplantnutrientsforfoodcropsweretobesuppliedsolelyfromorganicsources.Therecyclingofcarbonandnutrientsfromplantandanimalmanuresis,however,animportantbenefitoforganicproductiontechniques.Someconsumersprefertoeatfoodgrownwithoutchemicals,sotheyarewillingtopayhigherpricesforcropsthataregrownusingorganicmethods.

Nutrientsfromthesoilareabsorbedbyplantsmainlyintheformofions(chargedparticles).Whethertheseionscomefromaweatheringmineral,decayinghumus,orachemicalfertilizerisofnoconsequencetotheplants.Butthenutrientsshouldbeinaproperlybalancedproportion.

Composting

Compostingisaplannedandmanagedprocesstofostertheaerobicdecompositionoforganicmatter.Unlikeanaerobicdecomposition,compositingdoesn'tproduceodorsandseepingnoxiousliquids.

Compostingisoftenusedforasourceoforganicmatterforgardeners(Fig.8.19).Thebasicingredientsofacompostheapareorganicresidue,soil,moisture(50–70%),nitrogenfertilizer,andsomelimetocounteractacidityassociatedwithdecomposition.Thecarbon-to-nitrogenratioofthematerialmustbecloseto30:1.TheC/Nratiocanbeloweredbyaddingnitrogenfertilizerorbymixinghighcarbonaceousmaterialssuchasstrawwithlowcarbonmaterialssuchaslivestockmanure,greengrassclippings,andlegumehay.Air(oxygen)mustbeabletodiffusethroughthepiletothedecomposingresidueandpreventanaerobicdecompositionfromtakingover.Theheapisallowedtodecompose(rot)forseveralmonthsandisthenmixedintothesoil.Therottingprocessiscarriedoutbymicrobialactionthatisgreatlyhastenedinthepresenceofanadequateamountofnitrogen.Othermaterialssuchasrockphosphatepowder,woodashes,andmixedfertilizersmaybeaddedforimprovednutrientbalanceofthefinalproduct.

Compostingisalsobecomingmorepopularfororganicfarming.Atypicalsystemhaslargerowsoforganicmaterial(leaves,sawdust,ormanure)thatisamendedwithanorganicsourceofnitrogen,andothernutrientsifnecessary.Amachinemixesthematerialintherowsuntilthecompostisstable(partiallydecomposed).Theheatingofthecompost,assomeoftheorganicmaterialdecays,hasanaddedbenefitofkillingmostofthepathogenicordisease-causingmicrobes.Finishedcompostcanbebaggedforsaleinstoresorloadedontotrucksorspreadersforapplicationtofields.

Compostingisagoodwaytoturnmostorganicwasteproductsintoareliableandvaluableplantnutrientsource.However,certainwastesareproblematicandtheyshouldbeavoided.Forinstance;meatscrapsattractrodentsandotherpestsandproducenoxiousodorsaswell,catdroppingscarrymicrobesthatmaybeharmfultoinfantsandpregnantwomen,plasticsandglassarenon-biodegradable,whileplywoodmaybeladenwithheavymetals.

BiosolidsBiosolidsorsewagesludge,arenutrient-richorganicmaterialsresultingfromtheprocessingofdomesticsewageinatreatmentfacility.Biosolidscontainnutrientssuchasnitrogen,phosphorus,andpotassiumandtraceelementssuchascalcium,copper,iron,magnesium,manganese,sulfur,andzinc.Somebiosolidsmightalsobelime-stabilized,resultinginapHincreasewhenappliedtothesoil.Inaddition,theorganicmatterinbiosolidshasseveralbenefits(seeChapter4).Therefore,whenusedinaccordancewithexistingfederalguidelinesandregulations,biosolidscanbeappliedaspartofacropnutrientmanagementplantoproducecropsforhumanconsumptionwithnegligiblerisktoboththeconsumerandtheenvironment.Whenusedtofertilizecrops,thenutrientcontentofbiosolidsshouldbetestedandtheircontributiontothetotalcropnutrientneedsshouldbeaccountedfor.Furthermore,recommendationsfortheappropriateapplicationrate,method,andtimingshouldbefollowed.

Sincebiosolidsareregulatedbyproductsofwastewatertreatment,theyposevarioushealthandsafetyrisksthatmustbeaddressedwhentheyareappliedtothesoil.Theseinclude:presenceofdisease-causingorganismsandheavymetals,odor,andinsectproblemsassociatedwiththeapplicationofraw,unstabilizedwastes;andsurfaceandgroundwatercontaminationbynitrogen,phosphorus,andpathogens.

Chapter9SoilManagementPropersoilmanagementisanimportantpartofanyoperationthatinvolvescropproductionornaturalresourcemanagement.Thegoalofsoilmanagementforcropproductionistoestablishandmaintainthecorrectcombinationofallsoilfactorsnecessarytooptimizeandmaintainproductionefficiency.Effectivesoilmanagementwillensurethatfoodandfiberproductionaremaximizedandsustainedovertheyearswhileleavingthesoilinaproductivestate.

Inagriculture,soilmanagementiscloselyrelatedtocropmanagement.Itincludes(1)maintainingthesoilinagoodphysicalcondition;(2)keepingthechemicalcharacteristicsofthesoilintheproperbalance,suchasmaintainingsoilfertilityandthecorrectpH;and(3)influencingthebiologicalororganicportionofthesoilsothatmaximumbenefitsresult.

PhysicalConditionSoilwithgoodphysicalconditionisimportanttoplantgrowth.Thephysicalconditionofsoilasitrelatestoeaseoftillage,qualityoftheseedbed,andresistancetoseedlingemergenceorrootgrowthisreferredtoassoiltilth.Goodtilthhelpsthewaterinfiltrationrate,water-holdingcapacity,soil−airinterchange,androotdevelopment.Italsoaidsinreducingerosion.Soiltilthcanbemaintainedbycontinuingtoreturnplantresiduesandorganicmaterialstothesoilandbyusingtillagepracticesthatdonotcompactsoil.Physicalconditiondeterioratesifthesoiliscompactedbytillingwhenitistoowetorbyusingheavymachinerytoooften.

ResidueManagementCropresiduesinfluencethesoil'sphysicalcondition.Asoilthatislooseandfriableisgenerallyconsideredtohavegoodtilth.Managingtheresiduesonthesoilsurfaceisimportanttomaintainingthetilth.Oftentheoldcropresiduesareincorporated(buried)intothesoilduringtillage.Thefirstresiduemanagementstepisdecidingifandwhencropresidueincorporationisneeded.Thetimetoincorporateresiduewilldependonthecroppingsystem.Optionsforresiduemanagementrangefromcompleteincorporationofresidue—oftenreferredtoascleantill—tousingno-tillwherealltheoldcropresiduesareleftonthesoilsurface.

Residuemaybeincorporatedanytimefromimmediatelyafterharvesttojustbeforeplantingofthenextcrop.Ifthefieldistobeplantedthefollowingyear,itissometimesdesirabletoincorporateresiduesoonafterharvesttostartthedecompositionprocess.Thishelpstoensurethatasmoothseedbedcanbepreparedandthatthesoilisfreeofexcessiveresidueforplanting.Ifacropisnotgoingtobeplantedthefollowingspring,theresiduecanbeleftonthesoilsurfacetoactasacover.Thiswouldgreatlyreduceerosionbybothwindandwaterandconservesoilwater.

Wherewindorwatererosionisaproblem,partialincorporationofresidueintothesoilmay

bedesirable,withaportionleftonthesurface(stubblemulching).Somefarmersneverincorporateplantresiduesintothesoilandplantthesubsequentcropswiththeresiduestillonthesurface(minimumtillage).

AsdiscussedinChapter4,bacteriadecompose(breakdownorchange)cropresidueafterithasbeenincorporatedintothesoil.Bacteriabreakdowncorn,wheat,andgrainsorghum(allinthegrassfamily)residuesquiteslowly,whereasalfalfa,soybean,andclover(allinthelegumefamily)residuesdecomposemorerapidly.Theprincipalreasonforthedifferenceindecompositionbetweenthetwogroupsistheamountofnitrogen(inproteinform)intheresidue(Fig.9.1).Legumesarehighinnitrogen,whereasgrassesarelowinnitrogen.Ifitiseverdesirabletospeedupthedecompositionorbreakdownofalow-nitrogenresidue,addingnitrogenfertilizershouldhelp.

Figure9.1Grassesarelowinnitrogenatmaturityandaremoreslowlydecomposedthanlegumes,whichcontainmuchmorenitrogen.

TillagePracticesTillagepracticesusedtomanagesoilvarywidely.Thetypeofcropgrown,typeofsoil,erosionhazards,theuse(ornot)ofirrigation,andcostofthetillagepracticeareallconsideredindeterminingwhichpracticestouse.

Tillageofsoilmayvaryfromfarmsonwhichthereisnotillage(exceptplanting)—calledno-tillorminimumtillsystems—tofarmswherethesoilmaybetilled8−10timesayear.Becauseofexcessiveerosionandhighcosts,thetrendformostproducersistoreducethenumberoftillageoperations.Manyfarmersnowuseareducedorconservationtillagesystem,wheresomecropresidueisleftonthesurfaceandthesoilisonlymoderatelydisturbed.Thesesystemscanbethoughtofasacompromisebetweentheextremesoftillage,moldboard

plowing,andno-till.ReducedtillagewasinitiallyadoptedmostlybyfarmersinthehigherrainfallareasoftheeasternandmidwesternpartsoftheUnitedStates.

Inano-tillsystem,allthecropresidueisleftonthesurfaceandseedsareplantedwithaslittledisturbanceoftheresidueandsoilaspossible.Anexampleofno-tillplantingisillustratedinFigure9.2.Inadditiontocostsavings,minimumtillagealsodecreasessoilerosionandnormallyincreasesthewaterinfiltrationrate.Minimumtillagemayrequireamodificationofcertainpractices,includingplanting,fertilization,andapplicationofpesticides.Fertilizationrates,particularlyfornitrogen,mayneedtobeincreasedbecausesoiltemperaturestendtobelowerasaresultofthesurfaceresidues.Surfaceresiduecouldalsoharborinsects,therebyresultinginanincreasedneedforinsectcontrol.

Figure9.2Minimumtillageorno-tilloftenmeansplantingwhileresiduefromthepreviouscropisstillinthefield.

Soilerosionbywaterandwindhasbeenaproblemsincesoilshavebeentilled.Conservationtillagepracticesweredevelopedtoreducethelossofvaluabletopsoilbyerosion;thus,theyofteninvolveleavingaportionofcropresiduesonthesurfaceofthesoil.Bydefinition,conservationtillageisineffectwhen30%ofthesoilsurfaceiscoveredwithcropresidue.ConservationtillagehasbeenpracticedformanyyearsintheareasoflimitedrainfallandpotentialwinderosioninthewesternUnitedStates.Cropresidueisleftonthesurfaceofthesoilnotonlytodecreaseerosionbutalsotoconservewater(stubblemulching).

ReasonsforTillageTillagereferstothemoving,turning,orstirringofthesoil.Thesoilistilledtoaccomplishanumberofthings:

1.Incorporateresidue.Incorporationofsomecropresidueintothesoilhastensitsdecomposition.Withoutthedecayofcropresidues,itcouldbecomedifficulttoprepareagoodseedbedandplanttheseed.DifferentdegreesofresidueincorporationareillustratedinFigure9.3;theimplementsemployedarediscussedinthenextsection.

2.Improvephysicalcondition.Toooftensoilsareworkedorcropsareharvestedwhenthesoilistoowet,causingthesoiltocompactorformaplowpan(Fig.9.4).Somesoilshavenaturallycompactlayers;deepplowingorchiselingcanbedonetobreakupthecompactedsoilorplowpan.

3.Reduceerosionbywindorwater.Sometillagepracticesareusedtoreduceerosion(seeChapter10).Theyincludeplowingonthecontour,terracing,furrowdiking,stubblemulching,andcreatingridgestoroughenthesoilsurface.

4.Preparethesoilforplanting.Thismayconsistofcultivatingbedsinrowsinwhichtoplantseed.Oritmayinvolvealightdiskingorharrowingtobreakupasurfacecrustandatthesametimedestroysmallweeds.

5.Incorporatepesticides,fertilizers,andanimalmanures.Somepesticidesandfertilizersmaybeleftonthesoilsurfaceandstillbeeffective.Mostpesticides,however,areincorporatedeitherwithalightdiskingorbyusingarotaryhoe.Fertilizersandmanuresareoftenincorporatedintothesoilbyeitherdiskingorplowing.

6.Controlpests,includingweeds,insects,anddiseases.Eventhoughmanytypesofweedsarecontrolledbychemicals,tillagepracticesarealsoused.Alightdiskingbefore(orduring)plantingmaybeusedtokillearly-emergingweeds.Deepmoldboardplowingisoccasionallyusedtoturnuprootsofhard-to-killweedssuchasJohnsongrasssothattheywillbekilledbyfreezing.Shallowtillagemaybeusedonfallowlandtocontrolweeds.Tillageisalsousedtoincorporateresiduesorhostplantsthatmightharborinsectsanddiseases.

7.Increasewaterinfiltration.Inareaswheremoistureisoftenlimitingorwhereirrigationispracticed,certaintillagemethodsareusedsothatwater,particularlyrainfall,canmoveintothesoilmorerapidlyorstayonthesurfaceuntilitdoesso.Tillagethatbreaksupasurfacecrustcanincreasetheinfiltrationrate.Formingrowsonthecontourandterracingaswellasfurrowdiking(seeChapter10)helpssoilholdwaterlonger.

Figure9.3Diskingincorporatescropresiduetoashallowdepth,amoldboardplowcoverstheresidue,andachiselplowgoesdeepbutleavesnoresidueonthesurface.

Figure9.4Plowpanscanformatthedepthoftillageandinhibitrootpenetrationbecauseoftheirincreaseddensity.Chiselingorperiodicdeepplowingcanpreventthiseffect.

TillageImplementsSeveraltypesoffarmimplementsareusedtoaccomplishthetaskslistedabove.Thedegreeofsoildisturbancevariesbythetypeofimplement,howdeepitisplacedinthesoil,andhowfastitpassesthroughthesoil.Primarytillageimplementsrefertothosethatdisturbthesoilagreatdealandincorporatesignificantamountsofcropresidue(moldboardplow,chiselplow,anddisk),whilesecondarytillageimplements(disk,fieldcultivator,andharrow)generallytillthesoiltoashallowerdepthandoftenfollowprimarytillageoperations.

Amoldboardplow(Fig.9.5A)isusedtoliftthesoilandcompletelyorpartiallyturnitover.Thiscanbedoneatanydepth,butinmostareas,itis6-10in.(15-25cm).Developmentofamoldboardplowwithsteelshares(the“share”isatthebaseoftheplow;itcutsandliftsthesoilontothe“moldboard”whichinvertsthesoil)wasoneofthekeyfactorsthatenableddevelopmentoftheMidwestandGreatPlainsoftheUnitedStatesasmajorcrop-producingareasinthenineteenthcentury.

Figure9.5Implementsusedforfarming.

Moldboardplowingisoftendonetoincorporateresidueandbreakupdensesoil.Whenfollowedbysomesecondarytillage,moldboardplowinghelpscreateafineseedbedforplanting.Moldboardplowingincorporatesmostofthecropresidues,whichmaybegoodforresiduedecompositionandseedbedpreparationbutcanalsoleavethesoilvulnerabletoerosion.Itwasoncethemostcommontypeofprimarytillagebutitsusehasdecreasedsignificantlyduetothehighcostoffueltopulltheplowandconcernregardingexcessiveerosion.

Deepplowingtoadepthof18−36in.(46−92cm)isdoneincertainareas.Thisisusuallyusedonasoilthathasasandysurfaceandaclaysubsoil,withclaybeingbroughttothesurfacetomixwiththesandyportion.Insomeregions,itisalsousedtoincorporateunderlyingmineralsoilintoasurfacelayerofthickmuck(highorganicmaterial).

Adisk(Fig.9.5B)canbeusedforbothprimaryandsecondarytillagebyvaryingthedepthoftheimplement.Itisoftenusedtoincorporatematerialsintothesurfaceofthesoil,usuallythetop4−6in.(10−15cm).Thisoftenincludeschemicalsforweedcontrol,fertilizersorothersoilamendments,orcropresidues.Alightdiskingisalsousedtocontrolweeds.Diskingorshreddingofcropresidueswithanimplementcalledaflailchoppersometimesprecedesmoldboardplowingtoimproveresidueincorporationandpreventcloggingoftheplow.Onedisadvantageofdiskingisthatittendstocompactthesoiljustbelowthedepthbeingtilled.

Achiselplowisanothertypeofimplementusedinprimarytillage(Fig.9.5C).Itisanimplementpulledthroughthesoilusuallyatdepthsof10−14in.(25−35cm)andusedprimarilytobreakupahardpanorplowsole(adense,compactedlayerofsoilusuallycaused

byfarmimplementssuchasthemoldboardplow).Somechiselplowsaremadesothatthechiselvibrates,causingthesoiltoshatterorloosen.Recently,severalcompanieshaveproducedimplementsthatcombineadisk,chiselplow,andharrow.Theseimplementsarequitepopularbecausethedepth,spacing,andsizeoftheindividualelementscanbecustomizedoradjustedtosuitthefarmer'sneedsandthefieldconditions.Suchimplementsallowfarmerstouseasingleimplementformostoftheirtillageoperations.

AspringchiselorGraham-Hoemeplowisatillageinstrumentusedinthelow-rainfallareasoftheUnitedStates(Fig.9.5D).Itsprincipaladvantageisthatthesoilistilledtocontrolweeds(andpartiallyincorporateresidue)whileminimallydisturbingthesurfacesoilandtherebyreducinglossofsoilwaterbyevaporation.

Alistermayalsobeusedfortillage(Fig.9.5E).Itformsthesoilintobedsorrows6−8in.(15−20cm)highwheretheseedsmaybeplantedorthebedmaybelowered3−4in.(7.5−10cm)beforeplanting.

Aplanter(Fig.9.5F)andacultivator(Fig.9.5G)mayalsobeusedtotillthesoil.Eventhoughtheprincipalfunctionoftheplanteristoplacetheseedinthesoil,thefurrowopenersandotherattachmentsarecapableofcuttingthroughcropresiduessofertilizersandpesticidescanbeappliedduringplanting.Cultivatingwasusuallydoneinthepasttocontrolweedsthatgerminatedafterplanting.However,modernherbicideshavereplacedmuchoftherequirementforcultivation.Cultivationisstillusedinsomeareaswhenchemicalweedcontrolfailsandtobreakupcruststoimproveinfiltration.

ChemicalCharacteristicsItisalsodesirabletokeepthechemicalcharacteristicsofthesoilinproperbalanceorcondition.ThisincludesmaintainingsoilpHinanoptimalrange,providingasufficientandbalancedsupplyofnutrients,preventingoralleviatingsaline–sodicsoilconditions,andavoidingsoildegradationfromtoxicpollutants.

SoilpHSoilpH(seeChapter5)isimportantincropproductionandisanindicatoroftheacidityoralkalinityofasoilaswellasanindicatoroflevelsofcertainnutrientsandtheiravailability.Italsoinfluencesbiologicalactivitiesinthesoil.

Ifasoilistooacid(belowpH5.0),phosphorus,iron,andcertainothernutrientshavelimitedavailability,andlevelsofcalcium,magnesium,andpotassiumwillbelow.Ifasoilistooalkaline(abovepH7.8),phosphorus,iron,zinc,manganese,andothermicronutrientswillhavereducedavailabilitybuttherewillbeanamplesupplyofcalcium,magnesium,andpotassium.

SoilsintheeasternUnitedStatesandinmanyofthehigh-rainfallareasworldwidearemorelikelytobeacid.Ifsoilsintheseareasareincropproduction,thepHusuallyrangesfrom5.0toaround7.0.AsoiltestisthebestmethodfordeterminingwhetherpHneedstobeadjusted.MostsoilsarenotadverselyaffectedbyacidityiftheyareinthepHrangeof6.0–7.0.Ifthe

soilpHis6.0orbelow,itmaybenecessarytoraisethepH(makeitlessacid)bytheuseoflimestone(calciumand/ormagnesiumcarbonate).Limestoneforapplicationtosoil(alsocalledaglime)isrelativelyinexpensivebecauseitcanbequarriedfromabundantdeposits(Fig.9.6).

Figure9.6Agriculturallimeisproducedfromlimestonequarriedfrombedrock.

Theamountoflimestoneneededperacre(hectare)dependsonthecationexchangecapacity(CEC)ofthesoil(seeChapter5).TheCECisdependentontexture,typeofclaymineral,andorganicmattercontent.AtapHof5.5,asoilwithahighCECmightneed3tonsoflimestoneperacre(6.72Mg/ha[mega-grams/hectare])every2–3years,whereasasoilwithalowCECmayneedonly1tonperacre(2.24Mg/ha)toraisethepHtoadesiredlevel(Fig.9.7).Theapplicationratesandamountoftimebetweenapplicationsdependonthecropsbeinggrown,theamountofrainfallandleaching,andtheCEC.

Figure9.7Manyhumid-regionsoilsneedregularapplicationsoflimetocombatacidity.

LimestoneincreasespH(decreasestheacidity)byprovidingcalcium,andinsomecasesmagnesium,whichinturnreplaceshydrogenorotheracidicionsontheexchangecomplex.Withmorecalciumandmagnesium,whicharebases,andlesshydrogenontheexchangecomplex,pHincreases.

InthewesternUnitedStatesandotherlimitedrainfallareasoftheworld,soilstendtobeneutraltoalkaline.IfthesoilpHneedstobedecreased(madelessalkaline)andthesoilisnotsodic(tobediscussedlater),elementalsulfur(S)maybeapplied.UsingsulfurtodecreasethepHoftheentiresoilmasswouldbequitecostly;hence,theeconomicbenefitsusuallywouldbelessthanthereturns.WherehighpHduetocalciumisaproblem,thecommonpracticeistoapplysulfuroranacid-formingsulfurproductintoasmallbandorlimitedsoilarea.

Thesulfurformsanacidicmicroenvironmentinwhichnutrientavailabilitymaybegreatlyincreased.Inthisway,asmallamountofsulfuratalowcostcanbebeneficialforoneseasonincounteractingtheundesirableeffectsofhighpH.Theamountofsulfurrequiredtoeffectthischangeinamicroenvironmentmayrangefrom20to100lbofsulfurperacre(22–110kg/ha)ifproperlyapplied.Sulfurproductsmostcommonlyusedforthispurposeareprilledsulfur(80–90%elementalS),ammoniumthiosulfate(26%S),andammoniumpolysulfide(45%S).

NutrientSupplyKeepingplantnutrientsatadequatelevelsisimportantandpropernutrientbalancesneedtobemaintainedtoensurethatnutrientlevelsareadequatebutnottoohigh.Nexttoirrigation,thisisprobablythesinglemostimportantsoilmanagementfactorthatcaninfluenceyieldoverwhichtheproducerhascontrol.Iftoomuchofanutrientisapplied,notonlyisitaninefficientuseofresources(Fig.9.8)butitalsomayalterthebalanceofnutrients,adverselyaffectplantgrowth,andcreateapotentialsourceofpollution.Forathoroughdiscussionofnutrientsandtheirapplication,seeChapter8.

Figure9.8Anillustrationofhowprofitfromfertilizerismaximized.Inthishypotheticalexample,themostprofitablerateoffertilizationis100poundsperacre(about110kg/ha).Notethatthemaximumyielddoesnotcorrespondtothemostefficientrateofapplication.

SalineandSodicSoilsAnotherimportantmanagementpracticeistoreducetheeffectofsalineandsodicsoilsonplantgrowth.Asalinesoilisoneinwhichsolublesaltshaveaccumulatedinsufficientquantitytoadverselyaffectgrowth.Asodicsoilisonethatcontainstoomuchsodium,whichadverselyaffectsyields.Remediationofsalineandsodicsoilsmayinvolveacombinationofsoilmanagementpractices.

Salineandsodicconditionsmayoccurnaturally,butmostarisewhenirrigationwaterisappliedthatistoohighinsaltand/orsodium.AreasintheUnitedStateswherethesesituationsoccuraremainlyintheSouthwest,fromTexastoCalifornia.Itisapotentialhazardinanyirrigatedareaintheworld.

Irrigationwaterscanbechemicallyanalyzedtodeterminewhethersaltand/orsodiumarehighenoughtocreateproblemsifused.Chemicaltestsarehighlydesirableforanynewirrigationprojectorinareaswherethesalinityofwatermighttendtochange.

Soilsthatbecomesalineshowirregulargrowthofcropsinafieldandusuallyhaveawhitishcastfromsaltaccumulation,withthegreatestamountbeinginthetopsofthebeds(Fig.9.9A).Salinesoilsareoftencalled“whitealkali”becausetheyarelightincolorandhaveanalkalinesoilreaction.

Figure9.9Salinesoils(A)usuallyhave“whitecaps”ofsaltinthetopsofthebeds.Growthofcropsnormallyisspotted.Sodicsoils(B)areusuallydarkcolored(oftencalled“blackalkali”)andaregummyandslickwhenwetandcrackedwithapowderysurfacewhendry.

Ifsalineconditionsdevelop,leachingthesoilwithwaterwithlesssaltisacommonpractice.Thedownwardmovementofwaterinthesoilthatoccursduringleachingcarriesthesaltsbelowtherootzone,wheretheycauselessofaproblem.

Ifasoilbecomessodic,itishighlydispersedandinverypoorphysicalcondition(seeSoilAggregationinChapter5).Thesoilfeelsslickandgummywhenwet.Whendry,thesoilisdark,appearstobehighlydispersed,andhascracks.Sodicsoilsarecalled“blackalkali”becauseoftheirdarkcolorandalkalinesoilreaction(Fig.9.9B).SodicsoilsusuallyrangeinpHfrom8.5to10.0.

Tocorrectasodicsoilcondition,acalcium-containingcompound—specificallygypsum,whichiscalciumsulfate(CaSO4·2H2O)—wouldneedtobeapplied.Thecalciumwouldreplacethesodiumwhichcombineswiththesulfate.Leachingwithhigh-qualitywaterisrequiredtomovethesodiumsulfatedownwardandoutoftherootzone.Thecalciumthenhelpsthesoiltoreaggregateandimproveinphysicalcondition.Insomesodicsoils,calciummaybepresentascalciumcarbonateand,ifso,onlysulfur(anacid-formingtype)isneededtocorrectthesodicsituation.ThegeneralpHmanagementconsiderationsforthesoilsoftheUnitedStatesaresummarizedinFigure9.10.

Figure9.10ThegeneralizedpHmanagementconsiderationsforsoilsoftheUnitedStates.RegionAsoilsaregenerallyabovepH7.0-soilsmaybesalineorsodic.InregionBtheacid-baserelationshipsarecommonlyfavorable,andinregionCthebaseshavebeenleachedsothatlimeandfertilizerareneededinhighamounts.

BiologicalCharacteristicsLifeinthesoil,orsoilbiology,wasdiscussedinChapter4.Oneoftheprincipalpracticesrelatedtothebiologyofthesoiliscropresiduemanagement.Cropresiduedecomposestoreplenishsoilorganicmatter,aprocessthathasimportantandgenerallybeneficialeffectsonsoilbiologicalactivity.Othermanagementpracticesaffectingsoilbiologyincludetheapplicationofanimalmanureorotherorganicmaterialssuchasbiosolids(wastewatertreatmentresiduals)andcanneryorprocessingplantbyproducts.Physicalorchemicalconditionsinthesoilthatadverselyaffectsoilorganisms(pooraerationortoxiclevelsofchemicals)shouldalsobeavoided.

Plantresiduealsoservesasasourceofenergyfororganismsthatliveinthesoil.Ascropresiduedecomposes,nutrientsthatplantsrequirearereleased.Organicacidsareformedthatinturnmayenhancetheavailabilityofcertainplantnutrientsandalsoaidinthebreakdownofmineralsinthesoil.

Afterhumusisformed,additionalcationexchangecapacityispresent.Organicmatterhelpsasoilretainmorewater.Waterinfiltrationratesoftenincreasewithadditionalorganicmatterasaresultofanimprovedaggregationofsoilandenhancedstructure.Soiltemperaturemayalsotendtobeslightlyhigherbecauseincreasedorganicmattergenerallycausesasoiltodarken.Otherbenefitsofincreasedorganicmatterinclude(1)reducedtoxicityofcertainpesticides,(2)increasedbufferinginthesoil,and(3)adecreasedeffectofsaline–sodicsoilconditions.

Cropresidueonthesurfacedecreasesthedetrimentalimpactofraindropsandtherebydecreaseserosioncausedbyrunoffwater.Residuecoveralsodecreaseserosionbywindby

slowingthewindnearthesoilsurface.Inwinter,cropresiduecantrapandholdsnow.Asthesnowmelts,thewatermovesintothesoilwhereitcanbestoredandabsorbedbythecroplater.

Inareasoflimitedrainfall,thisextramoisturemaybecriticalforcropproduction.Surfaceresiduealsoconservesmoisturebykeepingsoiltemperaturesloweranddecreasingthelossofwaterbyevaporation.

Cropresiduecanhavesomepotentiallydetrimentaleffects.Itmayharbordiseasesthatwouldappearthefollowingyear.Weedseedsmayalsobepresentandthusmaygerminatethenextyear.Iftheresidueislowinnitrogen,whichisnormallythecasewithcropslikecornandwheat,temporarynitrogenshortagesmayoccurforthenextcroptobegrowninthatsoil.

Residueisoftenburnedtoavoidoneormoreoftheabove-perceivedproblems.Itisundesirabletoburnresiduebecausecertainnutrientscontainedintheresiduewillbelosttotheatmosphereandmustbereplacedthroughincreasedfertilization.Burningcropresidueisacommonpracticewithrice(Fig.9.11).InCalifornia,somericestrawisbaledandinjectedwithammoniaforanimalfeed.

Figure9.11Forsomecropssuchasrice,thestrawremainingafterharvestissothickthatburningmaybetheonlypracticalwaytomanageit.

CropProductionFactorsCropproductionisinfluencedbymanyfactors;somecanbecontrolledbythefarmerandsomecannot.Thesefactorscanbeclassifiedas(1)soil,(2)crop,(3)environmentalpractices,and(4)culturalpractices.

Themanyfacetsofthesoilarediscussedthroughoutthisbook.Itsphysical,chemical,andbiologicalcharacteristicsshouldbemaintainedunderoptimumconditionsforbestcropgrowth.

Thecroptobegrowninvolvesmanymanagementdecisions.Acropneedstobeadaptedtothesoilaswellastoclimaticconditions.Marketsmustalsobeconsidered.Choiceofcropping

system(whethertogrowonecropcontinuouslyortouseaparticularcroppingsequence)isimportant.AcroppingsystemmayinvolvecontinuouscornintheMidwesternUnitedStatesorasequenceofcorn/soybeansevery2years.IntheSouthwest,acroppingsystemmaybecontinuouscottonorasequenceofcottonandsomeothercropsuchasgrainsorghumorwheat.

Othermanagementdecisionsoncropsincludewhichvarietytoplant.Somevarietiesareadaptedforhigh-yieldingpotentialswherefertilityandmoistureareadequate,whereasothersarebetteradaptedtolowpotentialyieldsituationssuchasindrylandproductionareas.Theplantpopulationofmostcropsisimportantandneedstobemaintainedatanoptimumlevel.

Theenvironmentisgenerallythemostimportantaspectofcropproductionandoftencannotbecontrolledorinfluenced.Lengthofgrowingseason,altitude,daylength,lightenergy,andrainfallareamongconditionsoverwhichthefarmerhasnocontrol.Eventhoughrainfallcannotbechanged,certainmanagementpracticescanbeusedtoconservewater.Contourrows,cropresiduecover,furrowdiking,andsimilarpracticescanbeusedtoincreaserainfallretention.

AsonemovesfromthesoutherntothenorthernUnitedStates,thegrowingseasonshortens.Farmersneedtoplantannualrowcrops(cornandvariousbeantypes)asearlyaspossibletogetthehighestyield.Therefore,itisveryimportantthatthesoiltemperatureiswarmenoughforseedgerminationandgrowth.Onewaytomanagethisfactoristomovethecropresiduefromwithintherowtotheinter-rowareasofthefield.Rowcleanersarewheelsthattravelatanangletotheplantingdirectionandtheygentlymovetheresiduefromtherowareatotheinter-rows.Havingbaresoilintherowareaallowsthesunlighttodry(drysoilwarmsmorequicklythanwetsoil)andwarmthesoilwhichgreatlyimprovesearlyseasonplantgrowth.

Forsomespecialtycrops,managementpracticescanbeusedtomodifytheplantgrowthenvironment.Forexample,additionallightingcanbeusedtocontrolthegrowingseason,daylength,andlightenergyforgreenhouseplants,butlittlecanbedoneformostfield-cropproduction.Anexceptionwouldbetobaccogrownundercheesecloth,forexample,wheretheamountoflightreachingtheplantisreduced.

Usingtherightculturalpracticesattherighttime(management)isimportant.Suchdecisionsstartafterharvestandcouldinclude(1)whethertoincorporateresidueorleaveitonthesoilsurface;(2)whattillagepracticestouse;(3)whentoplantandhowdeeptoplant;(4)thetypeandrateoffertilizerandhowandwhentoapply;(5)therate,time,andmethodofapplicationofherbicides,insecticides,orfungicidesforpestcontrol;(6)whentoapplyirrigationwaterifitisavailable;(7)whentoharvest;and(8)whatmarketingstrategytouse.Andthealternativescouldgoonandon.Aproducermakesmanymanagementdecisionseachyear,withalmosteverydecisionbeingacriticaloneforyieldandprofit.

Conditionsthatcouldlimityieldarenumerous.Afewarecitedhereasachecklisttodeterminewhyyieldsarelessthanthepotential:(1)biologicalhazardssuchasweeds,diseases,orinsects;(2)nonbiologicalhazardssuchashail,excessiveorinsufficientrainfall,earlyorlatefrosts,orextremetemperatures;(3)inadequateplantpopulations;(4)imbalanceofplantnutrientsorimpropernutrientapplication;(5)poorphysicalconditionofsoil;(6)soilpHthat

istoohighortoolow;or(7)impropervariety(cultivar).

Chapter10SoilConservationandtheEnvironmentConservingthesoilhasbecomeincreasinglyimportantastheneedforfoodandfibercontinuestoincrease.Producersoffoodandfiberbecamemoreawareinthemid-l900softheneedtoconservetheirmostpreciousresource.Nowthepublichasbecomemoreandmoreawareoftheneed.Andawarenesswillcontinuetogrow.

Degradationofsoilworldwideisapparent.Awarenessofthisproblemisincreasing.Thoseconcernedwithproperlanduseandsustainedproductionhavecalledattentiontothisproblem.

Relatedtobothconservationanddegradationareenvironmentalconcerns.Thepublichasseentheconsequencesofalackofconservationresultinginnotonlythedegradationofsoilbutalsotheincreaseddetrimentalaspectsofsoilerosionsuchaspollution,waterqualityproblems,andsoon.

ErosionProcessesIfrainsaregentleonnativegrasslandsandforests,thewatersoaksintothesoilandpercolatesdownwardthroughthesoilprofile.Ifthereisenoughrain,iteventuallydrainstothewatertable.Whenrainfallsfasterthanthesoilcanabsorbit,somewatersoaksinandtherestrunsoverthesoilsurfacestothelowestpartofthefieldandeventuallyrunsoff.Therunoffwaterfromvegetatedlandmaylookclear,butitisactuallycarryingsomemineralandorganicmatterderivedfromexposedsoil,earthwormcasts,andthediggingactivitiesofants,moles,badgers,andsimilarorganisms.

Thisprocessrepresentsnormalorgeologicerosion.Insomeplaces,geologicerosionmaylowerthegroundsurfaceonlyaninch(2.54cm)in5,000years.Thisisprobablyanormalrateforhillsandvalleystoform.Indesertareaswherevegetationissparseandrainfallisinfrequent,therateoferosionisnormallyslowerpartlybecausemanydesertsoilsurfacesareoftencoarse-texturedandarmoredwithalayerofpebbles.Geologicerosionincludeslandslidesthatoccurinmountainousareasaswellasthecollapseofriverbanksintofloodwatersduringperiodsofexcessivelyhighrainfall.Geologicerosionisanongoingprocesswhereasacceleratederosionoccursduetohumanactivitiesthatincreaseerosionrates.Thischapteraddressestheimpactsofhumansonacceleratederosionprocesses.

Inareaswhereaprotectivevegetativecoverisnormal—suchasiscommoninprairiesandforests—erosionlossesareminimal.Unfortunately,itisusuallyhumanactivitiesthatcauseacceleratederosionduetotheremovalofvegetationbytillageorovergrazingbylivestock.Mostagriculturalsystemstendtoaccelerateerosion,butitistheobligationofthosewhomanagefarms,ranches,andforeststokeeperosiontoaminimum.Higherosionlossesdegradestreamsandrivers,thesourceofapproximately75%ofthedrinkingwaterintheUnitedStates.Thissedimentnotonlyimpactsaquaticorganisms,butalsoitisamajorcostforwater

treatmentplantstoremovethesedimentpriortowatertreatment.

EuropeanimmigrantstotheUnitedStatesclearedthenativevegetationtomakecultivatedfields.Theirlivestockoftenovergrazedrangelands,whichweakenedtheplantcoverandcompactedthesoil.Asaresult,theoriginalconditionofthesoildeterioratedrapidlyanderosionintensified.

Theworderosionliterallymeansgnawingaway.Inregionswherethesoilparentmaterialisloose,suchaswherethereisthickloessorglacialtill,thelong-termeffectsoferosionareseriousbutnotasdevastatingaswheresolidbedrockistheparentmaterial(Fig.10.1).Thelatterincludesmostoftheearthexceptforthevalleys,whichhavebeenfillingwithsedimentovermillionsofyears.

Figure10.1Loosesubstratum(A)slowlydevelopsintosoilifsurfaceerosiontakesplaceataslowrate.Wheresoilisthinoverbedrock(B),erosionofthesurfaceleavesabarrenlandscape.

Soilerosionisaseriousproblemonmuchoftheworld'scultivatedland(Fig.10.2).Alookatmostlargeriversshowsustheycarryaheavyloadofsedimentthatcomesfromwatersheds.Awatershedisallthelandareathatyieldswaterfromrainandsnowtoaparticularriver.Awatershedneednotbecomea“soilshed”too.Inaridregionswherevegetationissparseandrainsarequiteoftenintense,erosionmaybeseverewithouthumaninvolvement.

Figure10.2Agriculturalsystemscommonlyaccelerateerosion.

ErosionbyWaterBothwaterandwinderosionconsistofthreeprocesses:detachment,transportation,anddeposition.Soilerosionbywateroccurswhenparticlesareloosened(detached)andcarried(transported)bymovingwater.Thesoilparticlesareeventuallydeposited.Aslongassoilparticlesareclusteredintoaggregates,theyarenoteasilymoved.Butifthesoilisexposedtothebombardingforceofraindrops,theaggregatestendtobreakapartandthedetachedparticlesaresubjecttomovementintherunoffwater(Fig.10.3).Soilaggregatesareimportanttowaterinfiltrationandrunoff.Whentheaggregatesbreakapart,thesmallerparticlesplugthelargersoilporesimportanttoinfiltration.Keepingcropresiduesonthesoilsurfaceprotectstheseaggregateswhichkeepthesoilporesopenresultinginhigherwaterinfiltrationrates.Assoonasthewaterinfiltrationrateislessthantherainfallrateonthesoil,runoffoccurs.

Figure10.3Theimpactofraindropscontributestoerosionbybreakingupsoilaggregatesandsplashingsoildownslope.

Ithasbeencalculatedthattherateoffallofaraindropisabout20miles(32km)perhour.Thekineticenergygeneratedbya2-in.(5-cm)rainon1acre(0.46ha)isabout6millionfoot-pounds(4.4millionjoules).Thisisenoughenergytoraisea7-in.(18-cm)layerofsoil3ft(0.9m).Clearly,thisisnotallusedtomovesoil,butitdoesshowthattheimpactofraindrops

releasesalargeamountofenergythatcontributestosoildetachmentanderosion.Ona10%slope,60%ofthesoilsplashedbyraindropimpactmovesdownslopeandonly40%isthrownupslope.Thenetmovementdownhilliscalledsplasherosion.

TypesofWaterErosionThemaintypesofwatererosionaregullyerosion,rillerosion,sheeterosion,andstreambankerosion.

Sheeterosionistheplaningoffofalandsurfacebywateractionwithoutformationofchannels.Thisgenerallyhappenswherethereisnotenoughcoverofvegetationoverthesoiltopreventerosionbutenoughtopreventrilling.Sheeterosionisoftenunnoticedbecauseonlyafewmillimetersofsoilmaybelosteachyear.Onewaytodeterminethesheeterosionrateistofindanexposedsoilsurfaceandlookforasmallpebbleonthetopofthesoil.Ifthepebbleissettingonapedestal,thesheeterosionratecanbedeterminedifoneknowswhentillagelastoccurred.

Rillandsheeterosionalsooftengounnoticedbecausetillagedestroystheevidenceuntilmostofthetopsoilisgoneandthesubsoilisexposedatthesurface.Althoughlessspectacularthangullyerosion,rillandsheeterosioncausethelossofagreatdealofmoresoil.

Themostspectaculartypeofwatererosionisgullyerosion,whichoccurswhenwaterconcentratesinachannelanddeepensitrapidly(Fig.10.4).Usuallyagullyisdefinedasachannelsodeepthatfarmequipmentcannotsafelycross.ThisiswhathappenedoveralongperiodtotheGrandCanyonoftheColoradoRiver,anditishappeningonasmallerscaleonthousandsoffarmsthroughouttheworld.Agullygenerallystartsattheoutletofachannelandworksitswayupstreambywaterfallactionatitshead.Thegullyextendsitselfupstreambyundercuttingthefloorofthechannel.Onalargerscale,NiagaraFallsisalsoworkingitswayupstream,asshownbytheoccasionalfallofmassesoflimestonebedrockfromtherim.Bycontrast,rillerosionconsistsoftheremovalofsoilonasideslopebysmallchannels(Fig.10.5)thatarenotdeepenoughtointerferewithtillageequipment.

Figure10.4Gullyerosioncanbespectacular.

Figure10.5Rillandsheeterosioncanresultingreatsoilloss.

Streambankerosionisanothertypeoferosioncausedbyswiftmovingwaterthatoccurswhenchannelsarewidenedduringfloodornear-floodconditions.Theincreasedenergyofthestreamundercutsthestreambanktothepointthatlargeblocksofsoilactuallydropintothechannel.Thebestwaytopreventstreambankerosionistomaintainriparianzoneswithadequatevegetationsuchastreesandgrasses.Ifstreambanksaredisturbedthroughconstruction,overgrazing,orchannelization,streambankerosionratesskyrocket.

WaterErosionControlItisanoldadagethat“anounceofpreventionisworthapoundofcure.”Thiscertainlyappliestosoilerosioncontrol.Becausesoilparticlesdonotmoveuntiltheyaredetached,everyeffortshouldbemadetopreventthisfromhappeninginthefirstplace.Twofactorsshouldbekeptinmindinthisregard.Thefirstisthatthestrong,water-stableaggregatesusuallyassociatedwith

highorganicmattercontentallowwatertoinfiltrateharmlesslyintothesoil.Second,acoverofvegetationorplantresiduesfunctionstodissipatetheenergyofraindropssotheycannotstrikedirectlyonthesoilaggregates(Fig.10.6).

Figure10.6Twosafeguardsagainstsoilerosionarevegetativecoverandwell-aggregatedsoil.

Thefundamentalprincipleofpreventingthetransportationofdetachedsoilparticlesbyrunoffwateristoreducetherateofflowdowntheslopes.Slow-movingwaterdoesnothavetheenergytotransportalargeload,andtheslowerspeedallowsmoretimeforthewatertoinfiltrate.

ContouringandStripCroppingOneofthecommonwaysofcontrollingrunoffistosubdividetheslopingfieldsintocontourstrips(Fig.10.7).Eachstripisapproximatelyperpendiculartothedownhillpaththewaterwouldtake.Alternatestripsusuallycontainarowcropsuchascorninonestripandaforageorsmallgraincropinthealternaterow.Theridgesinthestripcropreducethewaterflowandminimizeerosionthere,andlittleerosioncanoccurinthedenseplantcoverinthealternatestrips.No-tillplanting,inwhichthepreviousyear'scropresidueisleftonthesurface,isanotherpracticethatreduceserosionevenwhenitisnotdoneonthecontour.Itpermitsuseofagriculturalequipmentthatistoolargeforcontourstrip-croppingoperations.

Figure10.7Contouringisveryhelpfulincontrollingrunoff.

GrassWaterwaysWherewaterconcentratesinchannelsandflowsinanuncontrolledmanner,thechannelsmaybecomegullies.Grasswaterways(Fig.10.8)canreducetheriskofgullyformationbecausethedensefibrousrootsystemofgrassesholdsthesoilsecurelyagainsttheforceoftheflowingwater.Grasswaterwaysmaybeplantedinnaturalchannelsthatleaddownslope,indiversionditchesonthesidesofslopes,andatthefootofadjoiningslopes.

Figure10.8Agrassedwaterwayofferserosionprotection.

ConservationStructuresIfthenaturalslopeofthechannelistoosteep,itcanbeflattenedoutbetweensteelorconcretestructurescalleddropspillways(Fig.10.9),whichactasstepsinthechannel.Ateachstep,thewaterdropsoveraheadwallandfallsonaconcreteapronfromwhichitflowsonwardthroughagrassedwaterway.

Figure10.9Adropspillwayisanerosioncontrolstructurethatpreventsgullyerosion.

Incommercialagriculturetoday,conservationpracticeshaveadvancedtothepointthatgulliesareseldomamajorproblem.Thesizeoffarmmachineryissuchthatifgulliesstarttoform,theyareeasilyremoved.Ifgulliescontinuetoform,however,farmersoftenareforcedtodividetheirfieldsinconvenientlybecauselargegulliescannotbecrossedwithmachinery.Fencesareundercut,androadshavetobereroutedaroundadvancinggullies.Insomeinstancesbuildingshavetoppledintothem.

Tostoptheadvanceofagully,thesoilmustbestabilizedatitsheadwherethewaterfallactionisoccurring.Sometimestheheadofthegullyisfilledinwithconcreteorrubble(riprap),andthesidesandbottomareplantedwithtrees,grass,andshrubs.Thesepracticesmaybecombinedwiththeuseofachutethroughwhichrunoffwaterpassestopreventanyfurtherundercuttingatthegullyhead.Variousothertechniquesmaybeemployed,dependingonthespecificproblemsathand.

ContourRidgesandTerracesAprincipalpointtokeepinmindisthatinmanycasesitisdesirabletokeepasmuchofthewateraspossiblewhereitfallsorisappliedbyirrigationsothatitcanmoveintothesoilandbeusedbyplants.Thisisveryimportantinaridandsemiaridareaswheremoistureisoftenthelimitingfactor.Contouringandstripcroppingaretechniquesinthiscategory.Anotherapproachisfurrowdiking,whichisanoldpracticethatisnowbeingusedmuchmorecommonlyindrylandareas(Fig.10.10).Adikeisplacedineachfurrowatintervalsof6–12ft(1.8–3.6m),witharesultingdepressionholdingwaterbetweenthedikes.Thisgreatlydecreasesrunoffand

improvesplantgrowth.Inthefurrowsthetractorfollowsduringharvest;thedikesmustberemovedbysweepsmountedinfrontofthetractor.

Figure10.10Furrowdikestrapmostofthewaterthatfallsasrainorbysprinklerirrigationsoitcanbeusedbythecrop.

Anothermechanicalerosioncontrolpracticeisterracing.Theremaybenooldererosioncontrolmethodthantheconstructionofterracessomeofwhicharemorethantwothousandyearsold.Insomepartsoftheworld,entiremountainsideshavebeenmodifiedintoaseriesofstepsbybenchterraces,eachonesupportedbyaretainingwallmadeofstones(Fig.10.11).Tremendousamountsoflaborwererequiredtoconstructthem,andtheystillrequirelabortomaintain,whichisverycheapinsomeareas.Terracesonamuchmoremodestscalehavebeenusedcompatiblywithmodernagriculture.Inthesecases,ridgesarebuiltseveralyards(meters)apartacrosstheslopeofthehillstotrapwaterfallingonthesoilbetweentheterraces.Presently,thetrappedwaterisusuallyallowedtoflowdownthehillthroughburiedtilessothaterosiondoesnotoccur(Fig.10.12).Indrierareas,thewatermaysimplybeheldabovetheterraceuntilitsoaksintotheground.

Figure10.11AgriculturalterracesforriceproductioninthehighlandsofVietnam.Theseterracesareirrigatedandhavebeenproductiveformorethan1,000years.

Figure10.12Parallelterracesmaybedrainedbyburiedtiles.

ErosionbyWindWhereasthelossofsoilbywatererosionismorewidespreadthanbywind,itwasthedevastatingwinderosionofthedroughtyearsofthemid-1930sintheUnitedStatesthatfocusedattentiononalltypesofsoilerosionandbroughtaboutlegislationdedicatedtosoilconservation(Fig.10.13).Inthelatepartofthenineteenthcenturyandintheearlytwentiethcentury,muchoftheprairieintheMidwestwasplowedwithhugesteamtractorsandplantedtowheat.Inaddition,muchoftheunplowedlandwasovergrazedsothatthesoilwasalsoleftwithaninadequatevegetativecovertoaffordprotection.Whenthedroughtandwindscameinthe1930s,soilfromtheGreatPlainswasblownaloftinominousdarkcloudsthatsometimesreachedtheAtlanticCoastandevenouttosea.Fenceswerecovered,roadditcheswerefilled,andfarmyardsweresmotheredwitherodedsoil,andtherewasmuchdespairintheheartsoffarmersandcityfolksalike.Eventoday,winderosioncontinuestomovevastquantitiesofthe

mostfertiletopsoil,therebyreducingtheproductivecapacityofcropland.

Figure10.13Althoughlesscommonthanwatererosion,winderosioncanbedevastating.

TypesofWindErosionWinderosionoccurswhenthewindissufficiently(a)strongtomovesoilparticlesalongthesoilsurface;(b)turbulenttokeepparticlessuspended;and(c)gustytokeepsoilmoving.Therearethreeprocessesthatoccurduringwinderosionevents:saltation,surfacecreep,andsuspension(Fig.10.14).

Figure10.14Winderosiontransportssoilparticlesbycreep,saltation,andsuspension.

Saltationisthebouncingofmediumandfinesandparticlesalongthesurfaceaftertheyinitiallystarttoroll.Thebouncingsandparticleshitothersoilparticleswhichthenprojectotherparticles(sand,silt,orclayparticlesorsmallsoilaggregates)intotheair.Surfacecreepistherollingofcoarsesandparticlesonthesoilsurface.Damageoccurstogrowing

plantswhensurfacecreepandsaltationoccur.Sandparticlesconstantlybombardthegrowingplantsandeventuallykillthemifthewindissevere.Rowcropslessthanabout4in.(10cm)tallcanbecutoffbytheblowingsoil.Toprotectthecrop,farmerswilloftentillstripsontheridgesofthefieldtobringmoistsoiltothesurfaceinthehopethatthehighwindswillbetemporary.

Theabrasiveactionofthesandparticlesloosensfinersoilparticles,siltandclay,andbringsaboutthethirdtypeofwinderosion,suspension.Thesefinesoilparticles,siltandclay,areoftensuspendedintheair,resultinginaduststorm,andmaybecarriedhundredsofmiles.Althoughthismaynotbetoodamagingtogrowingcrops,thelossofvaluabletopsoilwithitsorganicmatterandnutrientsiscostly.Damagemayalsooccurwhenthesoilparticlesaredeposited.Depositionmaybeattheedgeofafieldwheresandisdepositedormanymilesawayfromtheoriginoftheparticles.RecentstudieshaveshownthatdustfromdesertsinAfricablowsacrossthesouthAtlanticandsuppliesnutrientsforplantgrowthonotherwiseinfertilesoilintheAmazonbasinofSouthAmerica.

ControlofWindErosion

VegetativeCoversControloferosionbywindisachievedbyslowingdownthewindatgroundlevel,muchthesameascontrollingerosionbywaterisaccomplishedbyslowingtherunoff.Vegetativecoveristhemosteffectivemeansofcontrolbysimplykeepingthesoilcoveredsothatitcannotbeblownaway.Thevegetationmaybealiveintheformofagrowingcropcover,oritmaybedeadplantresidueshelteringthesoil(Fig.10.15).Oftencropresidueofthisnatureispartiallyincorporatedbutaportionisleftonthesurfacetopreventblowingofthesoil.ThecommontillagemethodtoachievethisstubblemulchingisshowninFigure10.16.

Figure10.15Anunprotectedsoilsurface(A)inviteserosion,butcropresidueonthesurface(B)givesprotectionfromwindandwatererosion.

Figure10.16Stubble-mulchingloosensthesoilbutleavesmostoftheplantresidueonthesurface.

RoughenedSoilSurfaceLeavingthesoilinaroughenedconditionalsohelpscontrolwinderosion.Anyobstacleinthewayofamovingsoilparticledetersitand,inmanycases,stopsit.Theobstaclemightbecloddysoil,pittedareasinthesoilsurface,orridgedrowsthathavebeenprepared.FarmersintheGreatPlainsoftheUnitedStatesuseasandfighter,whichisanimplementtakenoverafieldafterarainwhenthesoilsurfaceissmoothtocreatethousandsofsmallpitsandmoundstoimpedethemovementofsoil.Thisisusuallydoneinthespringwhenwindspeedsarehighestandcropshavejustbeenoraresoontobeplanted.Usingasandfightermaykillafewplantsifthecropisup,butdamageisminimal.

StripCroppingStripcroppingisanothermeansofminimizingsoilmovementbywind.Thepatternwouldbesimilartothatusedforerosioncontrolbywater,suchaspreparingaseedbedinthestandardmethodfor8–12rowsbutleaving1–2rowsoftheoldcropstanding.Stripcroppingisoftenusedonvegetablesgrownfollowingavolunteerwheatcrop.

WindbreaksWindbreaks,orshelterbelts(Fig.10.17),areusedinmanyareaswhereerosionbywindisaproblem.Aplantingoftrees,shrubs,orgrassstripshelpstoslowthewindanddecreasesoilmovement.Tobeeffective,thedistancebetweenwindbreaksshouldnotbetoogreat.Thedistanceforbestcontrolvariesfromoneareatoanother.Windbreaksareusedlesstodayduetotheadventoflargermachineryfortillageandharvestoperations.Farmingaroundwindbreakstakesmoretimeandthetreesinthewindbreakconsumesoilmoistureneededby

thecrops.ContactyourlocalNaturalResourcesConservationServicepersonnelforspecifics.

Figure10.17Shelterbeltsandroughsoilsurfacescanreducewinderosion.

SoilStabilizersSoilstabilizersmightalsoreduceerosionbywindorwater.Theseincludeorganicmatterthatactsasabindingagent,chemicalsthatcausesoilparticlestoaggregate,andwater.Waterstabilizesthesoilforthetimewhenitismoist,butwhenthesurfacedriesout,itisagainsusceptibletowinderosion.

ErosionbyMassWastingMasswastingisanothertypeoferosionthattakesplaceinseveralways.Massesofsoilsometimesmoveundertheforceofgravity,asbycave-insalongriverbanks(Fig.10.18)andslumpalonggullysidesandroadbanks.Soilcreepisacommontypeofmasswastinginregionswherethesoilfreezesandthawsrepeatedlyeachautumnandearlyspring(Fig.10.19).Frostcausesthesurfacesoiltoexpandperpendiculartotheslope;whenthesoilthaws,itdropsvertically.Thiscausesarippleeffectonthehillsides.Cattleoftenmakepathsthataccentuatetheseripples.Masswastingnotonlymayremoveportionsofcultivatedfieldsandpasturesbutalsosectionsofroadsandevenhomes(asinthemudslidesinCalifornia).

Figure10.18Erosionlosscanbegreatfromcave-insalongriverbanks.

Figure10.19Cattlepathsaccentuateripplesmadebymasswasting.

Diversionofwaterawayfromsusceptibleareasishelpful.Protectionofriverbankswithriprap,timberwork,andvegetationreducestheincidenceofmasswasting.Insomecases,channelizationisneededbut,ingeneral,isonlyatemporarysolution.Similarprotectionisalsobeingsoughtforshorelinesandbeaches;hugesand-filledplastictubesarelaidatthefootofanendangeredbluffatalakeorseashoretoblocktheforceofwaves.ThecityofMiami,Florida,hasspenthugesumsofmoneytryingtodecreasebeacherosionalongmuchofitswaterfront.

Wheneversoilismovedbywind,water,orgravity,itislikelytocauseimmediateproblemsinadditiontothoseofamorelong-rangenature.TheoccurrenceofmassivemudslidesinsuchplacesasCaliforniausuallyattractswidespreadmediaattention.Thisacceleratederosionisoftenattributabletothelossofvegetationduetofireorlogging.Housesmaybefilledwiththeflowingoozeorevensweptdownthehillsides.

SedimentasaPollutantErodedsoilfromuplandsiscarriedbyrushingwateruntilthewaterslowsdownanddropsitsloadofsedimentinriverchannels,harbors,andreservoirs(Fig.10.20).Dredgingthesesedimentstoremovethesedimentiscostly.Depletedchannelsincreasethethreatofflooding,whichcausesuntoldhardshipstohomeownersandbusinesses.Damsbuilttocontrolfloodwatersandprovideotherservicessuchasrecreationandelectricpowerlosetheirvalueasthecapacityoftheirreservoirsdecreases.Inextremecases,expensivenewdamshavebecomeaburdentothetaxpayerinaslittleas25years.

Figure10.20Erosionnotonlyreducesthevalueofcroplandbutalsocausesserioussedimentationproblems.

Sandcanbeasedimentsinceitfillsditches,coversroadsandfences,andcausessimilar

problems.Whenitisblownalongbythewind,itisaseriousproblem,whetherincentralWisconsinorinNewMexico.Valuabletopsoilislostandproductivitydeclines,andsmallplantsarecutofforabradedsothattheyfallpreytodisease.Theabrasiveactionofsandcanremovepaintfrombuildingsandautomobiles,andresultintexturalchangesinsoilsurfacehorizons.Thislossofsoilisalsoalossofvaluableplantnutrientsandcancarrypesticidesintoreceivingstreamsandlakes.

Itshouldbeapparentthatsoilconservationpays.Itpayswherethesoilisretainedbecausethelandcanbemoreproductive,anditalsocutscostsdownstreamwherethecapacityofreservoirsisprotectedandwaterqualityfordomestic,irrigation,andrecreationalusesispreserved.

OneofthefewbenefitsofsedimentationisthatitresultsintheformationofalluvialsoilsinfloodplainsandatthemouthofriverssuchasthedeltaoftheMississippiRiver.

ExtentoftheProblemAnaturalresourceinventoryreportedbytheNaturalResourcesConservationService(NRCS),in1992,revealedthatthenationalaveragesoillossperyearonallcroplandduetowatererosionwas3.1tonsperacre(T/A)equalto6.9megagramperhectare(Mg/ha)peryear.Thefigurevariesfordifferentpartsofthecountry,rangingfrom6.2tonsperacre(13.9Mg/ha)peryearinAlabamato0.1(0.2Mg/ha)inColorado.

Manyhighlyerodedareasexperiencelossesinexcessof20tonsperacre(45Mg/ha)peryear.Drierregionswithlowamountsofwatererosionmayexperiencelossesfromwinderosionthatmatchorexceedthesevalues.Nationwide,winderosionlossesaverage2.5tonsperacre(5.6Mg/ha)peryear.Comparativevaluesforyearspriorto1992were3.2T/A(7.2Mg/ha)in1987and3.3T/A(7.4Mg/ha)in1982.NRCSreportedthe1992winderosionfiguresfortheindividualstates.ThehighestwinderosionoccurredinNewMexico,whichlost13.5T/A(30.2Mg/ha)peryear,whileseveralstatesinthehigherrainfallareasshowednowinderosionlosses.SomeofthesestatesareAlabama,Arkansas,Connecticut,Georgia,Hawaii,andKentucky.

Toplacethesefiguresinperspective,itisworthwhilenotingthat1in.(2.54cm)ofsoilweighsabout150tons(136Mg).Byusingsomeofthesoil-lossfigurescitedabove,itiseasytocalculatehowlongittakestoloseaninchoftopsoilatthoserates.Forexample,at5tonsperacre(11.2Mg/ha)peryear,1in.(2.54cm)ofsoilislostevery30years.

Itisimportanttorememberthaterodedsoilisnotnecessarilyremovedfromafarmandtransportedtoalakeortothesea.Itismorelikelytobeshiftedfromthehighgroundtothelowground.Thehighgroundbecomesmuchlessproductive,andcropsonthelowgroundmaybedamagedbysedimentandeventuallythosesoilsmaybecomemoreproductive.

Despitethegreatworkofresearchersandgovernmentagencies,theNRCSinparticular,onlyaboutone-halfofthefarmersintheUnitedStateshaverequestedconservationplansfortheirland,andonlyaboutonehalfofthosehavecarriedthemout.Someofthebest-managedfarms

mayhavechangedownershipandresultedinthesoilconservationpracticesbeingabandonedbythenewowners.

AnexcellentUSDepartmentofAgriculture(USDA)publicationbyW.C.Lowdermilkin1953,“ConquestoftheLandthrough7,000Years,”citesmanyexamplesofOldWorldstateswhosefoundationswerebaseduponagricultureandwhosedemisecamewhentheirabusedlandwasnolongerproductive.Insomecases,populationincreasesplacedtoohighademandonthefragilesoil,andinothers,politicalinstabilityledtoneglectofboththesoilandtheirrigationsystems.

Itmayappeartosomefarmerstodaythatsoilconservationreallyisnotveryimportant.Forinstance,ayieldof200bushelsofcornperacre(12.5Mg/ha)canbeachievedontheblackprairiesoilsofIllinoisunderanannualrainfallof40in.(1,000mm).Whereversoildeterioratesbyerosion(andthatisjustabouteverywhereincultivatedfields)andbycompaction,whichreducessoilaeration,farmerscanstillobtainhighyieldsbyaddingmorefertilizer,byplantingmorevigoroushybridcornvarieties,andevenbyirrigatinginseasonsoflowrainfall.Thusgoodcropyieldsareachieveddecadeafterdecade,whilethesoilisslowlybutsurelywastingawayunnoticed.NewsreportsofcropsurplusesintheUnitedStatesandmanyothercountrieslullthepublicandpoliticiansintocomplacencyonthestateofthesoilresource.

OurU.S.populationcontinuestogrow.Withthisgrowththerehavebeenastronomicalratesofurbanizationthatdestroysmanyacresofprimefarmlandeachday.Hopefully,itwillbecomeanationalprioritytokeepourprimefarmlandgrowingneededfoodcropsandnotgrowinghousesandseptictanks.Ifcurrentpopulationgrowthcontinues,thislandwillbeneededtofeedfuturegenerations.

Chapter11ConservationAgricultureConservationAgriculture(CA)isanagriculturalmanagementapproachthatevolvedtosustainablyaddressissuesofsoilerosionasdescribedinChapter10andsoilmanagementasdescribedinChapter9forcropproduction.TheFoodandAgricultureOrganization(FAO)oftheUnitedNations(UN)definesCAasanapproachtofarming(cropproduction)thatsustainsandimprovesproductionforbothfoodsecurityandprofitswhilealsopreservingandenhancingenvironmentalresources(UnitedNations,2015).CAisbasedonthefollowingthreeprinciples:

1.Minimumsoildisturbance,

2.Permanentsoilcover,withcropresidues,mulch,and/orcovercrops,

3.Croprotation/diversity,

Theseprinciplesarewidelyapplicabletomanydifferentcrops,climates,andsoils;forexample,theyhavebeensuccessfullyappliedinarangeofclimatesfromtheArcticCircletothetropics.

CAPrinciplesMinimumSoilDisturbance

PlantingCultivationofcropsrequiresplantingofseeds.Therearemanyapproachestoplantingseeds,fromdirectseedingwhereseedisdroppedintoaslotthatisthenpressedclosedwithminimalsoildisturbancetointensivetillagethatpreparesa“clean”seedbedwithprimarytillageorinversionofthetoplayerofsoilwithamoldboardplowfollowedbysecondarychisels,disks,andharrowtillageresultinginacycleoferosionthathasseverelydegradedsoils.CAadvocatesminimumsoildisturbance,wherecropseedsareinserteddirectlyintothesoilthroughtheresidueofthelastcrop,omittingthestepsofseedbedpreparationwithintensivetillage.Directseeding/sowing,directdrilling,no-tillfarming,zerotillage,andno-tillagearealltermsusedtodescribethisapproach.Largeseedssuchasmaizeandbeanscanbeplacedatadesireddepthwithmechanizedequipmentcalledno-tillplantersordrillsthatcutthroughtheresidue,openaslotandplacetheseedintothatslot,ideallyclosingtheslotsothattheseedisingoodcontactwiththesoilandthesoiliscoveredbytheresidue.Verysmallseedsmaybebroadcastandwillgerminatewithoutincorporationjustasoccursinnativeecosystems.

Iftractor-drawnplantersarenotavailable,no-tillorminimumtillagecanalsobecarriedoutmanually,typicalofsmallscalefarming,usingseveralmethodsincludingplantingbasins,

plantingspots,rippers,plantingsticksorjab-planters,andanimal-drawnplanters.Aplantingbasinisaholedug6in.(15cm)wideby12in.(30cm)longby6–8in.(15–20cm)deepandspacedinrowsabout30in.(75cm)apart,usingahoeasthediggingimplement.Basinsarepreparedduringthedryseasonandafterbeingdug,inputssuchasmanure,fertilizer,andlimecanbeplacedinthebasin,whichisthenpartiallyfilledwithsoiltotheplantingdepth.Atseedingtimeafterthefirstrain,multipleseedsareplacedinthebasinandthencoveredwithsoiland/ormulch.Shortlyaftergerminationtheplantsareoftenthinnedto2–3plantsperbasin.Afterbasinsaredugthefirstyear,theycanbereusedinsubsequentyearsthussavinglabor.Basinsdirectrainfalltotheseedareaandprovideaframeworkformorepreciseplacementoffertilizer.

Plantingspotsaremoreshallowplantingholesdugtotheseedplantingdepthwithahandhoe,sowedwithseed,andthencoveredwithsoil,andcanbeplantedwithlesstimethanbasinsinthedryseasonorjustaftertherainsbegin.About2weeksafterthecropemerges,astickcanbeusedtomakeaholeabout2.5in.(10cm)awayfromtheseedlingtoplacefertilizerforplantgrowth.

Awoodenstickwithapointedend,calledaplantingordibblestickcanbeusedtocreateaholetoacertaindepth.Theseedisdroppedinthehole,whichisthensteppedontoclosetheholeandensuresseedcontactwithsoil.Jabplantersarelight-weight,hand-carried,and-operateddevicesthathaveaseedhopperconnectedtoashaftthatdelivers1–3seedsatatimetoasteelfurrowopenerattheendoftheshaft.Theactionofpullingthetwohandlesapartandsimultaneouslyjabbingthetipoftheshaftintothesoil,setstheseedintothetip.Asecondactionofpullingthehandlestogetherdeliverstheseedsand/orfertilizerintotheholecreatedbythetip.Jab-planterscancomewithoneortwohoppersforseedandfertilizer.Jabplanterscanalsobeusedtofillgapsthatarenoticedaftergermination.

Rippersarechisel-pointedtoolsthatopenanarrowslot(furrow)about2–4in.(5–10cm)deepforsowingseedsbyhand,amacheteorplantingstick.Amechanicalplanterattachedtotheripperitselfcanalsobeusedtoinserttheseedsintotheslot.Rippedlinesareusuallyspacedabout30in.(75cm)apart.

Severalvarietiesofanimal-drawn,hand-operated,automatic,andsemi-automaticplanterdevicesareavailableforpurchaseandusethatcanreducethetimeforseeding.

WeedControlWeedscompetewithcropsformoisture,nutrients,space,andsunlight.Weedscanalsoharbordiseaseandpeststhatcanattackcrops.Traditionaltillageagricultureusedtheplowasthemajorweedcontroltechniqueincorporatingweedsandresidueintothesoil,providingacleanseedbedinwhichtoplantandalsoprovidingnutrientsforthesubsequentcropfromtheincorporatedplantresidues.InCA,weedsarekilledandleftinplaceusingherbicides,byhandweedingorusingequipmenttocutorcrushweeds,avoidingsoildisturbanceasmuchaspossibleandleavingplantresidueontopofthesoil.

Herbicidesdonotdisturbthesoilandcanbequickandeasytoapply.Herbicidesrequirespecialapplicationequipmentsuchassprayersorwipersandtheyrequireknowledgeor

trainingtodeterminewhichherbicidesareappropriateandhowtoprepare,handle,andapplythemcorrectlyandsafely.Herbicidesmaynotbeaccessibleoraffordablebysmallholderfarmers.

Weedingwithhandtoolsorhoescanbeusedtomanuallydislodgeweeds.Thoughhandweedingisharderwork,itgenerallydisturbsthesoillessthanusingahoe.Kniferollerskillweedsandcovercropsbybendingandcrushingthem,andcanbeusedbeforeseedingthecrop.

InCA,weedsarealsocontrolledbyplantingcropsclosertogethertoshadeoutweeds.Idealcropspacingdependsonsoilmoisture,temperature,andfertility,andhastheobjectiveofavoidingcompetitionbetweenthecropplantsforwaterandnutrients.Weedsarealsocontrolledusingcovercrops,mulch,andcroprotationsasdescribedinthefollowingsections.

PermanentSoilCoverwithCropResidues,Mulch,and/orCoverCropsMakingsurethesoiliscontinuouslycoveredwiththemaincrop,mulch,cropresidues,and/orcovercropsprotectsthesoilfromtheerodingforceofraindropsandexcessiveheatingbythesun.Preventingtheerodingforceofraindropsonexposedsoilreducessoilcrustingandsurfacesealingenablinggreaterrainfallinfiltrationandreducedsurfacerunoffandflooding.Constantsoilcoveralsoreducessoilmoistureevaporationlossestherebyincreasingsoilmoisturecontentavailabletogrowingplantsduringdryperiods.

Therearetwomaintypesofsoilcover,including:(1)livingplantmaterialsuchascropsandcovercrops;and(2)mulch,compost,and/ordeadplantmaterial,whichincludescropresiduesandcuttingsorleavesfromgrass,shrubs,andtrees.Acombinationofmulchandcovercropsmaybeusedtokeepthesoilcovered.

Byreducingtheforce,speed,andsplasheffectsofraindrops,residues,mulch,andcovercropsallowhigherinfiltrationofwaterintothesoilandreducerunoff,whichalsodecreasessoilerosion.Theresiduesalsoformaroughphysicalbarrierthatreducesthespeedofwaterandwindoverthesurface.Reducingwindspeeddecreasesevaporationofsoilmoisture.

Soilcoverprovidesthefollowingbenefits:

Foodandhabitatforsoilmicro-andmacroorganisms(Chapter4),whichperformimportantrolesdecomposingorganicmatter,nutrientcycling,soilmixing,andthedevelopmentofsoilporesandstructure.

Insulationfrommaximumheatingandcoolingprovidingamoretemperatemicroclimateforoptimalgrowthofsoilorganismsandplantroots.

Weedsuppressionbycompetingwithweeds,restrictingsunlight,andreducingweedseedgermination.

Improvedwaterinfiltrationandretentionofsoilmoisture,makingmorewateravailabletocropsoveralongerperiodandincreasingavailabilityofplantnutrients.Byincreasingwaterinfiltration,waterrunoff,anderosionarereduced.

OneofthebiggestgeneralbenefitsofCAmaybereducedsoilerosion.Erosionofsoilintowaterwaysleadstofillingwaterreservoirs,lakes,andstreamswithsediment,reducingwaterstoragecapacityandtheusefullifeofreservoirsanddams.Sedimentinsurfacewaterincreaseswearandtearinhydroelectricinstallationsandpumpingdevices,whichresultinhighermaintenancecostsandnecessitatesearlierreplacement.

BecausemorewaterinfiltratesintothesoilwithCAratherthanrunningoffthesoilsurface,streamsarefedmorebysubsurfaceflowthroughsoilthanbysurfacerunoff.Thus,surfacewatersarecleanerandmorecloselyresemblegroundwaterinCAthaninareaswhereintensivetillageandaccompanyingerosionismoreprevalent.Greaterinfiltrationwillreduceflooding,bycausingmorewaterstorageinsoilandslowerreleasetostreams.Infiltrationalsorechargesgroundwater,whichcanincreasewatersuppliesofwellsandsprings.Sedimentsinsurfacewatershavetoberemovedfromdrinkingwatersupplies,soareductioninerodedsedimentinstreamscanleadtolowercostsforwatertreatment.

Acovercropcanbeplantedduringthecroppingseasoninbetweenthecroprows,knownas“intercropping.”Forexample,alowgrowingplantsuchasbeanscanmakeagoodintercropwithatallplantsuchascorn.Covercropsmaybeplantedfollowingthegrowingseasontocoverthewholefield,thoughinaridortemperateclimateswithcoldwinters,itmaybehardtogrowormaintainacovercropduringthenon-growing/dryseason.Acovercropcanalsobeplantedinthemaincropresiduetowardstheendofthegrowingseason,whichgivesthecovercropaheadstarttogrowatthebeginningofacoldordrynon-growingseason.

Covercropshavemultipleuses,includingasedibleseedsorvegetables,asanimalfodder,firewoodorfencingmaterial,medicine,ortoincreasesoilfertilityaswithnitrogen-fixinglegumes.Acovercropthatisusedtoaddnutrientssuchasnitrogentothesoilisknownasagreenmanurecovercrop(GMCC).Acovercropisselectedbasedonhowitaddressestheneedsofthefarmerorcroppingsystem.

Somecovercropsmaybeharvestedforfoodorascashcropssuchaswinterwheat(Triticumaestivum).Covercropsmayalsobeselectedbasedontheirabilitytoproducehighamountsofresidueforfodderorforstrongrootdevelopmentthatcanpiercehardpansandreducesoilcompaction,suchasradish(Raphanusspp.)orpigeonpea(Cajanuscajan).Somecovercrops,suchasmarigolds(Asteraceae),haveinsectpestrepellingcapability,orweedsuppressioncapabilitywithallelopathiccompounds,suchasryegrass(LoliumperenneL.),whichcaninhibitthegerminationandgrowthofsomeweedspecies.

CropRotationCroprotationisakeyprincipleofCAbecauseithelpstocontrolweeds,diseases,andpestsandithelpstoimprovesoilfertilityandstructure.Croprotationinvolvesthediversificationofcropspeciesandthesequencetheyaregrown,orthevarietyofcropsgrowntogether(inassociation)forperennialspecies.Rotatingcropsincreasesthebiodiversityofthesoilenvironmentandreducesthecarryoverandgrowthofcrop-specificpestsanddiseasesthatresultfromplantingthesamecropinsuccession(monocropping).Croprotationincreasesbiologicalactivityinsoilsbyprovidingdifferentfoodsourcesanddiverserootingstructures.

Somecropshavestrongdeeprootsthatallownutrientsandmoisturetobeextractedfromdeeperlayersinthesoil,whileotherplantshaveshallowfinerootsthatcreatestructureandchannelsathigherlayers.Thesedifferenttypesofrootsenablenutrientstobeextractedandrecycledoveralargerarea.Differentplantsextractandstoredifferentnutrients,sovaryingthecropsplantedimprovesthedistributionandbalanceofthemajorplantnutrients(nitrogen,phosphorous,andpotassium)andminorplantnutrientsavailableinthesoil,contributingtodiversityofthebiotaaboveandbelowthesurface.

Croprotationsareplannedaccordingtoobjectivesoffood,fodder,andresidueproduction;pestandweedcontrol;andnutrientuptakeandproduction.Somecroprotationsmayhaveacycleofsevenormoredifferentcropspeciesinasequence.Speciesdiversitycanbeachievedbyplantingamixtureofcovercropspeciesinoneseason.Cropselectionisalsobasedonsoilandclimateconditions.Croprotations,multiplecropsgrowninayear,and/orintercroppingcanreducerisk,sinceasinglecropcanfailduetoadroughtorattackbypests.

Croprotationhelpstoreplaceplowingofthesoilbyaeratingthesoilwithdifferenttypesofroots,addingtheorganicmatterfromrootsatvariousdepthsinthesoil,recyclingnutrients,andcontrollingweeds,pests,anddiseasesthatcanliveintheresidueandsoil.

SynergiesbetweenthePrinciplesThoughthesearewellknownprinciples,inCAtheyarecombinedtoworkinconcert,forexample,maintainingsoilcoverandreducingsoildisturbancebynottillingreducestheerosionthatresultsinlossofsoil,reducedsoilfertility,andsoilcompaction.Allthreeprinciplesworktogethertoincreasebiodiversityandsoilorganicmatter,whichincreasessoilfertility.Theseprinciplescapitalizeonnaturalbiological,chemical,andphysicalprocessesaboveandbelowground,especiallythephysical,chemical,andbiologicalpropertiesoforganicmatterthatholdswaterandnutrientslikeasponge,therebysupportingbothplantandanimallifeandnaturalecosystemservices.

Theconstantadditionofcropresiduesfromnottillingandkeepingcontinuoussoilcoverwithresidues,mulch,andcovercropsleadtoanincreaseintheorganicmattercontentofthesoil.Inthebeginningthisislimitedtothetoplayerofthesoil,butwithtimethiswillextendtodeepersoillayers.Organicmatterplaysanimportantroleinthesoil;fertilizeruseefficiency,waterholdingcapacity,soilaggregation,rootingenvironment,andnutrientretentionalldependonorganicmatter.

InCAsoilplants'rootsandmacrofaunasuchasearthwormsperform“biologicaltillage”alsocalledsoilbioturbation,inplaceofthemechanicaltillageofplows.CharlesDarwinestimatedinhisbookentitled:TheFormationofVegetableMouldThroughtheActionofWormsWithObservationsofTheirHabits(1881)thatearthwormscouldturnover16tonsofsoilperacre(40tonsperha)peryear,andDarwinpointsoutwithgreatforesightthat:

Theploughisoneofthemostancientandvaluableofman'sinventions;butlongbeforeheexistedthelandwasinfactregularlyploughed,andstillcontinuestobethusploughedbyearthworms.Itmaybedoubtedwhethertherearemanyotheranimalswhichhaveplayedsoimportantapartinthehistoryoftheworld,ashavetheselowlyorganizedcreatures.

Thoughallthreeprinciplesaregenerallyrequiredtorealizeoptimalresults,oftenCAisadoptedinastep-wisefashionwithoneortwoprinciples.Thoughitmayappeareasier,forexample,toimplementno-tilltostart,therecanbealargecostsuchastheproliferationofweedsandpestswhenonlyno-tillisapplied.

CAAdoptionHistoryofCATheeventsleadinguptothedevelopmentofCAbeganwiththeDustBowlintheGreatPlainsoftheUSA.FollowingtheU.S.FederalHomesteadActsinthelate1800s,whichsettledmuchoftheMidwesternplainstates,andtheincreaseinwheat(TriticumaestivumL.)pricesduringWorldWarI,farmersplowedmillionsofacreswithnewgasolinetractorsinthesoutherngreatplainsstatesoftheUSthroughthe1920s,referredtobymanyasthe“greatplow-up.”Whenan8-yeardroughtbeganin1931,agriculturallanddriedupand100millionacresofplowed,unprotectedcroplandlostmostorallofitstopsoilfromwinderosion.TheU.S.SoilConservationService,establishedin1935,addressedsoilerosionwithconceptstoprotectthesoilincludingwindbreaks,contourstrips,terraces,grassedwaterways,andcontourplowing,thefirstchangeinconventionaltillage.

OneofthefirsttoperceiveandpubliclyquestionthedamagecausedbytillageusingthemoldboardplowwasEdwardFaulknerinhisbookPlowman'sFolly(1943),pavingthewayforotherstoseriouslyconsiderandexploreno-till.SimultaneouslyMasanobuFukuokabeganexperimentinginJapanwithno-tillconcepts;however,hisworkOneStrawRevolution(1978)didnotgetglobalattentionimmediately.

Aftertheinventionoftheherbicides2,4-D,atrazine,andparaquatinthe1940sand1950s,no-tillageresearchgainedmomentuminthe1960sintheUSAandtheUK,andthefirstmechanizeddemonstrationfarmtrialsshowedeffectiveuseofno-tillin1961.Withthedevelopmentofequipment,severalUSfarmsanduniversitiesdemonstratedsuccessfulapplicationsofno-tillwithavarietyofcrops,andUSuniversitiessetupExtensionProgramstopromoteno-till.Equipmentmanufacturersbegansellingno-tillplantersinthemid-1970s,whichmadeno-tillpracticaltoadoptbylargeandmediumscalefarmers.

No-tillalsogainedafootholdinBrazil,duetoagovernmentpolicyencouragingashiftfromlivestockfarmingtocroppingsystemsinthehighrainfallhillyareasinsouthernBrazil,totakeadvantageofrapidgrowthinglobaldemandforsoybeansinthe1960s.Farmeruseoftheplowhadproducedseveresoilerosion,whichdramaticallyreducedyields.ABrazilianfarmercontactedtheUniversityofKentuckyintheearly1970s,whichprovidedaccesstoearlyno-tillequipment,launchingacollaborationinBrazilbetweenfarmers,researchers,andequipment

manufacturers.Duringthe1980sfarmersandresearcherssupportedbytheBraziliangovernmentandindustryadaptedequipmentforclaysoils;andfromtheirexperiencesaddedthepracticesofrotationandcovercropstono-tilltoformthebasisforthethreeCAprinciples.ThiswasfirstadoptedbylargerBrazilianfarmersinthe1980sandthensmallerfarmersinthe1990s.

Fromthe1970stothe1990s,farmers,researchers,andequipmentmanufacturersinBrazilandtheUSAdevelopedandadvancedno-tillfarmequipmentandmanagementpracticestoimproveperformanceoffieldoperationsandcrops,reachingabaseofadoptionthatenableddemandandcorrespondingsupplybyindustryofno-tillagefarmequipment.

Withalotoftheearliestresearchinto-tilltakingplaceintheUSA,thelargestno-tillareaintheworldasestimatedin2009wasintheUSAwith88millionacres(35.5millionha),thoughthepercentageofcroplandunderno-tillintheUSAis35%,andonly10%ofUScroplandisundercontinuousno-till,withtheother25%usingsomeformoftillage,likestriptillage.Table11.1showsthecountrieswiththegreatestpercentageofcroplandunderCAasreportedtotheFAObetween2008and2014(FAOSTAT,2014).Asof2007,about76%ofgloballandunderno-tillwasintheAmericas,12%inAustraliaandNewZealand,5%wasinAsia,4%inRussiaandKazakhstan,withonly1%inEurope,and1%inAfrica.

Table11.1Areaoflandunderno-tillinthetopcountriesasreportedtoFAOfrom2009to2014

Country Millionacres Millionhectares AreaofCAas%oftotalarablelandUSA. 88 36 22Argentina 67 27 71Brazil 79 32 44Australia 44 18 36Canada 45 18 39Paraguay 7 3 62Uruguay 3 1 36

ThoughinitialresearchalsotookplaceintheUKandothercountriessuchasNigeriaandKenya,adoptionhasbeenslowinEuropeandAfricadueinlargeparttotraditionalfarmerpractices.

TheUNFAObeganformallypromotingtheadoptionofCAaroundtheworldsince2002andprovidesanextensivesourceofinformation,educationalmaterialsandadoptionstatusonitswebsite(at:www.fao.org/ag/ca).

ChallengestoAdoptionofCAMakingamajorchangetoafarmingoperationcanhavehighstartupcostsincludingplantingequipmentandthetimerequiredtolearnandeffectivelyapplyanewsystem.Growingcrops

andmanagingafarmiscomplexanddespitetheeffectivenessandrelativesimplicityofCA,applyingthethreeprinciplesdependsontheconditionsandmustbetailoredtothespecificclimate,crop,andsoil.Inadditiontothecropandenvironmentalsetting,agriculturedependsheavilyonmanagementdecisionssuchastheplantingpopulationsandthetimingofplanting,fertilizerapplications,andweedcontrol.EffectivemanagementdecisionsarecrucialforachievingoptimalyieldswithCA.Becausetillagemineralizesincorporatedorganicmatterthroughmicrobialdecomposition,itprovidesaflushofnutrientssimilartoafertilizerapplication.Thishasashort-termbenefitfortheimmediatecrop,butoverthelong-termtillagereducesthisnutrientstockthroughthelossofsoilandorganicmattercontentfromerosionanddecomposition,whileCAbuildstheorganicmatterstocksandreduceserosion.Becauseofthisshort-termeffect,tillagehasbeenassociatedwithincreasedfertilityandhashadtheappearanceofbeingavaluabletool,untilmostoftheorganicmatterislostandthesoilisdegraded.Withoutaddingnutrientsbackintothesoil,suchaswithcompost,mulch,manure,and/orfertilizer,cropswillnothavesufficientnutrients.Thissoildegradationresultingfromtillagemaybeonereasonforthehistoricaluseoffallowperiodsandshiftingagriculturetoenablenaturalecosystemstorestorenutrientsthroughre-vegetation.

Addingnutrientssuchasfertilizer,canoftenmakeupforthelossoforganicmatterandnutrientsindegradedsoils,however,thisisoftencostly.Fertilizeralsodoesnotmakeupforthemanypositivequalitiesoforganicmatter,suchaswaterholdingcapacity.Evenwithfertilizeradditions,overtimefarmersfindthattheircropyieldsdeclinewithdegradingsoils.

Farmersusedplowingandburningofplantresiduestoincreasefertilityandtocontrolweeds,diseaseandpests.So,oneofthegreatestbarrierstoadoptionofCAischangingfarmers'intuitiveunderstandingabouttillage.Diseaseandpestscanincreasewiththechangefromtillageagriculturetono-till.Managingpestsandweedswithoutplowingorburningisnotsimpleorobvious.IntegratedpestmanagementiscriticalforsuccessinCA,anditrequiresknowledgeandtraining.Croprotationanddiversityhaltsthegrowthofspecificpestsandplantdiversitytakesadvantageofthechemicalandphysicalinteractionsofdifferentspecies.Whilesyntheticpesticides,especiallyherbicidesareindispensableespeciallyinthebeginning,afterbeneficialorganismsbecomeestablishedandorganicmatterisincreased,itispossibletoreducetheuseofchemicalpesticides,herbicides,andfertilizers.

BecausethechangefromconventionaltoCArequiressignificantstartupcosts,isachangefromdeeplyingrainedtraditions,hasasteeplearningcurve,andmostoftenproducesgreaterweedsandloweryieldsinthefirstfewyears,thereisasignificantbarriertoadoption.Weedsarenotedasoneofthebiggestproblemsandcantakeseveralyearstogetundercontrol.AlsoduringthetransitiontoCAsomepestsanddiseasecancreateproblemsuntilamorediversebiologicalcommunitytakeshold.ThemainbenefitsofCAmaytakefrom3to7yearstoberealized(Pope,1989).

ErosionandseverelydegradedsoilsinSouthAmericaandinthesouthernUSAhelpedtospuradoptionofno-tillinthoseareas,becausedecreasingyieldsonerodedanddegradedsoilsrepresentedagreatercost.Byofferingsubsidies,thegovernmentofBrazilwasableto

promoteadoptionbyfarmersandthedevelopmentofano-tillequipmentindustry.Withouteducationalprogramsandgovernmentsupport,itmaybedifficultfortheaveragefarmertoovercomethebarrierstoadoption.

Also,thereisnoexactrecipeforapplyingCAtoaspecificlocation(soilandclimate).Achievinganewfarmecosystembalancerequiresfarmerobservationandtesting,thatis,adaptiveresearchonthefarmerscaleandthesharingofknowledgegainedbetweenlocalfarmers.RealizingmanyofthebenefitsofCA,cantaketime,evendecades.ApprehensionaboutconvertingfromtillageagriculturetoCAcanbeovercomewiththeformationoffarmercommunitiesthatprovideaforumfortheexchangeofideasandknowledgeamongfarmerspracticingCA,buildingafoundationoflocalknowledge.

Thoughcontinuousno-tillprovidesminimumsoildisturbance,somefarmersthathaveadoptedCAwilloccasionallytillforvariousreasonssuchasincorporatinglimeindryclimates,orcombatingherbicideresistantweeds,increasingnutrientmineralization,ortocontrolcertainpests.

TrendsinNo-TillAdoptioninMechanizedAgricultureIn2012theUSDAestimatedno-tillpracticesat92millionacresintheUS.Advancementsmadeingeneticallymodifiedcrops,suchassoybeansresistanttotheherbicideglyphosate,andthedevelopmentofnewfertilizers,insecticides,andherbicideshavecontributedtoadoptionbylargeandmediumscalefarmers.

EquipmentandfuelcostsarethegreatestcostconsiderationforlargescalefarmersconsideringadoptingCA.Intheearly1990slighter,precisionseedingmodelswerereleasedthathelpedtospuradoption.InBraziltherearenow300differentmodelsofcommercialno-tillseeders(Calegari,etal.,2013).Whilethetractorsusedforno-tillplantingdonotrequireasmuchpowerorfuel,reducingthosecosts,theswitchtoCAdoesrequireaninitialinvestmentinnewplantingequipment.Analysesshowthatlabor,fuel,andequipmentcostsaresmallerovertime,butadditionalcostssuchasherbicideandpestmanagementcanoffsetthesesavings.ComparisonofCAsystemswithconventionaltillagesystemshasnotshownconsistentlyhigherfinancialreturnsexceptwhereerosionwasanissuedegradingsoilfertilityandwhereshorterfieldpreparationtimewithCAenabledmorecropsordoublecroppingduringagrowingperiod.

Overthelongterm,CAhasbeenshowntoincreaseprofitabilityoverconventionalfarmsinsomeareas,asa10-yearcomparisonof18largeandmedium-sizedfarmsinParaguayshoweda300%increaseinnetincomewhilethenetincomeonconventionalfarmsfellduringthatsameperiod(Sorrenson,1997).IntheUS,greateryieldswerefoundwithadoptionofno-tillinthewesternandsouthernregionswherethewaterconservingaspectsofCAhelpedaddresswatershortages,howeverinnorthernregions,especiallyonpoorlydrainedsoils,yieldscouldbelowerwhereno-tillpostponedplantingdatesandshortenedthetotalgrowingseasonwithsoilsthatstayedcoolerlongerinthespring.

No-tillplantingequipmenthasevolvedtoincreaseefficiencyandyieldwithmoreprecise

seeding.Toincreasetheeffectivenessofplantersonheavyclaysoils,doublediscopenerswerereplacedwitharippertineonsomeplantingequipment.Seedingspeedscanresultinbouncingandmisplacedseeds.Equipmentmanufacturersdevelopsolutionstotheseandotherproblemswithdevicessuchasrebounderattachmentsandotherinnovations.Preciseseedspacingiscriticaltoreduceseedgaps,weedpressure,andcompetitionbetweenplants,andcanbeaccomplishedwithmeteringsystems.Seedfirmershavebeendevelopedtoincreaseuniformityinseeddepth.Specialclosingwheelshavealsobeenaddedtocoverseedswithsoilthatislessdense.

Becauseresiduecaninterferewithplantersandseedplacement,andcanbelargeforsomecropssuchascorn,doublediscopenershavebeendevelopedtoslicethroughresiduetoimproveseedplacement.Somecombinesusedforharvestingalsohaveimplementsthathelptochopandevenlyspreadtheresidue.Stripheadershavebeendevelopedtostripthegrainfromthestalkandsomecombineshaveresidueshreddersthatshredresiduesastheypassthroughthecombine.Recentequipmentinnovationshaveincreasedspeedsofplanting,toincreasetheamountoflandthatcanbeplantedperdayforlargerfarms.

Somefarmersbothintillageagricultureandno-tillareadoptingprecisionagriculturewithgeographicinformationsystems(GIS)andin-fieldsensorsthatcanprovidefeedbackabouttheneedsofthecropandproblemsinthefield.Thisdataassistswithoptimaltimingofplanting,fertilizationandharvesttoimproveyieldandprofitsandlowercosts.Equipmentmanufacturersandfertilizercompanieshavealsocomeforwardwithinteractivedecisionsupporttoolsforprecisionagricultureandotherservicestoassistgrowers.

Inmanydevelopedcountries,especiallyinEurope,reducedandno-tillhasnotbeenadoptedinlargepartduetochallengesmanagingweedswithouttillage,especiallywheregovernmentpolicyrestrictselevateduseofpesticidesandherbicides.Someresearchhasexploredalternativestoherbicidalcontrolofweedstoaddresstheneedsofno-tillandorganicagricultureandinconsideringtheadventofherbicidalresistantweeds.ResearchinBrazilhasshownthatsomecovercropssuchashairyvetch(ViciavillosaRoth),blackoat(AvenastrigosaSchreb.),andoilseedradish(RaphanussativusL.)areeffectiveatreducingweedpopulationsandreducingtheamountofherbicideneeded.Methodssuchasincreasingcovercropbiomasstosuppressweedsandallelopathicinteractionshavebeenstudiedwithmixedresults.

TrendsinHoe-TillAdoptioninSubsistenceAgricultureMostprogressinadoptionbysmallholdersubsistencefarmershasbeeninSouthAmerica;adoptionhasbeenmarginaloutsideofBrazil,Paraguay,andUruguay,wheregovernmentprogramshaveprovidedsupportandeducationthroughextensionservicestopromoteadoptionbysmallfarmers.StudiesofnetfarmincomeofsmallholdersinSouthAmericaweregreaterwithCAthanconventionalpractices.AnimaldrivenrippersandseedersweredevelopedinBrazilforsmallscalefarmers,andhavebeenexportedtoAfricaforsmallholderfarmermechanization.

Other“homemade”equipmentdevelopedbyandforsmallfarmersincludesthekniferoller

(roller-crimper)thatisdesignedtobendoverandcrushcovercropsandweeds,andothervegetationflattenerstopresstheresiduedownbeforeplanting.Woodenandmetalsubsoilerswherethemoldboardplowshareisreplacedwithametalpointtoreducesoildisturbancehavebeendevelopedthatcanbeusedwithdraftanimals.Alsoametalchiselbasedontheconventionalsubsoilerdesignhavereplacedthemetalpointwithathinnerandlongermetalspinedevelopedtopiercethroughcompactedsoillayerswhiledisturbingthesoilless.

Weedcontrolbecomesanissueifthereisnotenoughlaborornoaccesstoherbicides.ThereisalsoacommonviewthatCArequiresincreasedfertility,whichinthepasthasbeenaccomplishedbyplowingresiduesintothesoil.Withoutthatflushofnutrients,fertilizerinputatleastintheinitialstageswouldberequiredtoavoidareductioninyieldwithCA.Fertilizerinputsforthesmallholderfarmerareoftenexpensiveornotaccessible.

ManysmallholderfarmersinAfricamustconsidertheuseofresiduesasfeedandbeddingforlivestockwheretheyareintegratedintofarming.Residueshaveotherpurposessuchasconstructionandasafuelsource.Smallholderfarmersalsofaceissuesinadoptingcroprotations,whentherearenomarketsforalternativecrops,especiallylegumes.

Intercroppingoflegumesmaybeonesolutiontoincreaseresidueproductionandnutrientrequirements.However,thetimingandcropselectionareimportanttoavoidcompetitionbetweencropplantsandloweryields.

WithrespecttoequipmentinSouthAsia,two-wheeledtractor-mountedplantersarereplacinganimaldrawnplowsandplanters.

SummaryofCABenefitsByincreasingbiodiversity,increasingwaterinfiltration,andreducingsoilerosion,CAhasmultiplebenefitsforagricultureandecosystems.Insummary,CA:

reducessoilerosionanddegradation,which:

maintainssoilstructure,quality,andfertility

improveswaterandairquality

reducesvegetation/ecosystemloss

reduceslossofnutrients

reducessedimentbuildupinreservoirs

increaseswaterinfiltration,which:

enhancesresiliencetodroughtandotherextremeweather,suchashurricanes

reducessurfacerunoff

reducesflooding

reducessoilerosion

rechargesgroundwaterandaquifers

increasessoilorganicmatter,which:

improvessoilnutrientsandwaterstoragecapacity

improvessoilfertilityandstructure

reducesdependenceonchemicalinputsreducingenvironmentaldegradation

restoresagroecosystemhealth

sequesterscarbon

maintainscropyieldsandoftenincreasesyieldsonsoilsthathavebeendegraded,which:

increasesfoodsecurityandreducesruralpoverty,

minimizesexpansionofagricultureintolesssuitableareas,reducingdeforestationandnativeecosystemdestruction

reducescostsoffarming,byreducingfueluse,equipmentmaintenance,andcanreducelaborandproductioncostsovertime,which:

increasesprofitsforeconomicsustainability

reducesgreenhousegasemissionsfromtillageandreplacingtillageequipment(tillingthesoilconsumesmoreenergythananyotherfarmingoperation)

Enhancesnaturalresourcesandbiodiversityofagroecosystems,which:

increasesthevarietyofsoilorganisms,includingbothfaunaandflora,includingwildlife.

BibliographyCalegari,A.,GuilhermedeAraujo,A.,Costa,A.,Lanillo,R.F.,CasaoJunior,R.,&RheinheimerdosSantos,D.(2013).InR.A.Jat,K.L.Sahrawat,&A.H.Kassam,ConservationAgricultureinBrazilofBookConservationAgriculture:GlobalProspectsandChallenges,(Chapter3,p.85.Oxfordshire,UK:CABI.

Darwin,C.(1881).TheFormationofVegetableMould,ThroughtheActionofWorms,WithObservationsonTheirHabits.London:JohnMurray.

UnitedNations(2015).“WhatisConservationAgriculture?”Retrievedfrom:FoodandAgricultureOrganizationofTheUnitedNations:Helpingtobuildaworldwithouthunger.Availableat:www.fao.org/ag/ca/1a.html(accessedonFebruary9,2015).

FAOSTAT.(2014).AQUASTATdatabase–UNFAO.Availableat:www.fao.org/nr/water/aquastat/data/query/index.html(accessedonJuly16,2014).

Faulkner,E.H.(1943).Plowman'sFolly.London:MichaelJosephLtd.

Fukuoka,M.(1978).TheOneStrawRevolution:AnIntroductiontoNaturalFarming.Emmaus,Pennsylvania:RodalePress.

Pope,R.O.(1989).Age-hardeningbehavioroftwoIowasoilmaterials.Doctoraldissertation.IowaStateUniversity.

Sorrenson,W.,Portillo,J.L.,Derpsch,R.,&Nunez,M.(1997).Economicsofno-tillageandcroprotationscomparedtoconventionalcultivationcroppingsystemsinParaguay.BibliothecaFragmentaAgronomica,2.

Chapter12SoilClassificationandSurveysSoilclassificationmakessafeandproductiveusespossibleforeachkindofsoil.Inthistimeofincreasingpressureontheland,thesystematicapproachofmodernsoilclassificationisagreathelpinavoidingabuseofsoilsandmistakeninvestmentsinlandandoperationsthatareincompatiblewithsoilconditions.Soilclassificationandmapping(surveys)permitthetransferofsoilsinformationfromoneplacetoanotherandfrompresenttofuturegenerations.

Scientificsoilclassificationisgenerallyrecognizedtohavebegunin1885whentheRussianscientistV.V.DokuchaevundertookthestudyandclassificationofsoilsnearMoscow.Ataboutthesametime,intheUnitedStates,similarconceptsofsoilsasnaturalbodiesinthelandscapewerebeingformulatedbyE.W.Hilgard.C.F.Marbut,thedirectoroftheU.S.SoilSurveyDivisionforthefirst35yearsofthetwentiethcentury,introducedmanyoftheRussianconceptsofsoilscience.Marbut'ssuccessorwasC.E.Kellogg,whowastheprimaryauthorofthefirstU.S.systemofsoilclassificationthatwaspublishedintheU.S.DepartmentofAgriculture(USDA)1938YearbookofAgriculture.

Itsoonbecameclearthatthissystemwasinadequate,inpartbecauseitdidnotincorporatespecificboundariesonsoilpropertiesinorderforsoilstobeclassifiedwithinacertaingroup.Asaresult,thesoilsurveystaffbeganworkonamodernsystemofsoilclassificationinthe1950s.Theproductoftheirinvestigationsanddeliberations,undertheleadershipofGuySmith,wentthroughsuccessiveapproximations,andin1960SoilClassification,7thApproximationwaspublished.

Taxonomyisthescienceofclassification.Thespecificsubdivisionofsoilsciencethatdealswithsoilclassificationispedology,andthosewhospecializeinsoilclassificationarepedologists.

Aftermuchworldwidescrutinyandmanyamendments,the1975editionofSoilTaxonomywaspublished.Atthattime,10orderswererecognized.Therefinementscontinued,andin1998thecurrentrevisionofSoilTaxonomy,with12soilorders,becameavailable.

TheSoilClassificationCategoriesThesoilclassificationsystemoftheUSDAishierarchicalwiththesesixcategories:

Order

Suborder

Greatgroup

Subgroup

Family

Series

Thehighestcategory,soilorder,isthemostgeneralized.Asanindividualsoilisclassifieddownthroughthesystemtothelowestcategory,soilseries,anincreasingnumberofspecificpropertiesarerecognized.Thissystemattemptstopreciselycategorizesoilsovertheentirefaceoftheearthinoneofthe12soilorders.Itshouldberecognizedthatthereareextensivelandareasthatarenotsoil.Theseincluderockyland,shiftingsand,andice/glaciers.

The12SoilOrdersInthisbook,thesoilordersaregroupedaccordingtonaturalcharacteristicsbasedonthefivesoil-formingfactorsinanefforttomakethemeasiertoremember.Thesefactorsareparentmaterial,climate,livingorganisms,topography,andtime.

Eachsoilorderhasaformativeelementthatisadescriptivetermconsistingoftwotothreelettersthataresetinitalics.Theselettersformthelastsyllableofthenamesinlowercategoriesinthatorder.Aftereachsoilorderheading,theestimatedpercentageoftheworld'ssoilinthatsoilorderisgiven.Table12.1givesasummaryofthe12soilorders,theircharacteristics,andapproximateclassificationintheFoodandAgricultureOrganizationoftheUnitedStates(FAO)systemthatisusedinmanypartsoftheworld.Aworldmapofthe12soilordersbaseduponSoilTaxonomyispresentedasFigure12.1.

Table12.1The12soilordersusedinSoilTaxonomy,1theirformativeelements,correlatingFAOclassification,andU.S.andworldwidedistribution

Formativeelements

Resultingsoilorders

Description Commonmasterhorizons

Commonsubsurfacehorizons

Descriptionofgenetichorizons

FAOtranslation

Time Entisols Young,thinsola,relativelyunderdevelopedsoils

AC

None ThinAhorizonoverparentmaterial

FluvisolsLithosolsArenosolsRegosols

Inceptisols OlderthanEntisols

ApBwC

Ap(plowed)Bw(cambic)

WeakBhorizon

GleysolsSolonchaksRankersCambisols

Oxisols Veryold,thicksola,highlyweathered

ApBo(several)BtC

Ap(plowed)Bo(oxic)Bt(argillic)By

WeatheredBhorizon,weakArgillic

Ferralsols

Climate Aridisols Arid,canbehighlydeveloped

ABk,Btk,By,orBzBtkC

Bt(argillic)Bz(accumulationofsaltsmoresolublethangypsum)By(gyspsic)

Duetolimitedleaching,accumulationofvarioussalts

XerosolsYermosolsSolonchaks

Spodosols Formedunderacidforests

OieAEBhsC

E(eluvial,ifverylight,Albichorizon)Bh(illuvialorganicmatter)Bs(illuvialsesquioxides)

Acidlittercauseseluviationofmanycompounds

Podzols

Gelisols Permafrost GleysolsRegosols

Topography Histosols Organic,wetlandsoils

OapOaOe

TheOhorizonsareorganicmaterials(Oaarehighlydecomposed,Oeareofintermediatedecomposition,Oiareslightlydecomposed)

Differentiatedbasedupondegreeofdecompositionoforganicmatter

Histosols

Biota Mollisols Darksoilsformedunderprairies,highbasesaturation

ApBw,k,t,y,orzC

Ap(plowed)Bt(argillic)Bk(carbonates)Bn(sodium)

DarkAhorizon,Bhorizoncanhaveavarietyofendopedons

GleysolsSolonchaksAndosolsRendzinasPlanosolsSolonetzKastanozemsChernozemsPhaeozemsGreyzemsPodzoluvisolsSolonetzLuvisolsPlanosolsNitosols

Alfisols Brownsoilsformedunderdeciduousforest,highbasesaturation

ApEBtC

Ap(plowed)Bt(argillic)Bx(fragipan)

Thin,browncoloredAhorizon,musthaveanargillichorizon

Ultisols Brownsoilsformedunderdeciduousforest,lowbasesaturation

ApEBtC

Ap(plowed)Bt(argillic)Bx(fragipan)

Thin,brownAhorizon,musthaveanargillichorizonand/orakandichorizon

AcrisolsNitosolsPlanosols

Parentmaterial

Andisols Formedfromvolcanicash,youngsoils

ApBwC

Ap(plowed)Bw(cambic)Bh(humus)

Bhorizonsarecommonlyweaklydeveloped

Andosols

Vertisols Haveexpandingclays

ApBwC

Ap(plowed)BhorizonsareweaklydevelopedduetoconstantchurningDuetoclayexpansionandcontraction

Self-churningsoilsduetoexpansionandcontractionofclays.

Vertisols

1NaturalResourcesConservationService.Landuseareasareapproximateanddonotsumto100%duetounknownfactors.

Figure12.1Worldsoilmap.

TimeIstooShortforStrongSoilDevelopmentEntisols:SoilsHavingMinimalDevelopment(16.2%)TheorderEntisol(therootwordistheEnglishword"recent")includessoilsthataresoweaklydevelopedtheymayhaveonlyathinochricepipedonoveraChorizon(Table12.1,Fig.12.2).Therearetworeasonswhythesesoilslackgreaterdevelopment:(1)Theparentmaterialconsistsofsuchhighlyresistantmineralsthattherateofweatheringisveryslow;forexample,droughtysandsremainpoorlydevelopedbecausetheycontainanabundanceofquartz;or(2)Theexposedlandsurfaceisyoungasaresultoferosionorburialundernewmaterialbroughtinbywindorsomeotheragent.Unprotectedsoilsonslopesaresubjecttowatererosion,andonmore-levelplainswindmayerodethetopsoil,thusexposingnewmaterialbelowandkeepingthesoilyoung.Theoppositeprocesskeepsalluvialandwetlandsoilsyoung.Newmaterialisaddedlayerbylayerasfloodwatermovesoverthesoilsortemporarypondsspreadacrossthem.

Figure12.2Entisolsareweaklydeveloped.

Inceptisols:ImmatureSoils(9.8%)Inceptisols(fromtheLatinwordinceptumwhichmeans"beginning")showmoredevelopmentthanEntisols,butcomparedtoothersoilsinthesameregion,theyareimmature(Fig.12.3).Theyarefoundinmostclimaticzonesbutareexcludedfromaridregionsandwherethereispermafrost.MostInceptisolshaveanochricepipedonandacambicBhorizon.

Figure12.3Inceptisolsarerelativelyimmature.

SlopingmountainsidesarecommonlyoccupiedbyInceptisolsbecausegeologicerosionandleachingbyrainfallisineffectiveatsuchsites.Inceptisolsarealsocommonindepressions.

Onereasonisthatsoildevelopmentisslowedwherethesoildoesnotdryoutperiodically.

ClimateIstheDominantFactorinSoilDevelopmentAridisols:DesertSoils(12.0%)Aridisols(fromtheLatinwordariduswhichmeans"dry")aredesertsoilsthataredrynearlyalltheyear(Fig.12.4).Theyhaveanochricepipedonandmayhaveeitheracambicoranargillicsubsurface(B)horizon.EntisolsarecommonlymappednearAridisols.Some,butnotall,desertsoilsaresalty.Aridisolsoftencontaincalciumcarbonatethatcanresultinacalcicsubsoilhorizoncalledacalcichorizon.Petrocalichorizonsarecementedcalcichorizonsthatareimpenetrablebyroots.Aridisolsoftenhavesilica-cementedhardpanscalledduripansthatarerock-likeandalsoimpenetrablebyroots.

Figure12.4Aridisolsareveryfragile.

Thedesertsareveryfragileregions,andoncetheyaredisturbedtheyareslowtorecover.Forthisreason,intensiverecreationaluseofthedesertisviewedwithalarmbyenvironmentalists.EarlyrecordsindicatethatmanyAridisolregionswereoncequitegrassywhereasnowonlyscatteredshrubsgrowasaresultofovergrazingandotheruses.

Gelisols:AlwaysFrozenSoils(8.6%)ThecentralconceptofGelisols(fromtheLatinwordgelarewhichmeans"freeze")isthattheycontaingelicmaterialunderlainbypermafrostwithin40in.(100cm)ofthesoilsurface(Fig.12.5).Gelicmaterialsaremineraland/ororganicmatterthathasbeenmixedinvariouspatternsduetothechurningcausedbyfreezingandthawingintheactivelayerabovethepermafrost.Theyusuallyalsoexhibiticesegregationinthislayer.GelisolsarefoundextensivelyinAlaska,Canada,Greenland,Iceland,andSiberia.

Figure12.5Gelisolshavepermanentlyfrozensubsoil.

Oxisols:HighlyWeatheredTropicalSoils(7.5%)Oxisols(fromtheFrenchwordoxide)arehighlyweatheredsoilsandformmostcommonlyfromsedimentaryrocksandbasiccrystallinerocksthatarerelativelysusceptibletoweathering.Anochricepipedonoverliesanoxicsubsurfacediagnostichorizon.Oxisolsdevelopintropicalareas(Fig.12.6)andhaveahighcontentofinertclays,mostlyamorphousoxidesofironandaluminum.Theonlykindoflayeredsilicateclayfoundinmorethantraceamountsiskaolinite.Ironstonenodules,whichmaycontainconsiderableamountsofmanganese,aresometimespresentinthesoilprofile.

Figure12.6MostOxisolsareintropicalregions.

Oxisolsusuallyhaveagranularstructurethroughout,whichallowsthemtoabsorbwaterreadilyandmakesthemeasytotill.NutrientsarequicklylostfromOxisolswhentheyaretilledbecausetheirhumusdecomposesquicklyandtheirclayshaveaverylowcationexchangecapacity.Historically,farminghasbeencarriedoutbyasystemofshiftingcultivation,whereinthelandislefttogrowtreesandshrubsforseveralyears.Thisallowsnaturalincorporationofnutrientsabsorbedbytreeswithinthesoilintotheleavesofthetreesandsubsequentlybecomeorganicresidueatornearthesurface.Clearing,burning,andafewyearsofcroppingfollowswhilethehumusdecomposes.Thisiscalledslashandburnagriculture.Whencropyieldsdecline,thecycleisrepeated.ManyOxisolareasareusedsuccessfullyforpermanentcropssuchascocoabeansandoilpalm.SomeareasofOxisolsareplantedregularlytosugarcane,pineapple,andothertropicalcropswiththehelpofmodernagriculturaltechniques.

ParentMaterialIsSpecificAndisols:VolcanicSoils(0.8%)Andisols(fromtheJapanesewordandousedtodescribedarksoil)arevolcanicsoils.Andisolsformedfromvolcanicejecta(pumice,cinders,lava)andcloselyassociatedparentmaterialsinhillyormountainousareas(Fig.12.7).FreshvolcanicashwouldnotqualifyasanAndisolandgeologicallyolddepositsgradeintoothersoilorders.Andisolshaveearlyweatheringproductsinthecolloidalfraction(verysmallclayandhumusparticles),namely,allophane,imogolite,ferrihydrite,andaluminum-humuscomplexes.

Figure12.7Andisolshavemanylayersofvolcanicash.

Duetotheresistancetodecompositionofthemetal-humuscomplexes,theorganicmattercommonlyreaches10–20%.Thisgivesamelanicepipedonthathasaverydarkcolor.Andisolshavealowbulkdensitythatpromoteswaterinfiltration,lowwatererosionpotential,

andeaseoftillage.Theyarenotedfortheirhighnaturalfertilityandareoftenthemostproductivesoilsintheirregion.Cropssuchascoffeearegrownextensivelyonthem.Nevertheless,productionmaybeinhibitedbythetie-upofphosphateontheiranionexchangecomplex.

Histosols:OrganicSoils(1.2%)Histosols(fromtheGreekwordhistoswhichmeans"tissue")areorganicsoilsthatformedfromaccumulationsoforganicmatter(Fig.12.8)inwet/coolenvironmentsthatslowedresiduedecomposition.ManyHistosolsformedinshallowlakesortidalflatsthataccumulatedplantresidues.Thesesoilsactlikeaspongeandaresaturatedmostofthetime.Histosolsoncoolmountainslopeshaveafolisticepipedonanddrainfreely.

Figure12.8Histosolsareaccumulationsoforganicmatter.

PoorlydecomposedHistosolsarecommonlycalledpeatandarenotgoodforfarming,eveniftheyaredrained.Theyare,however,sometimesharvestedforuseingreenhousesandnurseries.Well-decomposedHistosolsarecalledmuckandareoftendrainedforspecializedfarming(vegetables,forexample).ThedrainageandtillageofHistosolscausestheporestofillwithair,whichspeedsdecompositionresultinginsubsidenceatarateofabout1-ft(0.3-m)dropofthesurfaceevery10years.TheEvergladesAgriculturalAreainsouthernFloridaisanexampleofanareawithhighagriculturalproductivityduetothesuccessfuldrainingoftheHistosols.Unfortunately,thispracticeisnotsustainableduetosubsidence.Figure12.9showsthesubsidencepostatBelleGlade,FL.In1924thetopofthispostwasevenwiththesoilsurface;in2002thetopofthepostisnearly6ftoutofthesoilsurface.ThisHistosolformedinnearly9ftoforganicmaterialabovethebedrock.Intheseplacesthesoilmustnowbemovedintoridgestohavesufficientsoildepthtoplantcrops.

Figure12.9OneauthorstandsbesideasubsidencepostatBelleGlade,Florida.

Vertisols:CrackingDarkClaySoils(2.4%)Vertisols(fromtheLatinwordvertowhichmeansto"turn")aredarkclaysoilsformedmostextensivelyinthewarmtemperateandtropicalareaswithanusticmoistureregime,buttheymayalsobefoundincoolerclimates(Fig.12.10).Forexample,intheUnitedStates,mostVertisolshavebeenmappedinTexasandSouthDakota.

Figure12.10Vertisolsarerichinclay.

Vertisolsowetheiruniquepropertiestotheshrinkingandswellingofclays.Widecracksopenduringthedryseasonandsomesoilislikelytofallintothem.Whentherainsreturnthecracks

swellshut,butiftheyhavebeenpartiallyfilled,thereisnotenoughroomforthecrackstoclose.ThiscausesachurningactionthatbringsupfreshlimymaterialfromtheChorizonandtherebyrejuvenatesthetopsoilfasterthanitcanbeleachedbyrainwater.

TheswellingactioninVertisolsalsocauseslens-shapedblockswithinthesoiltoslidepasteachotheranddeveloppolishedsurfacescalledslickensides.Thesameforcesbucklethelandscapeintomoundsandhollows,whichresultsinamicrotopographycalledgilgai.

Vertisolsareparticularlywelladaptedtosugarcaneandpaddyriceculture.InTexas,Vertisolsarelargelyusedforpasturedespitethefactthatindryseasonsopencracksmakefootinghazardousforcattle.Withinafewyears,fenceposts,telephonepoles,andbuildingsmaybecometippedandtwistedonthesesoils.Whenhousesarebuiltonthem,thebuildermayprovideawaytokeepthesoilsunderthefoundationmoistatalltimestopreventtheheavingactionthatcanruinabuilding.

VegetationIsaGrassland(Prairie)Mollisols:GrasslandSoils(6.9%)Mollisols(fromtheLatinwordmolliswhichmeans"soft")aregrasslandsoils.Theyareamongthemostproductiveagriculturalareasoftheworld(Fig.12.11).IntheUnitedStatesmostMollisolsarefoundinthecentralnorthernprairiestatesthatcomprisetheCornandWheatBelts.Thegeneralpropertiesofsoilsdevelopedunderprairievegetationwerediscussedearlierinthischapter,andnowherearetheymorestronglyreflectedthanintheMollisols.Thedensefibrousrootsystemofthegrassesandforbshasresultedinthedevelopmentofathick,dark,humus-enrichedAhorizon(mollicepipedon)withanabundanceofplantnutrients.ThesesoilswerefirstdescribedinRussia,wherethedarkestwerecalledChernozems,meaningblackearth.

Figure12.11Mollisolsareveryproductive.

InthehumidpartoftheMollisolareainthemidcontinentalUnitedStates—theCornBelt—thesubsoilhasanaccumulationofclay(argillichorizon).Thispropertyisminimalorevenabsentindrierorcolderpartsofthegrasslandswhereacambicdiagnosticsubsurfacehorizonprevails.InNorthAmerica,Mollisolsaremostextensiveonthetall-andshort-grassprairiesthatextendeastwardfromtheRockyMountains.Manysettlersbuilttheirfirsthousesofsodinwhichsoilwasboundtogetherbygrassroots.

ClimateandVegetationCombinationDominatesAlfisols:High-BaseStatusSoilsofHardwoodForests(9.7%)Alfisols(fromtheterm"pedalfer"in1938system)aretypicallyfoundunderdeciduousforestswhererecyclingofplantnutrientsiseffective(Fig.12.12).Calcium,forexample,isabsorbedbyplantsandmovedintotheleavesandisreturnedtothesoilwhentheleavesfall.Normally,theparentmaterialcontainscalciumcarbonateandismediumtofinetextured.Thehumus-enrichedAhorizonisnotthickandisthereforecalledanochricepipedon.Thesubsoilhasanaccumulationofclaywithdarkclayfilmsonthestructural(ped)surfaces.Thisistheargillichorizon,whichholdsmoistureandnutrientswithintheupperpartoftherootzoneandis,therefore,beneficialtoplants.

Figure12.12Alfisolshaveahighbasecontent.

Inthepartsofthetropicswheregeologicerosionpreventsthelandsurfacefrombecominghighlyweathered,thesoilsmaygradefromInceptisolstoAlfisolswiththeincreasingstabilityofthelandscape.WhenAlfisolsareclearedoftheirtimberandplacedundercultivation,theyareusuallyquiteproductiveandrespondwelltofertilization.Thegoodsupplyofwater,timber,andagriculturallandinAlfisolregionsthroughouttheworldaccountsforthedevelopmentoflargecentersofpopulationonthem.

Ultisols:Low-BaseStatusForestSoilsofWarmRegions(8.5%)Althoughcyclingofbases(calcium,magnesium,potassium)goesoninUltisols(fromtheLatinwordultimuswhichmeans"last")underforestcover,itislesseffectivethaninAlfisolsbecausethegeologicsubstratumusuallylackscalciumcarbonate.Leaching,whichoccursyear-round,hasremovedmanyplantnutrientsfromtherootzone.Thereisadefiniteaccumulationofclayinthesubsoil(theargillichorizon),butitishighlyweathered.InUltisolstheultimateweatheringoflayeredsilicateclayshastakenplace.Asaresult,kaoliniteisabundant.ThesesoilsareusuallyconsideredolderbytensofthousandsofyearsthantheAlfisols.Well-drainedUltisolsarebrightlycoloredbystainsofyellowandredironoxides.PoorlydrainedUltisolsaregray.IfUltisolsarecultivated,theyquicklybecomeimpoverishedunlessfertilizationandcarefulmanagementarepracticed(Fig.12.13).

Figure12.13Ultisolslackbasesandquicklybecomeimpoverishedundercultivation.

Historically,inthesouthernUnitedStates,manyUltisolswereplantedtocottonwhichhaveverylowyieldsafterafewyearsofproduction.Oftensevereerosionfollowedandsometimesabandonmentoftheland,whichcontinuedtoerode.Byuseofmodernmethods,manyofthesefarmsarebeingbroughtbackintoproduction;manyevenyieldtwocropsperyearduetothelonggrowingseason.Intropicalareasthreecropsperyeararepossiblewithintensivemanagement.

VegetationandParentMaterialDominateSpodosols:AcidSoilsofSandyPineLands(2.7%)Spodosols(fromtheGreekwordspodoswhichmeans"woodash")aremostcommoninthesandyoutwashregionsoftheborealforestsandinquartzose(sandy)coastalmarinedepositsextendingtothetropics.Undertheacidhumuslayer(Fig.12.14),thereislikelytobeawhitishalbichorizonoverlayingadarkbrownspodichorizoninwhichhumusandorironoxidescoatthesandgrains.Sometimesthesecoatingscementthishorizonintoapancalledanortstein.The

villageofWhiteEarth,Minnesota,wasnamedforthiswhitesoilhorizon.MuchofthepinelumberthatwenttobuildtownsandfarmsteadsthroughoutNorthAmericaandEuropecamefromtheextensiveforestsoftheSpodosolregions.However,whentheforestswerecleared,thesesoils,whichhadyieldedsuchbeautifultimber,didnotprovetobegoodforbase-loving,shallow-rootedagriculturalcrops.Manyfarmsfailedandwerereplacedbypineplantationsandmixedforestsforlumberandpulpproductionaswellasforusebywildlifeandforrecreation.

Figure12.14Spodosolsareveryacid.

LowerCategoriesoftheClassificationSystemAswasstatedearlier,therearesixcategoriesintheUSDAsoilclassificationsystem.Theyareorder,suborder,greatgroup,subgroup,family,andseries.EachofthesecategoriesisregulatedbytheNaturalResourcesConservationServiceoftheUSDA.ThefirstfivecategoriesaredefinedinSoilTaxonomy,andindividualsheetsarepublishedforeachsoilseries.Inaddition,twoothercategories,typeandphase,maybedescribedlocallyforland-useplanningpurposes.Witheachsuccessivecategory,moreinformationisrevealedaboutthesoilbeingclassified.

Inthefollowingparagraphs,eachcategorybelowtheorderofthesoilclassificationsystemisexplainedandexamplesaregiven.

SubordersThesubordercategoryusesaformativeelement(syllable)fromtheordernameandplacesanewsyllablebeforeittogivemoreinformationaboutthesoil.Inmanycasesthenewsyllablemayindicatesuchfeaturesastheusualmoistureconditionandaparticularpropertyoftheparentmaterial,orasinthecaseofHistosols,thedegreeofdecompositionoforganicmaterials.Forexample,the"oll"fromMollisolisusedtoidentifythesoilorderinsoilclassification(Table12.1).

"Oll"isjoinedwithanothersyllablesuchas"ud"(fromudic)tomakethetwo-syllable"udoll."Audollisasubordercommoninhumidregions(commonintheCornBelt)whereasanustollwouldbeintermediateinmoisturebetweenahumidandaridclimate,soilscommonintheWheatBeltstates.Similarly,an"aquoll"isawetsoilwherethewatertablewillbenearthesoilsurfacesometimeduringtheyear.Mostaquollsrequiredrainagebeforetheycanbeintensivelycropped.Somesyllablesareusedatmorethanonelevelinthesoilclassificationsystem.Table12.2illustratestheprocessforclassifyingsoilprofiles.

Table12.2SoilTaxonomyclassificationscheme

Order Suborder Greatgroup Subgroup Family Series Description

Gelisols Permafrostwithin1mofsoilsurface

Turbels Profileswithcryoturbation

Histoturbels Morethan30%organicmaterialstoadepthof0.5m

TypicHistoturbels

FitwithintheconceptofaHistoturbel

Loamy-skeletal,mixed,superactive,subgelicshallowTypicHistoturbels

Hasloamtexture,rockswithintheprofile,mixedmineralogy,high-activityclays,cold,shallow

Ester Poorlydrainedsoilformedinthinloessoverschistbedrock

Histosols OrganicsoilFibrists Organicfibric

materialsthatarenotidentifiable

Cryofibrists Meantemperatureis<8°C,nopermafrost

LithicCryofolists Bedrockat<0.5m

DysicTypicCryofolists

Acidic

Reggad Deep,excessivelydrainedsoilformedindecomposingorganicmatterwithamixtureofashandpumiceoverrubble

Spodosols Haveaspodichorizon

Cryods Meantemperatureis<8°C,nopermafrost

Humicryods Have>6%organicCinspodic

AndicHumicryods

Formedfromvolcanicashandhaveandicproperties

Sandy-skeletal,mixedAndicHumicryods

Sandytexturewithrocksintheprofile

Chugach Verydeep,welldrained

soilsformedinathinmantleofashoverlyingglacialoutwash

Andosols Formedinvolcanicmaterials

Torrands Soilprofileisdrymostoftheyear

Duritorrands Haveacementedhorizon

PetrocalcicDuritorrands

Haveapetrocalcichorizon

Medial-skeletal,amorphicisohyperthermicPetrocalcicDuritorrands

Soilformedinvolcanicashunderlainbycobblesandlava;andicproperties,temperatureis>22°Cthroughouttheyear

Hapuna Deep,well-drainedsoilthatformedinvolcanicash

Oxisols Haveanoxichorizon

Ustox Haveanusticsoilmoistureregime

Eutrustox Havehighbasesaturation

KandustalficEustrustox

Havelow-activityclays,characteristicsofanalfisol

Veryfine,mixedisohyperthermicKandiustalficEutrustox

Clayey,mixedmineralogy,temperatureis>22°Cthroughouttheyear

Saipan Verydeep,welldrainedsoilsformedfromlimestone

Vertisols Formedinlacustrinedepositswithexpandingclays

Xererts Haveaxericsoilmoistureregime

Calcixererts Haveacalcichorizon

TypicCalcixererts

FitwithinthecentralconceptofaCalcixerert

Fine,smectitic,frigidTypicCalcixererts

Siltyclaytextureinsoilprofilecontrolsection,2:1expandingclays,meanannualairtemperatureof5°C.

Niter Deep,well-drainedsoil

formedinlacustrine.

Aridisols Haveanaridicsoilmoistureregime(soilprofiledryinallpartsmostoftheyear),hasasalichorizon(saltaccumulation)

Salids Haveasalichorizon

Haplosalids Fitswithintheconceptofthesalidssuborder

TypicHaplosalids

FitswithintheconceptoftheTypicHaplosalidsGreatGroup

Fine-silty,mixed,superactive,hyperthermicTypicHaplosalids

Siltyclaytexture,mixedmineralogy,high-activityclays,soiltemperature>22°Cduringthesummer

Yahana Deep,well-drainedsaline–sodicsoilthatformedinfloodplainalluvium

Ultisols Oldersoilwithan

argillicorkandichorizonorafragipanandargillicorkandichorizons

Udults Haveaudicsoilmoistureregime

Fragiudults Haveafragipan

GlossicFragiudults

Haveagraymatrixcolorbetweenthekandicorargillichorizonandthefragipan

Fine-silty,siliceous,semiactive,thermicGlossicFragiudults

Siltloamtexture,silica-cementedfragipan,low-activityclays,meanannualtemperatureisbetween15–22°C

Dickson Deep,moderatelywell-drainedsoilformedinloess

Mollisol Grasslandsoils

Udoll Udmoistureregime

Argiudoll Hasanargillichorizon

TypicArgiudoll TypicalMollisolwithanargillichorizon

Fine,mixed,active,mesicTypicArgiudolls

Clayey,mixedmineralogy,mesicsoiltemperature

Woolper Deep,well-drainedsoilsonfootslopes,alluvium,orcolluviumparentmaterials

Alfisol Deciduousforestsoils

Aqualf Poorlydrained

Epiaqualf Soiliswetfromwaterdrainingontothesite

MollicEpiaqualf Epipedondarkenoughtobemollicbutusuallytoothin

Fine-loamy,mixed,superactive,mesicMollicEpiaqualfs

Textureofthesoilcontrolsection,mixedmineralogy,mesicsoiltemperature

Havana Deeppoorlydrainedsoilformedinloessor

loamycalcareousglacialtill

Inceptisols Hasacambichorizon

Udepts Hasaudicsoilmoistureregime

Dystrudepts Anacidicudept

FluvaquenticDystrudepts

Formedinfloodwateralluvium

Coarse-loamy,mixed,active,frigidFluvaquenticDystudepts

Coarse-texturedprofile,mixedmineralogy,high-activityclays,meansoilprofiletemperature<8°C

Podunk Verydeep,moderatelywell-drainedsoilformedinalluvium

Entisols Nosubsurfacehorizons

Psamments Loamyfinesandorcoarser

Quartzipsamments Morethan90%resistant(quartz)material

SpodicQuartzipsamments

Hasahorizoncementedbyaluminumandorganic

matter,withorwithoutiron

Hyperthermic,uncoatedSpodicQuartzipsamments

Soiltemperature>22°Cduringthesummer,nocoatings

Orsino Verydeep,moderatelywell-drainedsoilsformedinsandymarineoraeoliandeposits

1AdaptedfromNRCSsoildescriptions.

2Eachlevelofclassificationishighlightedinitalicsanddescribed.

GreatGroupsGreatgroups,thethirdcategory,areformulatedbyaddinganothersyllableinfrontofthesubordernametogivemoreinformationaboutthesoilproperties.Fiveexamplesofthesearegivenbelow:

Arg—Latinargilla,whiteclay

Ust—Latinustus,burnt(dryclimate)

Hapl—Greekhaplous,simple

Gloss—Greekglossa,tongue

Pale—Latinpaleos,old

Sometimesavowelisplacedbetweenthesesyllablesandthesubordernametomakethewordeasiertosay:

Argiudoll—aUdollwithclayaccumulationintheBhorizon

Ustipsamment—aPsammentofdryregions

Haplorthod—asimpleOrthod

Glossocryalf—aCryalfwiththeEhorizonirregularlyprotrudingintotheBhorizon

Paleaquult—ahighlydevelopedAquultevidencedbygenetichorizonthicknessornumber

Subgroups

Thesubgroupsareformedbymodifyingthegreatgroupnamewithoneortwoadjectives.Thisadjectivemaydepictanormalcondition,oritmayindicatesomespecialfeaturesaboutasoil.Insomecasesagreatgroupofoneordermaybeintegratedtowardanother,andthiswouldbeshownbythesubgroupadjective.Fiveexamplesaregivenbelow:

Typic—fitsthecentralconceptofthegreatgroup

Aquic—havingpropertiesofwetness

Alfic—gradingtowardanAlfisol(withanargillichorizon)

Fragic—havingafragipan

Aeric—periodicaeration

Thesetermsmaymodifygreatgroupstoformsubgroupsasfollows:

TypicArgiudoll—atypicalMollisolwithanargillichorizoninahumidclimaticzone

AquicUstipsamment—aslightlywet(seasonally)sandyEntisolinadryclimaticzone

AlficHaplorthod—asimple,ordinarySpodosolhavinganargillichorizonbelowthespodichorizon

FragicGlossocryalf—acoldAlfisolwithhighlyirregularhorizonboundariesandafragipan

AericPaleaquult—anold,wetAquultwithcolorsindicatingperiodicaeration

Thereaderisremindedthateachofthesesyllablesindicatesspecificsoilproperties,aspresentedintheUSDApublication,SoilTaxonomy.

FamilyThefifthcategoryoftheclassificationsystemisthesoilfamily.Thisisnotnamedwithstrange-soundingGreekandLatinsyllablesbutinsteadhasdescriptivetermsindicatingsuchpropertiesasparticlesize,mineralogy,cationexchangeactivity,andtemperatureregime.Acommonfamilynameformanysoilsthatareformedfromglacialtillinmid-Americaiscoarse-loamy,mixed,superactive,mesic.Afiner-textured,highlyweatheredsoilinthesoutheasternUnitedStatesmightbeinthefine-loamy,kaolinitic,subactive,thermicfamily.

SeriesThesoilseriesisthesixthcategoryandisthenamegiventosoilswithverysimilarprofiles.Thenamethatisgivenisderivedfromthetownorcommunitywherethesoilwasfirstofficiallydescribed.Morethan20,000soilserieshavebeendescribedintheUnitedStatesandmanymoreinotherpartsoftheworld.OneexampleofaseriesistheFayette,whichisnamedafteratowninIowa.ThissoilformedindeeploessforsomedistanceoneachsideoftheMississippiRiverinIowa,Minnesota,Illinois,andWisconsin.

TypeandPhase

Eventhoughnotcategorizedinthesoilclassificationsystem,thetypeandphaseareaddedtofurtherdefineasoil.Soiltypegivesthetextureofthetillagezone.Soilphasegivesinformationaboutsoilpropertiesthataffectlanduse;slopeandstoninessareexamples.Thetypeandphasearenotnumberedbelowbecausetheyarenotapartofthesix-categorysystemforclassifyingsoils.However,typeandphaseareusefulinland-useplanning.

AsummaryisgivenbelowforthenamingofasystemusinganAlfisol:

1.Order—Alfisol

2.Suborder—Udalf

3.Greatgroup—Hapludalf

4.Subgroup—TypicHapludalf

5.Family—Fine-silty,mixedsuperactive,mesic

6.Series—Fayette

Type—Fayettesiltloam

Phase—Fayettesiltloam,nearlylevel

SoilHorizonsPedonsandPolypedonsSoilhorizonsweredescribedinChapter2asthelayersthatforminthesoilduringthelongperiodofsoildevelopment.Whenviewedinatwo-dimensionalcross-section,suchasthesideofapit,theyrepresentasoilprofile.Asoilprofileextendsfromthegroundsurfacetothedepthofsoildevelopment.

Theconceptofthesoilpedonconsidersthesoilprofileinthreedimensions.Thepedonisdescribedasthesmallestthree-dimensionalbodyofsoillargeenoughtoillustratethenatureandarrangementofsoilhorizonsandtheirvariability.Thesurfaceareaofapedonisarbitrarilysetatfrom1to10sqm,dependingonthesoil'suniformityorcomplexity.Itislikeacolumnofsoilthatwouldbeleftstandingifabulldozerremovedallthesoilexceptforthatbeneathasmallpatchofground.Sometimesthatisdoneifexcavationtakesplacebeforethetelephonecompanycanreroutetheirlines.

Apolypedonisdefinedasasetofcontiguouspedonsfallingwithintheacceptedrangeofcharacteristicsforaspecificallynamedsoilonthelandscape.Thepolypedonmayalsobetermedasoilbody.Itislikehavingmanycolumnsofsoil,sidebyside,extendingtoaboundarywhereadifferentkindofsoilisencountered.Thesoilprofile,pedon,andpolypedonareillustratedinFigure12.15.

Figure12.15Anillustrationofapolypedon,pedon,andasoilprofile.

DiagnosticSoilHorizonsManytypesofsoilhorizonshavebeendescribedbycharacteristicsthatfallwithinquantifiablephysicaland/orchemicalparametersandmeetaspecifiedminimumthickness.Atthehighestlevelofsoilclassification,thesoilorder,therearethreediagnosticsurfacehorizonsthataregermanetosoilclassification.Thesenamedsurfacehorizonsmayalsobecalledepipedons.Oneoftheepipedons,themollic,hasthreenamedvariationsthatwillbediscussedbelow.

Similarly,therearefourdiagnosticsubsurfacehorizonsusedintheclassificationofmostsoils,attheorderlevel,unlesssubsurfacesoildevelopmentisminimal.Variouscombinationsofthesesevendiagnosticsurfaceandsubsurfacehorizonsdeterminemostsoilorders.Therearethreemajorexceptions.Thefirstiswhereextremeclimate,verydryorverycold,makes

traditionalagricultureimpractical.Thesecondiswherethesoilparentmaterialisvolcanicash.Thethirdiswherethereisathickaccumulationofplantdebris,suchasinbogsandtidalflats.

Sevencommondiagnostichorizonsforsoilordersareasfollows:

Diagnosticsurfacehorizons(epipedons)

Ochric(paleorthintopsoil)

Mollic(thickdarktopsoil,neutraltoalkaline,fertile)

Histic(thickorganicmatovermineralsubsoil)

Diagnosticsubsurfacehorizons

Cambic(onlymoderatesoildevelopmentevident)

Argillic(enrichedinclayleacheddownfromabove)

Spodic(enrichedincolloidalhumus,aluminum,andusuallyironleachedfromsurfacehorizons)

Oxic(severelyweathered,infertile,highinsesquioxideclays,usuallyreddishcolored)

Afewotherdiagnostichorizonsthatarevariationsofthoselistedabovewillbediscussedwiththemajordiagnostichorizons.

Inmostcases,theepipedonshaveanAhorizonsymbol,whereasthediagnosticsubsurfacehorizonsnormallycarryaBhorizonsymbol.Lowercaselettersareusedtoindicatemorespecificallythetypeofsoildevelopmentthathasoccurred.Table12.3explainstheuseofsymbolsinsoilhorizonnomenclature.

Table12.3Examplesofsoilhorizons

SymbolsSoillayer General Detailed PropertiesSolumOrganiclayer OLeaflayer Oi PlantfiberrecognizableHumus Oa SaprophyteshavedecomposedthefibersTopsoil AHumusenriched Ap DarkenedplowedlayerSubsoilLeachedlayer E Lightcoloredduetofineparticlesbeingwashed

downward,eluviatedAccumulationzone

B Bt Whereclayhasmovedinfromabove

Parentmaterial C LittlechangebysoilformationBedrock R Thesolidsubstratum

DescriptionoftheDiagnosticSurfaceHorizons(Epipedons)OchricEpipedonTheochric(fromtheGreekwordochrosewhichmeans"pale")epipedon(Fig.12.16)isthemostcommontypeofAhorizon.ItmaybeathinAhorizonoronethatiseitherpaleordarkcoloredifithas<1.0%organicmatterbyweight.Thisistheusualconditionwherethenativevegetationisaforestortheclimateisarid.Theaveragetemperatureconditionsmaybeeitherwarmorcold.Whenthesesoilsareplowed,thefieldshaveagrayish-oryellowish-brownappearanceunlessthefarmerhasaddedalotofplantresidueandmanuretodarkenthem.

Figure12.16Ochricepipedon.

MollicandSimilarEpipedonsMollic(fromtheLatinwordmolliswhichmeans"soft")epipedonshaveathickAhorizonthatisverydarkbrownornearlyblackduetoanenrichmentofhumusto>1.0%byweight(Fig.12.17).Thisconditionisusuallymetwherethenativevegetationisprairiegrass.Thegrassescyclebasicionstothesurface,andthelimitedprecipitationpreventsrapidleachingandmaintainsahighbasesaturation.Thesesoilsdevelopastronggranularstructure,whichallowsthemtobecrumblyevenwhentheyaredry.

Figure12.17Mollicepipedon.

Threevariationsofthemol-licepipedonareasfollows:

Umbricepipedonsappeartobemollic,buttheyhavealowbasesaturationduetotheiracidity.ThenamecomesfromtheLatinwordumbra,meaningshade,whichalludestotheirdarkcolor.Umbricepipedonsarenotwidespread,buthumidorwetconditionsprevailwheretheyoccur.

Melanic(fromtheGreekwordmelanoswhichmeans"black")epipedonsareblack,humus-enrichedAhorizonsformedinloosevolcanicmaterials.Theyhavealowbulkdensity,arehighinaluminum,andhaveahighphosphateretentioncapacity.

Anthropic(fromtheGreekwordAnthropoeswhichmeans"human")epipedonsareahuman-inducedformofthemollicepipedon.Theymostfrequentlyoccurinaridregionsthathavealonghistoryofirrigatedagriculturewithorganicmatterincorporation.TheyareformedextensivelyintheOrient.

HisticEpipedonAhistic(fromtheGreekwordhistosewhichmeans"tissue")epipedonisanOhorizonmadeupofplantresidue8–24in.(20–60cm)thickovermineralsoil(Fig.12.18).Therangeinthicknessalloweddependsonthedegreeofdecomposition.Theseepipedonsdevelopinlowlandsthataresaturated>30daysperyear.Histicepipedonsaresubdividedaccordingtotheirdegreeofdecomposition.Fromtheleasttothegreatest,thetermsusedarefibric,hemic,andsapric.TheirnotationsareOi,Oe,andOa,respectively.

Figure12.18HisticepipedonontheOhorizon.

Folists(fromtheLatinfoliawhichmeans"leaf")areagreatgroupconsistingoffreelydrainedHistosolsthatarecomposedprimarilyofleaflitterandareunderlainbyfragmentedbedrockorgravelwithin1mofthesoilsurface.FolistepipedonsareOhorizonsthatcanbeasthinas6in.(15cm)andaresaturatedforonlyafewdaysfollowingheavyrains.Theyforminuplandpositionsandarecommoninalpineregionswherelowtemperaturescauseplantresiduedecompositiontobeslow.

DescriptionsofSubsurfaceHorizonsCambicSubsurfaceHorizonWhenfieldandlaboratoryinvestigationsofthesubsoilshowonlyamodestamountofweatheringandnotmuchaccumulationofmaterialsleachedfromabove,thehorizonisclassifiedasacambichorizonandhasthesymbolBw.ThewordisderivedfromtheLatincambire,meaningtochange.Toqualifyascam-bic,thehorizonmustnotbeverysandyandmustshowsomealterationbyprocessesofweathering.Thesechangesmaybeevidencedbychangesincolor,thedevelopmentofsoilstructure,ortheremovalofsomeofitsmoresolublecomponents.

InthesubhumidpartsoftheGreatPlains,soilsusuallyhaveacambichorizonbelowthemollicepipedonwhereprismaticstructurehasdevelopedandfromwhichcarbonateshavebeenleached.Figure12.17showsaBwhorizonofthiskind.Figure12.19showsanothertypeofcambichorizonwhereperiodicwetnesshasbroughtaboutamottledcolorduetotheformand

distributionofiron.ItisindicatedbythesymbolBg.The"g"isderivedfromtheRussianwordglei,meaningwetstickyclay,andthe"w"indicatesdevelopmentofstructureorcolorbutlittleilluvialaccumulation.

Figure12.19CambicBhorizon.

ArgillicSubsurfaceHorizonThroughoutmostofthehumidhardwoodforestregionoftheUnitedStatesandsomeofthedrierareasaswell,thesubsoilscontainmoreclaythantheAhorizonandusuallymorethantheChorizon.SomeoftheaccumulatedclaywasmoveddownfromtheAandEhorizons,andsomewasformedwithintheBhorizonbythealterationofprimarymineralsintoclayminerals.Thesmallletter"t"inthehorizonsymbolBtinFigure12.20istakenfromtheGermanwordtone,meaningclay.Argillichorizonsaresubsurfacediagnostichorizonswherehigh-activityclaysaccumulated.ThewordisderivedfromtheLatinargillus,meaningwhiteclay.Anargillichorizonusuallybenefitsplantsbyholdingmoistureandnutrientswithintherootzone.Tobeclassifiedasanargillichorizon,theremustalsobevisualevidenceofclayfilmsliningporesorbridgingsandgrains.

Figure12.20ArgillicBhorizon.

NatricSubsurfaceHorizonAnatrichorizon(Btn)musthaveallthepropertiesoftheargillichorizonplusabundantsodium(natriuminLatin)thatcausesthesoiltosealitselfagainstthepercolationofwater.TheimpervioushorizonillustratedinFigure5.20isanatrichorizon.

KandicSubsurfaceHorizonSimilartotheargillichorizonisthekandic(Bt)horizonfoundinsubtropicalandwarmerregions.Kandichorizonshaveaccumulationsoflow-activityclay,primar-ilykaolinite,andthereforedonotholdnu-trientswell.

SpodicSubsurfaceHorizonInboreal(northern)forestregionsandsomewetsandyareasofsubtropicalregions,thesubsoilisusuallyreddishbrowntoblack.Thiscoloriscausedbycoatingsofhumustogetherwithironandaluminumoxidesonthesurfacesofsandgrains.Thesecoatingsmaybreakoffandbecometinypelletswithinthesoilmatrix.Asubsoillayerwiththesepropertiesiscalledaspodichorizon(Fig.12.21).

Figure12.21SpodicBhorizon.

ItmaycarrythesymbolBh,Bs,orBhs;"h"representshumusand"s"representsironandaluminumoxides(sesquioxides).Whenthesesoilsareplowed,thelightgrayoverlyingalbicEhorizonassociatedwiththespodichorizongivesthefieldanashyappearance.ThenamecomesfromtheGreekspodos,meaning"woodash."

OxicSubsurfaceHorizonAveryimpoverishedsubsoilwithalmostnoprimarymineralsotherthanquartziscalledanoxichorizon(Fig.12.22).Itconsistsofquartzsandandaninertclayfractionofkaoliniteplusoxidesofironandaluminum,hence,thesymbolBo.Theentiresubsoiliscommonlyquiteuniformlyweatheredandlacksoriginalrockfeatures.Oxichorizonsarefoundintropicalregionswheresevereweatheringhasbeeninprogressforaverylongtime.

Figure12.22OxicBhorizon.

OtherDiagnosticSubsurfaceHorizonsAlbicSubsurfaceHorizonAlbic(fromtheLatinwordalbuswhichmeans"white")Ehorizons,similartothatshowninFigure12.20,arewhitishorgrayincolorwithbleached,uncoatedmineralgrains.Usuallytheyarebelowamatofforestresidue(Ohorizon)andatthesurfaceofthemineralsoil.Thebleachingis,atleastinpart,duetoorganicacidsleachedfromtheOhorizon.Inthiscase,thealbichorizonmaybediagnosticforasoilorder.Somemaybeshallowinthewetsubsoilduetochemicallyreducingconditions.

CalcicSubsurfaceHorizonAcalcichorizon(Bk)(fromtheGermanwordkalkwhichmeans"lime")isanilluvialhorizoninwhichsecondarycalciumcarbonateorothercarbonateshaveaccumulatedtoasignificantextent.Theyarewidespreadthroughoutgrasslandanddesertregions.

SalicSubsurfaceHorizonSalichorizons(Bz)(fromtheGermanwordzaltwhichmeans"salt")arelayerswithahighaccumulationofsolublesalts,typicallysodiumchloride,inaridregions,wheretheymaybediagnostic.Thesaltisderivedfromperiodicallyshallowsalinewaterinthesubsoil.

HardpansasDiagnosticHorizonsInmanypartsoftheworld,asubsoilhardpanexists.Thelayershaveaveryimportanteffectonthepotentialuseofthesoil.Theyarenottypicallydiagnosticattheorderlevel,buttheyarerecognizedasdiagnosticforlowercategoriesinthesoilclassificationsystem.Fourprominentonesaredescribedinthissection.Anexampleofahorizonnotationsymbolisgivenforeach.

Petrocalcic(fromtheGreekwordpetrawhichmeans"rock")horizons(Bkm)occurinthesubsoilonoldlandformsinaridregionswherecalcichorizondevelopmenthasprogressedtothepointofbecomingrocklike.Itiscomposedmostlyofcalciumcarbonatehardenedaroundsiliciousgravel(Fig.12.23).Petrocalcichorizonsmayextendtoadepthofseveralfeet,andareusuallyimpenetrablebyroots.The"m"indicatesinduration.

Figure12.23PetrocalcicBhorizon.

Duripans(fromtheLatinwordduruswhichmeans"hard")aremoredurablethanpetrocalcichorizonsbecausethecementinduripansincludesmuchsecondarysilica(SiO2).ThesymbolforthemisBq("q"isfromquartz).Theyformbestwherethereisorhasbeenvolcanicashandtheclimatehasalternatingdryandwetseasons.

Fragipans(fromtheLatinwordfragiluswhichmeans"brittle")thatoccurinsomeforestedregionsaresodensetheyrestrictthepenetrationofwaterandroots.Theclosepackingofgrainsofsandandsiltandweakcementationcausethehorizontobebrittlewhendryormoist,butnotwhenwet.MostareBxorBtxhorizons.

Plinthite(fromtheGreekwordplintoswhichmeans"brick")horizons(Bv)forminwarmhumidregionswhereironisabundantinparentmaterials.Theironbecomesconcentratedandcementedintoacontinuousreticulate(netlike)layer.Plinthitemayhardenirreversiblyintoanironpan(petroferriclayer)whenexposedrepeatedlytowettinganddryingoveralongtime.

SoilMoistureandTemperatureRegimesIntheclassificationofsoils,SoilTaxonomytakesintoaccountnotonlysoilpedoncharacteristics,butalsosoilmoistureandtemperatureregimes.Moistureregimesrelatetothewateravailabletoplantsinthemainpartoftherootzone.Temperatureregimesaremeasuredatadepthof20in.(50cm)ortoarestrictivelayerifitisshallower.Eachregimehasestablishedparameters,butinthisbookonlygeneralfeatureswillbepresented.Thetermsbelowareincorporatedintothenamesofmanyofthesoiltaxonomicunits.Thelettersinboldarethe"syllable"onefindsinthetaxonomicclassification;thosewithoutboldlettersindicatethatthesetermsareusedunabbreviatedatthefamilylevelofsoiltaxonomy.

Classesofsoilmoistureregimesincludethefollowing:

Aquic—Saturatedforenoughofthetimemostyearstocausereducingconditions(lackofoxygen)toprevail;soilmoistureregimeofwetlands.

Aridicandtorric—Bothtermsareusedtoindicatedrynessthatrestrictscropproductionwithoutirrigation;soilmoistureregimeofthedeserts.

Udic—Usuallymoist,soilmoistureregimeoftheCornBeltoftheMidwest.

Ustic—Seasonaldryperiods,butenoughprecipitationduringthegrowingseasonmostyearsforcropproductionwithoutirrigation,soilmoistureregimeoftheWheatBeltoftheGreatPlains.

Xeric—AlsocalledaMediterraneanclimatewithdrysummersandcool,moistwinters,soilmoistureregimeofCaliforniaespeciallyintheareaspronetowinterlandslidesduetosaturatedsoilconditions.

Classesofsoiltemperatureregimesincludethefollowing:

Cryic—Verycoldsoils.Withinthisregime,thecoldestsoilshavepermafrost,temperatureregimecommontonorthernclimatesespeciallyathigheraltitudes.

Frigid—Coldwinters,butsummersarewarmenoughforcropproduction.NorthernUnitedStatesisanexample.

Mesic—Warmerthanfrigid.IntheUnitedStates,theOhioRiverValleyisanexample.

Thermic—Warmerthanmesic.IntheUnitedStates,thesouthernstatesareanexample.

Hyperthermic—Warmestofthetemperatezonesoils.FoundinthehottestpartsofthecontinentalUnitedStates.

Isohyperthermic—Hottropicalclimatethroughouttheyear.

(Theprefixisocanbeusedwithmosttemperatureregimesifthesoiltemperatureisquiteuniformthroughouttheyear.)

SoilSurveys

Whenthevarioussoilsonthesurfaceoftheeartharedelineatedonmaps,itiscalledasoilsurvey.Usuallythemappingofsoilsisdoneonacountybasis.

IntheUnitedStates,theNaturalResourcesConservationService(NRCS)hastheoverallresponsibilityformakingsoilsurveystoprovideaninventoryofournation'ssoilresources.AgenciessuchastheU.S.ForestServiceandtheBureauofLandManagementalsomakesoilsurveysonlandsforwhichtheyarespecificallyresponsible.Throughcooperativeinteragencyefforts,theworkbytheseagenciesshouldblendinwiththatoftheNRCS.

Thesoilscientistswhomakethesoilmapsaremenandwomenwhohavegraduatedfromasoilscienceprogramatanaccrediteduniversityandcompletedatrainingperiodwithanexperiencedsoilsurveyor.Aknowledgeofsoilscienceformappingpurposesincludesanunderstandingofgeomorphologysothatnaturallandformscanbeidentified.Theboundariesofsoil-mappingunitscommonlycoincidewiththeboundariesofvarioussegmentsofalandscape,suchasridges,sideslopes,terraces,andfloodplains.Withsufficientexperienceinalocale,asoilsurveyorshouldbeabletopredictthetypeofsoilonanyportionofthatlandscapeandtakesoilcoresjustfrequentlyenoughtodetermineifthepredictionwascorrect.

Figure12.24showsasmallsegmentofalandscape(left)thathasbeenexpanded(right)toshowitsthreecomponentsoilbodies.TheDubuquesoilbodyhasasiltloamsurfacehorizonwithclaysubsoiloverlimestonebedrock.TheHixtonsoilbodyisaloamoveryellowish-brownsandstoneandsiltstone.TheChaseburgisadeepsiltloamformedinlocalalluvium.

Figure12.24Asoillandscapecanbebrokenintoseveralcomponents.

Afarmerbecomesfamiliarwiththesoilcomponentsofthelandscapeonhisfarmafteryearsoftillingthesoilanddiggingholesinit.Thesequenceofsoilbodiesdownahillsideiscalledasoilcatena.Intheexampleshown,allsoilsarewelldrained,althoughthewatertableisclosertothesurfacethefartherasoilbodyisdownslope.Inless-hillyterraininhumidregions,atypicalcatenamayconsistofdrysoilsattheridgetopandwetsoilsatthefootslopeposition.

LandscapePatternsThelandscapeisamosaicofsoilbodiesthatfittogetherasneatlyaspiecesinajigsawpuzzle.Somelandscapeshaveconcentric,circularsoilpatterns;somearecharacterizedbyloopedpatterns;andothershaveparallel,linearsoilpatterns.ThesketchesinFigure12.25are

simplifiedmodelsbasedonasoilmap,eachrepresenting0.4sqmile(1km2)oflandinsoutheasternWisconsin.

Figure12.25Varioussoilbodiesfittogethertoformthelandscape.

Thepatternsoflandscapesinfluencefarming,forestry,andwildlifemanagementpractices.Forexample,contourfarmingisnotaseasyonalandscapewithtightcircularpatternsasitisinareaswithlinearsoilpatterns.

MakingSoilSurveysSincethe1930s,soilsurveyorshavebeenutilizingaerialphotographsasabasemapuponwhichboundariesbetweensoilmappingunitsaredrawn.Themappingunitsaredeterminedbysystematicallytraversingthelandandaugeringsamplesofsoil,eachofwhichischeckedbysightandfeelforitsdistinguishingphysicalproperties.SimplefieldtestsmaybeconductedforpH,freelime,andsolublesalts.Thedepthtowhichthesoilisinvestigatedwillvarywithitscomplexityofhorizons,butitmaybeasdeepas80in.(2m).Eachmappingunitinthepatternofsoilsisgivenasymbolfortheunititrepresents(Fig.12.26).Thesoilsurveyorinterpretsallofthisinformation,delineatesthesoiltypeonabasemap(Fig.12.27),andmovestoanotherlocation.

Figure12.26Mappingunitsymbol.

Figure12.27Asoilmappermakesaugerholestoinvestigatethesoilandrecordsthefindingsonanaerialphotograph

Soilmappingisasmuchamatterofphotointerpretationasitissoilinvestigation.Thetoneofblack-and-whitephotosgivesastrongcluetothelandform,degreeoferosionofcultivatedfields,thevigorofcrops,areasofpoordrainage,sometimesthespeciesofnaturalvegetation,andmuchmoretothetrainedeye.

Photocoverageforsoilsurveyismadesothatitprovidesstereoscopiccoverage.Thisisaccomplishedbyhavinga60%overlapalongthelineofflight.Whenthephotosareplacedsidebyside,thesoilsurveyorcanseephotographedimagesforonepositionoftheairplanewiththelefteyeandanotherpositionwiththerighteye.Thiscreatesinthebraintheappearanceofathirddimensionwhereinobjectswithahigherelevationappeartoriseupfromtheflatsurfaceofthephotos.Inthisway,thesoilsurveyorcanproperlyrecordtopographicfeaturesoftheland.

Colorphotographyhasreplacedmuchoftheblack-and-whitefilminrecentyears.Insomeplaces,colorinfraredphotosofthelandareavailable.Theyprovideabetterviewofthepatternofvegetationand,byextension,thesoilthatfavorsaparticularvegetativetype.

Thesoilsurveyortakesadvantageofavailablegeologicinformation.Thisisespeciallyimportantwhenmappingsoilswheretheparentmaterialisresiduumorbedrockasopposedtothatwhichwastransportedbywind,water,orglaciers.Inresiduum,bedrockstronglyinfluencesthetypeofsoilthatwilldevelop.

Theapplicationofcomputershasprovidedsoilsurveyorswithimprovedbasemapsbyremovingdistortionsinherentinphotosmadefromanairplane.Thesecorrectedphotosarecalledortho(true)photos.Whenlaidsidebysidetheymatch.Theycanbedigitizedsothatthephotoimagecanbereproducedonacomputerscreenandzoomedinoroutforadetailedorgeneralview.

Theglobalpositioningsystem(GPS)wasintroducedinChapter8.Itispossibleforsoil

surveyorstocarryaGPSlocatorandacompactcomputerloadedwithadigitizedthree-dimensionalbasemap.Inthisway,moreaccurateinformationwillbeenteredintoadatabaseasthefieldworkprogresses.Experiencehasshownthatsuchtechnicaladvancesimprovethequalityandefficiencyofsoilsurveying.

UsesofSoilSurveysIntheUnitedStates,theNRCS,agriculturaluniversities,andcountyextensionofficescanusuallysupplyinformationaboutthesoilsofagivencounty.Forexample,forColumbiaCounty,Wisconsin,thereisa156-pagepublishedsoilsurveyreportwith122mapsheetsshowingsoilmappingunitsinthecounty.Eachunithasitsowncombinationofsoilhorizons,slope,andmoistureregimecharacteristics.Asanexampleofthekindofinformationgathered,twoofthesesoilsaredescribedinTable12.4.

Table12.4TwosoilsofColumbiaCounty,Wisconsin

KindofhorizonSoilname Topsoil

(A)Subsoil(B) Parentmaterial

(C)Slope Typicalcornyield

inbushelsperacrea

Plainfieldloamyfinesand

Thick,pale(ochric)

None Sand,acidtoneutral

Undulating(2%to6%gradients)

45b

Planosiltloam

Thick,dark(mollic)

Stickytofirm,blocky(argillic)

Siltoversandyloamglacialdeposit

Nearlylevel(0%to2%gradients)

130c

Withoutirrigation.45bushelsperacre=2,800kg/ha.

130bushelsperacre=8,160kg/ha.

Justasthemakingofsoilmapsischanging,soisthepublicationofsoilsurveyreports.Recenthardcopyreportsaremoreflexible,withonlythephotosectionboundandtheinterpretivesectioninloose-leafformforeaseofmodification.Thereisgettingtobelessemphasisonhardcopyintheformofbookscontainingphotos,printeddiscussion,andinterpretivetables.Moreemphasisisbeingplacedondigitizedphotosandcomputerizedinterpretiveinformation.Thiswillextendtheusefullifeofthereportsbecausetheywillbeeasilyupdatedandtailoredtothespecificneedsoffutureusers.

IntheUnitedStates,soilsurveyshavebeenpublishedformostoftheagriculturalregionsaswellasmuchoftheforestandrangelands.Insomecases,countiessurveyedmorethan30yearsagohavetoberecheckedtogatherinformationthatwasnotcollectedatthetimeoftheoriginalsurvey.TherevisedsurveysarebeingdigitizedinaccordancewiththeSoilSurveyGeographic(SSURGO)databaseandmustmeetthenationalcartographicstandards.Astheseeffortsprogress,moresoilsinformationwillbeaddedtowhatiscurrentlyavailableoncompactdiscs

andWebsites.

ExamplesfromthemapsectionofatraditionalsoilsurveyreportofRandallCounty,Texas,areshownheretoillustratesomeoftheinformationavailable.Figure12.28showsaportionofadetailedsoilsurveymapthatmightbeusedasthebasisformakingafarmplan.Figure12.29isageneralizedmapoftheentirecountysuitableformakingdecisionsaboutbroadareasofcropandrangemanagement.

Figure12.28AdetailedsoilmapofonesectionoflandinRandallCounty,Texas.Itis1mile(1.6km)oneachside.

Figure12.29AgeneralizedmapofRandallCounty,Texas.

Soilsurveysnolongerbenefitonlyagriculturebutareofvaluetoanyonewhomakesdecisionsabouttheland.Thisincludesfarmersandrancherswhowanttomaximizeproductionefficiency.Fertilizerdealersbetterservetheircustomersthroughanunderstandingofthesoilsbeingmanaged.Engineerswhobidonearth-modifyingprojectssuchasroadsandairportsfindsoilsurveysusefulforplanning.Landdevelopersmustconsiderthesoilforfoundations,streets,lawns,andsometimesthesepticsystems.Bankersandothermoney-lendingagenciescangetabetterfeelingforthesecurityoftheirloansiftheyknowthepotentialofthelandbeingusedbytheborrowers.Forestersusesoilmapsinspeciesselectionforregeneration,andtheyalsoplantheharvestingoperationsbased,inpart,onthebearingcapacityofthesoilanditssusceptibilitytoerosion.Parksandotherrecreationalfacilitiesmustbeplannedaroundthesoil'ssuitabilitytosupporthumanandvehiculartraffic.Taxassessorsandappraisersarebecomingincreasinglyawarethattheycanmakemoreequitableassessmentsonfarmlandandranchlandiftheytakeadvantageoftheinformationprovidedinsoilsurveyreports.Ascanbeseen,thereisawealthofinformationavailableinthesereports;onlyafewhavebeengivenheretoillustratethepoint.

Evenifthereisapublishedsoilsurveywithacompletemapofanarea,itisagoodideatoexaminethesoils.Thereismorevariationonthelandscapethanthepublishedmapcanshow.Itishelpfultolearnhowtorecognizethepropertiesofthespecificsoilhorizons.

LandCapabilityClasses

Soilsurveymapsareinterpretedformanyusesinthesoilsurveyreport,butinagriculturetheyareusedmostextensivelytodeterminelandcapabilityclasses.Allsoilbodiesareplacedinoneofeightclasses.Class1landiseasilymanagedforcropproductionwithouthavingtoovercomeanyappreciablelimitations.Classes2through4haveincreasinglimitationsiftheyaretobetilledforcropproduction.

Landinclasses5through8isnotrecommendedascropland,andinclass8theprimaryvalueisaestheticandasawatershed.

Thefourpossiblesubclasses—E:erosion,W:wetness,S:rootingzone(e.g.,shallow),andC:climate(e.g.,arid)—showthedominantlimitationthatcausesthesoiltobeplacedinaparticularclass.Themostobvioustocroppingistheerosionhazard,whichisbasedonslope.

Severaldrawingsinotherchapterscanbeusedtoillustratesomeofthelandcapabilityclasses.Class1landisdepictedinFigure3.8becausethelandislevelandnocrop-limitingcharacteristicsareshown.Figure6.15mightbeclassed2Wbecauseofitsneedfordrainage.TheerosionprobleminFigure10.2ispresentbecauseclass3Elandisbeingimproperlytilledupanddownthehill.TheshallowsoiloverbedrockinFigure10.1Bcouldbeinclass6S,andtheboginFigure12.9mightbestbeinclass7W.

Landhastobeevaluatedforseveralpossiblelimitationstoplaceitinthepropercapabilityclass,butbylookingattheseexamplesthereadershouldbeabletounderstandbetterhowthesystemworks.Similarsystemsareusedinthesoilsurveyreportfortheland'spotentialforforestryandwildlifehabitat.

SoilLandscapeAppreciationTheeminentbiologistAldoLeopoldwroteinhisbook,ASandCountyAlmanac,"Whenweseelandasacommunitytowhichwebelong,wemaybegintouseitwithloveandrespect."Ithelpstoknowenoughaboutthesoiltoevaluatesoillandscapesintelligentlyandavoidmistakesinlandmanagementdecisions.Landscapeappreciationforitsownsakeisofinteresttotheordinarycitizenwhodoesnothavedirectresponsibilityforsoilmanagementbutfeelsanaffinitytoit.

Chapter13SoilandItsUsesSoilisamediuminwhichplantsaregrownforfoodandfiber—theprincipaluseofsoil.Itgivesmechanicalsupportforplantsroots—anecessaryfunction.Soilactsasadecomposerforvegetativeandanimalremains—anecessitytokeeporganicmaterialsfromcontinuingtoaccumulateonthesurface.Soilisinvolvedinwaterandairmovement—vitaltoplantgrowth.Inaddition,soilisbeautiful.Itisanaestheticresourcewhichispleasingtomanypeople.

Inadditiontothesegeneralfunctionsofsoil,therearemanymorespecificusesofsoilotherthanforproducingfoodandfiber.Theyrangefromfoundationsforstructures(roadsandbuildings)touseinwatertreatmentandwastedisposalfacilitiesbymunicipalitiestoitsuseinurbanlandscapesforlawns,flowers,andvegetablegardenstodams,leveesandponds.

Thepurposeofthischapteristodiscussthevarioususesofsoilswhenusedforpurposesotherthanforagriculturalproduction.Chapters3through7providethepropertiesandcharacteristicsofsoilthatcanbeusedinitsmanagementwhetherincropproduction,inurbanlandscapes,orforfoundationsforstructuresoritsmanyotheruses.

PrinciplesofsoilmanagementinChapter9aremainlyforagriculturalproduction,buttheycanalsobeappliedtomanyotherusesofsoil.

UrbanSoilsAspopulationincreasestherearenotonlymanymorepeopletofeedbutalsomanymorepeopletohouse.Asaresult,landonceinfoodproductionisbeingtakenbyexpandingcitiesandsuburbandevelopment.Asurbanitespopulatethecommunitiesthatincludetheirhomes,workplaces,industry,andinfrastructure,thenaturalsoilbodywillbegintoundergochangestosuiteachownersneedswithonlysomeregardtotheconsequencesofsuchchangestothesoilorthebroaderenvironment.

Soilsaresoils:theyareanaturalbodyofmatterthatoncewereleftonlytotheforcesofnaturetoevolveintowhattheyaretoday.Anurbansoilisonethathasbeendisturbed,modified,orcontaminatedthroughhumanactivityandurban/suburbandevelopment.Theynolongerhavecharacteristicslikethenaturallandscapesurroundingtheurbanarea.Themodificationstotheurbansoilhavebroughtaboutsignificantvariabilityinsoildevelopmentalcharacteristicsduetochangesinvirtuallyallofthesoil-formingfactorscomparedtonaturalsoils.Thehydrologyofanurbanareaalsochanges,resultinginaneffectonplantgrowth.Theintensityofthesechangeswilldependonpastlanduseandorsite-specifichumanactivity.

VariabilityandHydrologyTheonecommonthemeofurbansoilsisthedegreeoflateralandverticalvariabilityover

shortdistances.Whilelateralandverticalvariabilityiscommoninnaturalsoilprofiles,asthelandbeginstobecomemoreurbanizedthereisaneedtoexcavatesoiltoinstallundergroundutilities,foundations,basements,roads,orchangeelevationstoaccommodatedesignfeaturesofvariousstructures.Asaresultsoilsthatareexcavatedmayhavematerialremovedandplacedatadifferentlocationon-site,hauledoffsite,eroded,backfilled,orhauledinfromanotherlocation.Inaddition,humanartifactsusedinconstructionarecommonlyfoundburiedinurbansoilsandcanbeaproblemwhenplantingshrubs,trees,andsoon.

Insomecasesthenaturalsoilisburiedunderon-siteoroff-sitematerials,whileinothersitismixedwithsoilfromadifferentlocationordepth.Urbansoilsaredisturbedsoils,madeupofamixtureofsoilmaterialsobtainedfromoneormorelocationsandplacedinlayersofvaryingthicknessesmakingitdifficulttopredicttheirhorizoncharacteristics.Theextentofdisturbancewilldependontheexistingsoilproperties,theintendeduseandreasonfordisturbance,andthepropertiesofexternalsoilmaterialshauledin.

Therearemanyareasofurbanlandthatremainrelativelyundisturbedandretaintheirnaturalsoilcharacteristics.Theseundisturbedareasaretypicallyrepresentedbyurbanforests,orareasbeyondtheinfluenceoftheconstructionzone.Thesevariouseffectscreatea"mosaic"ofsoilconditions,rangingfromnaturaltohighlydisturbed"anthropogenic"soilprofiles.

Thehydrologyofanareacanchangeconsiderablyundertheimpactofurbandevelopment.Thisinturnimpactsplantgrowth.Twohydrologicalaspectsthatareimportantareinfiltrationandoverlandflow.

Urbanareasarecharacterizedbyimpervioussurfaces(roofs,driveways,parkinglots),layeredsubsoils,modifiedslopes,andlandsurfaceswithorwithoutvegetativecover.Rainwaterfallingonimpervioussurfacesmayormaynotbedistributedevenlyontheadjacentlandsurface;insteaditmaybecollectedfromrooftopsandsumppumpsanddischargedinlocalizedareasonthelandsurfaceormaybedirectlydischargedtoastorm/sanitarysewer.Whenthewatercollectedfromrooftopsandotherimperviousareasisallowedtorunfreelyontotheadjacentland,waterwillsoakintothesoilinanunevenmanner(alteringthesubsurfacewatermovement).Excesswaterthatdoesnotinfiltratethesoilisdistributedfartherdownslopefromthepointofdischarge.Wateravailableforinfiltrationimmediatelyadjacenttothedischargepointsofthepavedareasdependsontheslopecharacteristicsandtheclaycontentofthearea.Insoilswhereinfiltrationisrapid,theamountofwaterenteringasoilprofileishigherthanfromanaturalrainevent.Comparedtoruralsoils,localizedurbansoilsareexposedtomoreinfiltrationwaterthanthatwhichwouldhaveresultedfromnaturalprecipitationinthearea.

Whentheadditionalwaterpercolatesthroughthesoil,itnotonlyimpactswateravailabilitytourbanvegetationbutalsoimpactsfuturehorizondevelopment.Additionally,theexcesswatercanhelpmovesolublechemicalsfartherdownthesoilprofileandeventuallyintothegroundwater.Duringshortrainevents,rainwaterusuallysoaksintothesoilplusanyoverlandflowthatcomesfrompavedsurfaces.Duringintensestorms,theinfiltrationrateofthesoilmaynotberapidenoughtoabsorballtherain;therefore,moreoverlandflowiscreatedfromunpavedsurfaces.

Restoringsoilstructurebytheadditionoforganicamendmentssuchascompostandmulchesareknowntoincreaseinfiltrationratesbyafactorof6-10timesthanthatofasimilarsurfacesoilwithouttheamendments.

Thehydrologyofanareacanalsohaveanimpactonflooding.Modificationscanbemadetolessentheimpactofexcesswater.Inmosturbansoils,thesurfaceslopes(aspectandsteepness)aremodifiedtoefficientlyremoveexcesswateralongthelandsurface.Slopemodificationscanchangethequantityandthedirectionofwaterflow.Oftendrainageditches,swales,roofdrains,curb,andguttersaredesignedtocarrywaterrapidlyawayfromareasofhumanactivityimmediatelyafterarainevent.Additionally,therapidremovalofwaterfromlocationsintheupperelevationsofawatershedwouldmeanpotentiallyexcesswaterinlowerelevations.Adequateprecautionsmustbetakentoavoidfloodingoflow-lyingareas.

Manyurbanplannerswilldiscouragepropertydevelopmentinurbanfloodzonesbecauseofaperiodicseriousriskofbeingflooded.Landinfloodzonesisnormallynearwater,oftenhasflatslopesandthereforepreferredforconstructionofhomesandoffices.Manyurbaniteshavechosentobuildonsuchsitesandinstalledexpensivefloodcontrolmeasures.Withthehydrologyofthehigherelevationsofthewatershedconstantlychangingduetodevelopment,thefloodcontrolstructuresoftenturnouttobeineffectivesubjectingthepropertyownertoperiodicfloodingandlossoflifeandproperty.Theproximitytosurfacewateronfloodplaindevelopmentsalsohasanegativeimpactonsurfacewaterquality.

SoilPropertiesInadditiontothelateralandverticalvariationinmaterialsthatremaininplaceinanurbansoilenvironment,thephysical,chemical,andbiologicalpropertiesofthesoilaresignificantlyalteredwhensoilsaredisturbed.Allthesehaveanimpactonurbansoilqualityandtheenvironment.

PhysicalPropertiesThephysicalpropertiesofsoilthatarealteredincludetexture,compaction,temperature,waterstorageandmovement,air,structureandporosity,andslope.Thefirstthreelistedarediscussedingreaterdetail.Erosion,afunctionofmanyofthephysicalcharacteristicsofsoil,isalsodiscussed.

Textureinurbansoilswillvarysignificantlyindisturbedconditions.Thetextureofthematerialplacedinthegroundwilldependonwhetheritisofnaturalsoiloriginormanmade(suchasashes,demolitionwasteordredgedmaterialfromapondorwetland).Sometimesthechoiceofthesematerialsisbasedoncostandaspecificsoilpropertyintendedtobealtered(drainage,elevationchanges,plantgrowth,etc.).Inspecificlocationswherethelocalsoildoesnotmeetaspecificengineeringneedsuchasphysicalsupport(roadbed,driveway),watermovementorretentionmaybedesired(gravel,sand,orclaymaybehauledin).Forintensivelylandscapedareas,soilandamendmentsrichinorganicmattermaybehauledinthusmodifyingsoiltextureinthesurfaceorsubsurfacelayers.

Compactioninurbansoilsvariessignificantlyanddepends,forexample,ontheuseofheavyequipmentandintensivepedestrianorvehiculartrafficduringandafterconstruction.Compactionisintensifiedwhentheseactivitiesarecarriedoutonwetsoils.Compactedsoilswillaffectrootpenetration,airmovement,watermovement,andtheamountofstoredwateravailableforplantormicrobialgrowth.Suchimpactsareobviousinareascoveredbylawnsthatareconsistentlysubjecttohumanfoottrafficorvehiculartraffic.Soilpathsgetsocompactedthattherootshavedifficultygrowinginthesoil,andwaterhasdifficultyinfiltratingthesoilcausingpuddlesafterarainevent.

Compactioncanoccurinexcavatedorunexcavatedsoilsfromtheweightofconstructionequipment.Oftenthesoilsmaybecompacteddeeperintotheprofileaffectingthestructure,porosityanddensityofthesoil.Whilesurfacecompactioncanbeundonebytillingtoacertaindepth,itismoredifficulttoundothecompactionatdeepersoildepths.Suchdeepcompactioncanaffecturbantreegrowth,oftenstuntinggrowth(comparedtotheircounterpartsgrowinginundisturbedforestedconditions).Oneofthecausesforstuntedgrowthcouldbealackofoxygenorphysicalrestrictions(smallervolumeofpores)forrootgrowth.

Whensoilsarecompacteddeeperintheprofile,waterpenetrationcanbeaffectedresultinginabuildupofaperchedwatertableaffectingtreerootpenetrationandoxygenneedsoftreeroots.Suchasoilsituationcouldresultinashallowrootsystemandpotentialfortippingoverunderstrongwindconditions.Inotherurbansituations,thelandadjacenttoanexistingtreetrunkmaybecoveredwithadditionalsoilburyingthefeederrootsdeeperandcuttingoffoxygensupplytotheroots.

Itmaytakeseveralyearsbeforethesubsoilinanurbanareareturnstoitsnaturaldensity.Theuncompactionprocesswilldependonthetextureofthecompactedsoilmaterial,therootinghabitsofthevegetation,thefrequencyofwetting/dryingorfreeze/thawcycles,andtheextentofadditionalcompactingactivitiesatthesurface.Compaction,however,isnotofconcerninwell-managedflowerbedsandturfgrassareaswherethereislittlehumantrafficaftertheornamentalplantshavebeeninplace.Theseflowerbedsmayalsoperiodicallyreceiveorganicmatteradditionsthroughmulchorcompost.

Soiltemperaturewillbeimpactedbyurbandevelopment.Concretesurfaceswillabsorbsolarradiationandretainheatthatmaybelaterreleasedtothesurroundingsoils.Inadditionheatreleasedfromhumanactivitiestendstomakeurbanenvironmentandurbansoilswarmerthansoilsinnonurbanareas,thereby,extendingthegrowthcycle.Urbanareasoilsmayseefewerfrost-freedaysextendingthegrowingcycleandincreasingthedecompositionrateoforganicmattertherebyaffectingnutrientflux.Tallstructuresmayhaveashadingeffectononeormoresidesofthestructuremakingthesoilcoolerinareasprotectedfromthesun.Urbaneffectsofshadingfrombuildingsislikelytomodifymoistureandtemperatureregimesinisolatedsituations.Applicationofmulchesinlandscapingwillalsohaveasimilarcoolingeffectimpactingorganicmatterdecompositionandnutrientreleaseintheshadedareas.

Erosioninurbanareascanbeaproblem.Thereisasignificantpotentialforerosionwhenabaresoilisexposedtowindandrain,especiallyifthesubsoil(withlittleornoorganicmatterasbindingagents)isexposedtotheelements.Erosionatconstructionssitescouldbeaserious

problemamountingto100timesmoreerosionthanwhenthebareAhorizonofthesamesoilisexposedtotheelements.Erosioninurbanareasisfurtheracceleratedduetoalterationofslopeswhenthesoilisexcavatedandplacedinapilewithsteepsideslopes.Precautionsshouldbetakentopreventthesoilbeingwashedawayanddepositedonneighboringproperty.Oftensiltfencesareplacedatconstructionsitestotrapthesoilyetallowthewatertopassthroughthefenceandpreventanydepositsonadjacentland.Excavatedbaresoilexposedtoprolongeddryperiodscanresultinseriousdustissuesonwindydays.Itisoftenrecommendedthatearth-movingactivitiesbecoordinatedwithappropriateweatherconditions.Ifbaresoilmustbeleftexposedforprolongedperiodsoftime,allprecautionsmustbetakentosomehowbindthesoilbyhydromulching,sprayingwaterondrysurfaces,orestablishavegetativelayeratthesurfaceofthedisturbedsoil.

Onceanurbansoilislandscaped,theslopesmeticulouslymodified,andthelandsurfaceiscoveredwithvegetationduringmosttimesoftheyear,thesesoilstendtobelesserosivethantheirnaturalcounterpartsunderagriculturallanduse.

ChemicalPropertiesThereisahigherdegreeofchemicalvariabilityinurbansoilsascomparedtoruralsoils.Twomajorreasonsforsuchvariabilityarethedisturbed/mixednatureofurbansoilsandtheextentofchemicaladditionfromexternalsources.

AmongthechemicalpropertiesalteredarepH;ionicconcentrationsofthesoilsolution;nutrientbalanceandfertilizeradditions;organicmattercontent;additionofairpollutantsfromcars,industriesandtheburningoffossilfuels;andpossiblycontaminantsreleasedtothesoilfromhumanactivityandmanufacturingindustries.Addingtothealterationofchemicalpropertiesisforeignmattersuchasconcreteorconstructionmaterials,ashes,pesticides,soilamendments,andthedisposalofhumanwastesviasepticsystems.

Urbanenvironmentsreceivehigherconcentrationofatmosphericchemicalsintheformofdryorwetatmosphericdeposition.Thesourcesofthesechemicalsareburningoffossilfuels,emissionsfromindustry,andexhaustfromautomobiles.Atmospherictemperatureinversionsinurbanenvironmentsoftenkeepthesechemicalssuspendedovertheurbanareas.Heavymetalssuchaslead,copper,nickel,andairpollutantssuchassulfurandnitrogencompoundsareoftenpresentinthesedeposits.Thesechemicalsaccumulateinthesurfacehorizonsofurbansoils.

ThepHofurbansoilsissubjecttomanyfactors.ManyoftheairpollutantsmaylowerthesoilpHinsoilsthathavealowbufferingcapacity.Theacidicconditionspromoteleachingofbasesinurbansoils.HeavymetalspresentinurbansoilscouldbereleasedatlowerpHandabsorbedbytheplant.Inextremesituationssomeoftheheavymetalscanreachtoxicconcentrationsforplantsandanimals.SoilpHcanvarybetweenlocalizedareasmanagedforturfgrasssuchastransportationandcommercialcorridorsandthosearoundresidences.SoilsincontactwithorcloseproximityofbuildingandpavingmaterialsrichincalciumtendtoexhibitahigherpH.OftenlimeadditionsarepracticedinhumidregionstomaintainanadequatesoilpH.ThehigherpHlimitsheavymetalsfromcomingintosolution;therefore,theyremainlockedintothesoilmatrixandreducetherisktoplantsandanimals.

Thelevelsofplantnutrientsinurbansurfacesoilsaregenerallyfoundtobeadequatebecauseinmostinstancesacertainamountoftopsoilisplacedondisturbedland.Inintensivelymanagedlandscapeswherefertilizer,compost,andorganicmulchesareaddedonaregularbasis,nutrientlevelsmaybeathigherthannormallevelscomparedtoruralsoils.Insomeurbansoilsrepeatedfertilizeradditionscontainingadisproportionateamountofonenutrientcanoftencreateanimbalanceinnutrientavailabilitytoplants.Theoverapplicationofevenabalancednutrientsource,whilehelpingvegetationtogrowfaster,mayhaveanegativeimpactonsurfaceandgroundwater.Excessphosphorusinurbanlakesandstreamshasbeentracedtoexcessivefertilizationoflawnswithphosphorus.Thisimpactsbiodiversityinthereceivingwater.Evidenceshowsthatoverfertilizationwithnitrogencombinedwithirrigation(practicedofteninurbanenvironments)mayleadtoflushingofnitrogenintothegroundwater.

Additionofsolublesaltsisalsoafactorinurbansoils.Irrigationwater,excessiveuseofinorganicfertilizers,saltsusedfordeicingpavedsurfaces(streets,driveways,andsidewalks)incoldclimates,watersofteners,andsoon,haveanimpactonthesolublesaltcontentofthesoilwaterandthevegetationgrowingatthatlocation.Inareaswherethesaltconcentrationishigh,salttolerantplantsgainacompetitiveedge.Saltconcentrationwillalsoimpactthetypesofmicroorganismsthatwillbeactiveinthesoil.

Heavymetalsinurbansoilscomefromvarioussources.Atmosphericadditions,originalrock(copper,mercury,lead,zinc),andpastindustrial/miningactivities(cadmium,nickel,copper,lead)serveaspotentialsourcesofheavymetalsinthesoil.Leadinparticulariscommoninurbansoilsasaresultofautomobileexhaustwhenleadwaspartofgasoline,andleadpaintusedintheexteriorandinteriorofoldbuildings(priorto1978).Urbanfiresanddredgedmarinesedimentsalsoaddmetalcontaminantstolocalsoils.Presenceofmetalsdoesnotprecludeplantgrowth,onlyinsituationswheretoxiclevelsaccumulatewilltherebenovegetativegrowth.

Extraneouschemicalscanbeaproblemattimesforurbansoils.Modernindustrialsocietieshavedevelopedneworganicandinorganiccompoundsforvariousindustrial,agricultural,andmanufacturingapplications.Someofthesechemicalsareinertandharmlesswhilemanyofthesecanbeextremelytoxictohumansandotherorganisms.Someofthesechemicalsarenotbiodegradable.Manyhaveaccidentallymadetheirwayintoourenvironment(soil,water,air,andplantandanimallife).Theintensityofhumanactivityanduseofthesechemicalsbyhumansinvariousapplicationsleadstoincreasedconcentrationsofsuchchemicalsinurbansoilenvironmentsthaninruralenvironments.

Thesoilisoftenexpectedtomiraculouslydecompose,alter,treat,detoxifysomeorallofthesechemicalsandsomehowmakethesoilenvironmentunharmedandyethavelittleimpactonsurfaceandgroundwater.Oncethesematerialsenterthesoil,however,theyaffectallformsoforganismsinonewayortheotherandimpacttheoverallecologicalcycleintheseenvironments.Eachsoilorganismhasacertainthresholdconcentrationbelowwhichitsactivitiesareminimallyaffected.Oncetheselimitsareexceeded,ecologicaldamageisenhanced.

BiologicalPropertiesTheorganicmattercontentofurbansoilsishighlyvariableandwilldependonseveralfactorsincludingintensityoflandscapemanagement;additionsoforganicamendments;degreeofsoildisturbance;extentoforganicmattercontributionsfromvegetation;andrateoforganicmatterdecomposition.Litterandorganicmatterdecompositionrateisdependentonthesoilenvironmentalconditions,microbialandinvertebratepopulationpresent,andthesource/typeoforganicmatter.Typically,organicmatterdecompositionratesarefasterinurbansoilsthaninruralsoils.Thisisattributedtothehighertemperaturesandadequateamountsofnitrogencontributedfromfertilizeradditionsaswellasatmosphericdeposition.Otherstudieshaveshownthatduetothehigherozoneconcentrationinurbanair,thevegetativematterexposedtoozoneissubjecttoslowerdecompositionrates.

Soilorganismsinurbansoilsareimpactedbysoilconditions.Theconsequenceofchangesinsoilphysicalandchemicalpropertiesinurbanareasmeansthatthehabitatfornativespeciesmayhavechangedallowingnon-nativespeciesofplantsandanimalstobecomedominant.Thefragmentednatureofurbanlandscapesreducesthehabitatavailableforanygivenanimalandplantspeciesallowingnativespeciestobedisplacedandreplacedbynonnativespecies.Manynon-nativespeciesareplantedinurbanareasthatmayperformwellunderdisturbedsoilconditionswhilethenativespeciesmaynotbeabletodoaswellunderthedisturbedsoilconditions.

Sometimestheseplantspeciesareintroducedviaseedspresentinsoilorcompostmaterialhauledinfromexternalsources.Atothertimesplantspeciesarepartofalandscapingplan.Unlesstheplantcanhandlethephysicalandchemicalconditionsofthesoil,itmaynotthrive.Theinteractionofsoil,plants,insects,andmicroorganismsamongeachothercandeterminethesoilnutrientfluxunderthenewenvironment.

GrowingPlantsinUrbanLandscapesToobtainoptimumgrowthofplantsintheurbanlandscape,propermanagementpracticesarejustasimportantaswiththeproductionoffoodandfiber.Theidealsituationistoestablishandmaintainthecorrectcombinationofallsoilfactorsnecessarytooptimizethegrowthofplants.

Thephysicalconditionofthesoilisthestartingpoint.Startingwithandkeepingthesoilingoodphysicalconditionmeansthatitwillbeeasytotillandthatseedgerminationwillbeoptimumorthattransplantswillresumegrowthquickly.Therateatwhichwatersoaksintothesoilandthewater-holdingcapacityofthesoilwillbeoptimal.Rootgrowthandsoil-airinterchangewillbenefitfromgoodphysicalconditionofthesoil.Applyingandincorporatingorganicmaterialsisonewaytoassuregoodphysicalcondition.

Nextthechemicalcharacteristicsofthesoilneedtobeinproperbalance,whichmeansmaintainingsoilfertilitylevelsandthecorrectpH.StartwithsoilpH—theacidityoralkalinityofthesoil.ManyplantscanachieveoptimumgrowthoverawiderangeofsoilpH.Someplantspecies,however,preferanacidsoilconditionwhilesomewillperformbestinanalkaline

soilcondition.ApplylimestonetocorrectthepHofsoilifitistoolow,orusesulfurforasoilthatistooalkaline.Limestoneandsulfurneedscanbedeterminedbysoiltests.

Nextassureaproperbalanceandsupplyofplantnutrients.Ifneeded,applyfertilizer.Itcanprovidetheplantnutrientsnecessaryforgoodgrowthprovideditincludesthoseneededtobalancethenutrientlevelsinthesoilandtoprovidethenutrientsneededbyplants.Fertilizermaycontainonlyonenutrientoranycombinationofthe14elementsrequiredforgrowth(otherthanC,H,andO).

Nutrientsotherthannitrogennormallyneedtobeappliedpriortoplantingortransplantingbecausetheynormallymoveverylittleinthesoilafterapplication.Anexceptionispotassiuminsandysoils.Highnitrogen-useplants,suchasgrasses,mayneedarelativelylargeamountappliedatseedingorsoddingwhileforlownitrogen-useplants,suchasflowersandshrubs,nitrogencanbeappliedminimallyatplantingandtopdressedastheseasonprogresses.Applyplantnutrientsaccordingtosoiltests.

Soilamendmentssuchaslimestoneandorganicmaterialsshouldbeappliedpriortoplanting.Itwouldbeconvenienttoapplytheseatthesametimethatfertilizerisapplied.Amendmentsandfertilizershouldbeincorporatedintothesoilbeforeplantingortransplanting.

Thebiologicalcharacteristicsofthesoilareimportantforgoodgrowth.Organicmatterandhumusneedtobemaintainedatanadequatelevelsothatmaximumbenefitsresult.Materialsthatservethispurposearepeatmoss,compost,manure(preferablytreatedandpartiallydecomposed),oralmostanytypeofplantresidue(leaves,grassclippings,etc.).Organicmaterialsneedtobeincorporatedpriortoseeding,sodding,ortransplanting.

Aswithanycropinagriculturalproduction,properselectionofavarietyorspeciesisdesirable,whetheritistrees,grass,shrubs,orflowers.Waterneedsshouldbeconsidered.Forexample,inthelow-rainfallareaoftheWesternUnitedStates,usespeciesofgrassthathavealowwaterrequirementsuchasBuffalograss.Inthecaseoftrees,shrubs,andornamentals,aXeriscapeapproachisoftendesirable.

Keepinmindthattheneedsofsoilsusedinurbanlandscapesarethesameasthoseinagriculturalproduction.Chapter9onSoilManagementgivesamuchmorecompletediscussiononsoilmanagement,mostofwhichappliestoturf,trees,ornamentals,vegetablegardens,andshrubs.

Therearemanyaspectsoflandscapesoilsandplantsandtheirmanagementthatoftenrequireprofessionalassistanceinsoilpreparationandinplanting.Thisisparticularlytrueforturfplantedassodandfortreesandshrubs.Foralong-termusesituation,itisoftenbettertoaskforprofessionalassistancetoassurelong-termsuccess.

Predictingsoilbehaviorofanurbansoilisdifficultbecausethephysical,chemical,andbiologicalchangesbroughtaboutbyonelandownertosuittheirownneedsmaynotservetheneedsofanewlandowner.Thesechangesinlanduseormodificationofsoilconditionsarenotalwaysaccuratelyrecordedandtransmittedtothenextlandowner.Therefore,itiscriticalthataprospectivelandownershouldexerciseduediligenceinevaluatingsoilconditionsthatmeettheneedsofaspecificproject.

EngineeringUsesSoilisasourceofmaterialthathasawidearrayofusesforengineeringpurposes.Someexamplesincludefillfordamsandlevees;foundationmaterialforroads,runways,andbuildings;aggregate(sandandgravel)formakingconcrete;clayforsealingthebottomsandsidesofponds,canals,andsolidwastelandfills;covermaterialovertanks,utilitylines,tunnels,culverts,andconduits(sewers,drains,pipesforwater,oil,andgas);andaporousmediumfortreatingliquidwastes.

SoilhasbeenusedtomakehomesliketheadobesoftheSouthwestandthesodhousesofpioneerssettlingontheAmericanprairies.Soilssupportenormousloads,bothinanimate(roadsandbuildings)andanimate(people,animals,andplants).

Totheengineer,soilisanysurficialmaterialoftheearththatisunconsolidatedenoughtobedugwithaspade.Soilmechanicsisthefieldofengineeringdevotedtotheuseofsoilasabuildingmaterial.Inthevocabularyofengineers,soilincludesboththesoilofthesoilscientistandanyloosesubstratumthatmaybepresent.Theconceptsofthesoilasconsideredbythescientistandtheengineeraremergedinthissection.

Anadvantageofusingsoilforengineeringpurposesisthatthereissomuchofit,anditmaybealreadyonthesite,whichavoidstheexpenseofhaulinginothermaterial.Anotheradvantageisthatsoilcanbesoreadilyshapedintoalmostanydesiredform.Dependingonhowitismanipulated,soilcanallowthepassageofwaterthroughitoritcanbemadealmostimpermeable.

Therearealsosomedisadvantagesforusingsoilinengineering.Soilisextremelyvariable,bothgeographicallyandovertime.Cyclesofwettinganddryingaswellasfreezingandthawingchangetheengineeringpropertiesofsoil.Unlikeknowntypesofsteelorwood,soilisnotauniformmaterialforwhichreliablestrengthscanbecomputed.Stabledryloammaybeadjacenttounstablewetclayinalowland.Duringarainyperiod,thedryloammayalsobecomewetandunstable.Inwinterbothsoilsmayfreezeandheaveinsuchawayastocrackpavementsandbasementwalls,especiallywherethemoisturecontentishigh.Table13.1comparesthesuitabilityofthreesoilsforvariousengineeringuses.Eventhoughthesesoilsareusuallyfoundnexttoeachotherinthelandscape,theirsuitabilityfordifferentengineeringapplicationsvarieswidely.

Table13.1SuitabilityorlimitationratingforsoilsoftheClarion-Nicollet-Websterassociation

Clarion Nicollet WebsterUSDAclassification Fine-loamy,mixed,

mesicTypicHapludolls

Fine-loamy,mixed,mesicAquicHapludolls

Fine-loamy,mixed,mesicTypicHaplaquolls

Shrink-swellpotentialRoadfill

LowGood ModerateFair(wetnessandlowstrength)

ModerateFair(lowstrength,wetness,andshrink-swell)

Embankments,dikes,andlevees

Severe(piping) Moderate(piping) Severe(wetness)

Dwellingswithbasements

Slight Moderate(wetness) Severe(wetness)

Septictankabsorptionfield

Slight Severe(wetness) Severe(wetness)

Sewagelagoonarea Moderate(slopeandseepage)

Severe(wetness) Severe(wetness)

Sanitarylandfillarea

Slight Severe(wetness) Severe(wetness)

Dailycoverforlandfill

Good Fair(wetness) Poor(wetness)

Source:Nelson,G.D.1990.SoilSurveyofMurrayCounty,Minnesota.USDA-SoilConservationService,U.S.GovernmentPrintingOffice,Washington,DC.Reasonsforlimitationsaregiveninparentheses

EngineeringPropertiesofSoilsPrecedingchaptershavediscussedsoilphysicalpropertiesfromtheperspectiveoffactorsrelatingtocropproduction.Soilmechanicsalsodealswithphysicalpropertiesastheyrelatetotheuseofsoilasabuildingmaterial.Somephysicalpropertieslikeparticlesizedistributionandbulkdensityareimportanttobothsoilscientistsandengineers.Evenso,engineershavedevelopeddifferentsoilclassificationsystemsspecificallyforengineeringapplications(Fig.13.1).

Figure13.1Soilclassificationsystemsusedbyengineers(AASHTOandUSC)havedifferentrangesforparticlesizedistributionsthantheUSDAsystem.

TheUnifiedSoilClassification(USC)SystemwasdevelopedduringWorldWarIIfortheconstructionofmilitaryairfieldsandhassubsequentlybeenmodifiedforuseinfoundationengineering.TheAmericanAssociationofStateHighwayandTransportationOfficials(AASHTO)SystemiswidelyusedbystatetransportationdepartmentsandtheFederalHighwayAdministrationforthedesignandconstructionoftransportationlines.BoththeUSCandAASHTOclassificationsystemsincludeseveraltestsinadditiontoparticlesizedistribution.

Twostandardizedtestsareoftencompletedtotestasoil'ssuitabilityasabuildingmaterial.ThesetestsarecalledtheAtterberglimits,ortheliquidandplasticlimits.Atahighwatercontent,asoilpossessesthepropertiesofaliquid.Asitdries,itactsmorelikeaplastic,thenlikeasemisolid,andfinallylikeasolidwhenitisdry.Theliquidandplasticlimittestsarecompletedtoidentifythemoisturecontentatwhichasoilchangesfromtheconsistencyofaliquidtoaplastic(liquidlimit)andfromaplastictoasemisolid(plasticlimit).

TheAtterberglimitshelpengineersdecidewhetherthesoilmaterialunderconsiderationissuitablefortheirprojectorwhetheradifferentsoilisneeded.LiquidandplasticlimitvaluesareusedwithparticlesizeinformationandotherteststoclassifysoilsintheUSCandAASHTOsystems.

SomeofthesoilcharacteristicsthatshouldbeknownbeforedecisionsaboutengineeringusesaremadearegiveninTable13.2.Manysoilsurveyreportsincludetablesshowingthesuitabilityofthedifferentsoilsforvariousengineeringpurposes.TheAmericanSocietyforTestingandMaterials(ASTM)publishesofficialmethodsformeasuringengineeringpropertiesofsoils.Itisimportanttounderstandthesoilsofanareabystudyingsoilsurveymapsandreports,talkingwithsoilscientistsandengineers,andmakingthepropermeasurements.

Table13.2Characteristicsofsoilsforengineeringpurposes

KindsofInformationaboutsoils

Comments

Soiltexture Theinorganic(mineral)partofsoilsisamixtureofsand,silt,clay,andcoarsefragments,evenincludingboulders.theUSDA,USC,andAASHTOsystemsarecomparedinFigure13.1.

Kindsofclay

Clayspeciesvaryindegreeofshrink-swellpotentialandotheractivity.

Depthtobedrock

Veryshallowsoilsareusuallyunsuitableforexcavationforbasements,ditchesalongroads,orutilitylines.

Kindsofsurficialbedrock

Bedrockmaybeveryhard(granite)orporous(sandstone,shale).

Soildensity Soilhorizonsrangeindensityfromporoustocemented.Thedensersoilshavethehigherbearingcapacitiesandrateoftransmissionofvibrations(soundmaytraveltwiceasfastthroughdensesoilasthroughair).

Contentofrockfragments

Soilwithmanyrockfragmentsisusuallydifficulttoexcavateandcompactuniformly.

Erodibility Manysandysoilsaresusceptibletowinderosion.Siltysoilgullyeasily.Someclaysoilsaresubjecttopiping,whichissubsurfaceerosionbyspontaneoustunneling.

Surfacegeology

Thelayofthelandaffectslanduse.Proportionsofsteepandlevellandvaryaswellassoilpattern(linear,circular)

SoilpH(reaction)

Degreeofacidityoralkalinityinfluencessoilbehaviorphysically,chemically,andbiologically.Tostabilizesoil,engineerssometimesaddhydratedlimetoit,whichraisesthepHintothealkalinerange.

Salinity Highsaltcontentofsoilaffectsitsstabilityandthatofvegetativecover.Corrosivity Soilsdifferincapacitytocorrodeburiedpipesandtanks.Wet,acidsoilsare

usuallyverycorrosiveDepthtoseasonalwatertable

Soilswithaseasonallyhighwatertableprovideinadequatesupporttoroadsandstructures.Frostactionismostsevereinwetsoils.

Plasticity Clayeysoilsarecommonlyquiteplasticand,withwetting,becomefluidlikeathickliquid.The"Plasticindex"istherangeofpercentmoisturecontentinwhichasoilisplastic.

Contentoforganicmatter

Tosupportgrowthofprotectivesod,asoillayercontainingorganicmatterisneeded.Otherwise,organicsoils,paritcularlypeatsandmucks,areusuallyremovedinengineeringprojects.

Roads,Residences,andStructuresNearlyallroadsandsmallbuildingsareplacedonsoils,manyofwhicharesoftinwetseasons.Insomeareassubjecttoseasonalfrosts,thesoilheaves(lifts)anddirtroadsmaybecomeimpassableincertainseasons.Modernengineersfindthatnaturallywell-drained,verysandy,andgravellysoilsprovidethemosttrouble-freebasesforroadsandbuildingfoundations.Onclaysoils,ablanketofsandandgravelisplacedovertheclaybeforepavementislaid,andditchesaredugoneithersidetodrainawaystormandseepagewaters(Fig.13.2).

Figure13.2Useofsandandgravelprovidesastablebaseforastructureonpotentiallyunstableground.

Water,soessentialforplantgrowth,isoftenundesirableinsoilsonwhichstructuresareplaced.Thelonglifeofaroadorbuildingfoundationdependsonmaintainingsoilconditionsthatpermitittobehavelikeacompact,well-drainedsandorgravel.

Roadsidesoilsalsoperformspecialfunctionsunrelatedtotheirengineeringuses.Certainorganismsinsoilcanmetabolizevariouscomponentsofautomobileexhaust.Embankmentsofsoilalongmajorhighwaysabsorbmuchofthesoundoftraffic,therebyreducingnoisepollutioninthearea.

Thehighcapacityofsandandgraveltosupportweightarisesfromtheparticle-to-particlecontactwithoutlubricationofsiltandclaybetweenthem.Gravelmadeofstrongandstable

quartziteislonglastingunderthestressofheavytrafficonpavementabovethegravelbed.Concretepavementitselfcontainsasmuchas50%graveland25%sand.

DamsandLeveesSoilisthemostreadilyavailableandleastexpensivematerialforthebuildingofdamsandlevees.Numerousreservoirs,lakes,andpondsbackupbehinddamslargelymadeofsoil.Manycitieslocatedinfloodplainsaredefendedagainstoverwhelmingfloodsbymiles(kilometers)ofleveesbuiltofsoil.

Suchearthworksneedtohavetwoqualities:stabilityandimpermeability.Fourkindsofsoilmaterialsareneededtoachievethesepropertiesinadamorlevee:(1)aclaycoreandblanketarecompactedandkeptmoistasasealagainstleakageofwater;(2)asandymassisadded,surroundingtheclay,todrainwateraway;(3)alayerofstoneandrubble(riprap)ispiledonthesurfaceexposedtomovingwatersuchaswavesandrivercurrents;and(4)agoodloamisusedtocoverremainingsurfacesoftheearthworktosupportthegrowthofprotectivevegetation(Fig.13.3).

Figure13.3Variouskindsofsoilmaterialsareusedinconstructionofanearthwork.

Sandisthemosteasilyexcavatedandtransportedconstructionmaterialbecauseitislooseanddoesnotbecomestickyuponwettingorhardupondrying.Baggedsandisusedinemergencyenlargementofleveesduringexceptionalfloods.Becausereservoirsandriverfloodplainstendtograduallyfillwithsedimentwashedinfromupstream,thismaterialoftenmustberemovedfromthereservoirorchannel(bydredging)every20–50yearsorso.Thedredgedsedimentcanbeusedtobuildartificialislandsorprotectedmoundsonthefloodplains.Itcanbereturnedtothefarmlandbutusuallyatgreatexpense.Reducingerosionisthereforedoublyimportantforbothsoilconservationandreducingsedimentbuildupinwaterbodies.

PondsandCanalsBottomsandsidesofpondsandcanalscommonlyneedtobesealedtopreventleakage.Variouskindsofliningmaterialsmaybeusedthatarelessexpensiveandlessbulkythanconcrete.Ifclayisavailablelocally,itmaybemixedwithbentonite,aspecialtypeofswellingclay,tomakeatightclayliner.Fabriclinerscontainingplasticorrubberorcoatedwithasphaltmaybeusedintheabsenceofclay.

UndergroundStructuresandLines

Earthshelteringofhomesandburialofutilitylines(Fig.13.4)andtunnelsprotectstructuresandfacilitiesfromunfavorabletemperaturesthattakeplaceatornearthesurfaceoftheground.However,someprecautionsshouldbefollowed.

Figure13.4Twoexamplesofearthshelteredinstallations.

Undergroundinstallationsmustbedesignedtosupportthegreatweightofsoilcover.Thestrainonburiedstructuresfromtheweightofoverburdenmaybeheightenedinwetseasonsbyexpansionofclaysinthesoil.Seepageofwaterintocracksinburiedstructuresmaybearecurrentproblem.Growthofdepositsofcalciteorironoxidesincracksmaygraduallyshatterconcretebelowground.

Concreteandmetalpipesandtanksaresubjecttocorrosioninsomemoistsoils(Fig.13.5).Tinyelectriccircuitsmaydevelopspontaneouslybetweenthesoilandironpipesinsuchamannerastoliterallyboreminuteholesinthepipesbydissolvingthemetal.Inregionsofpermafrost,watermainsmustbeinsulatedagainstfreezingbothfromaboveandbelow.

Figure13.5Cross-sectionofaburiedpipeandtheeffectofseverecorrosioninawetlandposition.

MunicipalWasteWasteTreatment

Humanactivitiesgenerateliquidandsolidwastesthatrequiretreatmentand/orcontainment.Liquidwastesincluderesidentialandindustrialwastewaters.Solidwastesincludehouseholdandofficewastesthatconsistofyardwaste,paper,plastic,glass,andmetal,andindustrialwastesthatincludemanufacturingby-productssuchassludgesfrompapermills.Manytypesofliquidandsolidwastescanbedisposedofinsoil.

Soilisaporousmediumwithenormousinternalsurfaceareapopulatedbymicroorganismsthatarecapableofdecomposingbiodegradablematerials.Becauseofthischaracteristicwithmicroorganisms,whenbiodegradablewastesaredisposedofinsoil,theyarebrokendownandtransformedmostlyintowater,carbondioxide,andothergases.

Nondecomposablewastessuchasrock,metal,glass,plaster,andplasticremainburiedorstoredinthesoil.Caremustbetakentomakesurethatliquidandgaseouscontaminantsderivedfromwastesdecomposinginsoilsdonotcontaminategroundwater,lakes,andstreamsorcometothesurfaceinunacceptableamounts.

WastewatersandBiosolidsInurbanareas,residentialandindustrialwastewatersaredischargedintosewersandaretreatedatawastewatertreatmentplant.Thetreatedwastewateroreffluentistypicallydischargedtoariverorlake(ortheeffluentcanbeappliedtofarmland).Oneoftheby-productsgeneratedatwastewatertreatmentplantsissewagesludge,nowcalledbiosolids.Biosolidscanbedewateredandhandledasasolid(10%ormoresolids)orasaliquidtypicallyover90%water.Biosolidscontainplantnutrientsandtheorganicremainsoftreatedwastewater.Biosolidsaretypicallyspreadonagriculturallandasasourceofplantnutrientsandcarbonforsoilorganicmatter.Biosolidsapplicationisnotwithoutenvironmentalorhealthrisk;weedseeds,humanpathogens,odors,andindustrialcontaminantsmaybepresentinimproperlytreatedbiosolids.

Somebiosolidsfromindustrialwastewaterscontaintraceamountsofmetalssuchascadmium,chromium,lead,andzincthatmaycontaminatesoilandlimittheuseofbiosolidsonagriculturalland.Driedorcompostedbiosolids(aslowas20%watercontent)aresoldasafertilizer.Itcanbeagoodsoilamendmentinthesamemannerasanimalmanure.

Inruralareas,septictanksandsoilabsorptionsystemsordrainfieldsareusedtotreatresidentialwastewaters(Fig.13.6).Wastewaterisdischargedfromahouseintoaseptictankburiedbeneaththesoilsurface,wheresolidsaredegradedandthewastewaterundergoesprimarytreatment.Thewastewateristhendischargedtoagravelbedandthesoilbeneaththebedisusedforfinaltreatmentandremovalofcontaminants.Thetextureandstructureofsoilbeneaththedrainfieldandtherateofwaterdischargeddeterminethedegreeoftreatment.Whilemostpathogensinthewastewaterareremoved,thereisariskofnitrogenleachingintothegroundwater.Whensoilscontaintoomuchclayorgravelorthedepthtobedrockorgroundwateristooshallow,anartificialsoilmoundofsandyorlocalsoilmaterialisconstructedabovetheoriginalsoilsurface.Thesetypesofsystemsremovesomepathogensandnitrogencompoundsfromthewastewater.

Figure13.6Residentialwastewatersgeneratedinruralhomesarerecycledbysoilabsorptionofseptictankeffluent.

Industrialwastewatersvaryincompositiondependingontheprocess.Somewastewaters,suchasthosegeneratedatcanneries,areoftenspray-irrigatedonagriculturallandafterprimarytreatment(Fig.13.7).Manyotherindustrialwastewaters,ifnotdischargedtowastewatertreatmentplants,aredisposedofinstabilizationpondsorabsorptionponds.Theby-productsofindustrialwastewatertreatmentsuchasbiosolidsorpondbottomsludgemaybeland-appliediftheycontainadequateplantnutrientsandaminimalamountofindustrialcontaminants.Stabilizationpondscontainimpermeablelinersandrelyontreatmentwithinthepondandevaporation.Wastewaterinabsorptionpondsistreatedasitinfiltratesthroughthesoilbeneaththeponds.Someindustriesgeneratehazardouswastesthatmustbetreatedanddisposedofatspecialhandlingfacilities.Researchersarecurrentlydevelopingalternativestothesemethodsofwastewatertreatment,includingdischargetoconstructedwetlands.

Figure13.7Irrigationoffarmlandisbeingtriedonalimitedbasisasameansofdisposalforwastewatergeneratedbysomesmallindustriessuchascanneries.

SolidWastesSolidwastesaretypicallyburiedinanengineeredlandfillthatismadeupofseveralmodulesthatarefilledinsequenceoveraperiodofseveralyears.Acompactedclayand/orasyntheticlineratthebottomofeachmoduleformsasealtopreventinfiltrationofliquidsthatmightcontaminatethesoilandgroundwaterbeneaththelandfill.

Precipitationthatcomesincontactwiththesolidwasteandtheliquidgeneratedbythedecompositionofsolidwasteiscalledleachate.Leachateiscollectedinpipesleadingtoacontainmenttankandthentreatedatawastewatertreatmentplant.Solidsplacedinacell(smallareaofthelandfill)arecompactedandcoveredwithsoildaily.Thesoilonthewasteservesseveralpurposes:trapsodors,keepspaperandplasticfromblowingaway,trapscontaminantsinleachate.Whenthemoduleisfull,itiscoveredwithaclayand/oraplasticcaptominimizetheinfiltrationofprecipitation.Severalfeetofrootingsoilisplacedaspartofthefinallandfillcovertoestablishvegetation.Ventsareinstalledinthelandfilltoallowgastoescapefromslowlydecomposingwaste(Fig.13.8).Landfillgasesthattypicallycontain65%methaneand35%carbondioxideareeitherburnedon-siteorconvertedtouseableelectricity.

Figure13.8Cross-sectionofalandfillcellwhenfilled.

Soilandgroundwaterqualitybeneathandsurroundingalandfillareintensivelymonitoredtopreventenvironmentalcontamination.Thecontaminantsofmostconcernaretracemetals,nitrogencompounds,andorganicsolvents.Duetothelargetractsoflandneededandthepotentialforcontamination,manymunicipalitiesareusingalternativestolandfills,suchasincineration,composting,andrecycling.

DisturbedorContaminatedLandsDisturbanceofland,eitherbynaturalprocessesorhumanactivity,andthecontaminationoflandareoftenofconcerntoengineers.Disturbedandcontaminatedlandsresultinasituation

wheresoilqualitymaybeimpacted,thegrowthofplantsmaybeseverelylimited,orplantsmaynotgrowatall.Reclamationandremediationofadisturbedareaisachievedbyrestoringittoaproductivestate.Thismayincludeartificiallandformingandtheapplicationofsoilamendmentsand/orfertilizertobefollowedbyreestablishmentofvegetativecoverorenhancementofthesoilenvironmenttopromotedegradationofcontaminants.

NaturallyDisturbedLandSoilmaybedisturbedormadeunproductivebynaturaleventsincludinglandslides;floodsthatdepositsedimentonlowlands;sandduneanddustinvasions;blowdownoftreeswithconsequentexposureofsoil;andburialoflandunderfreshlavaflowsandvolcanicashfalls.

ArtificiallyDisturbedLandMostartificialdisturbanceoflandiscausedbyhumanactivities,including(1)mechanicalstripminingforcoal,oilshale,ormetallicore;(2)hydraulicminingofsoilmaterialforgoldorphosphorus-bearingmineralsusedinfertilizer;(3)concentrationofliquidandsolidwastes(includingminetailings)onlimitedacreage;(4)contaminationofsoilareaswithoilbrinearoundoilwellsandwithtoxicmaterialsnearchemicalplantssuchassmeltersandoilrefineries;(5)contaminationofsoilbeneathandsurroundingleakingundergroundstoragetanks;(6)sterilizationwithresiduesofagriculturalchemicalsinlowspotsinfarmland;(7)quarrying;(8)construction(usingcut-and-filloperations)inlandscapesfordevelopmentofresidentialandcommercialbuildingsaswellasroadsandotherfacilitiesinurbanareas;(9)operationofvehiclesonfragilesoilsindesertsandtundraareas;(10)overgrazingofrangelands;and(11)overcultivationofcroplands.

Manydisturbedlandsareleftintheformofdeepholes,mudholes,driftedsand,ormountainsofoverburdenthataresteeplyslopingandwhererunoffisrapid.Ifthisisthecase,landformingisthefirststepinreclamation.

Disturbedsoilsarenotnecessarilylosttoagriculture.AtonesiteinWisconsin,asandyclayloamsubsoilwasneededbyafoundrycompanyformakingmoldsinwhichtopourandcoolmoltenmetal.Theyfirstremovedthetopsoil,andthenremovedtheneededsubsoil.Thefertileloamtopsoilwasreturnedtothesite,anditnowproduceshigh-qualitycropsjustasitdidbeforetheexcavation(Fig.13.9).

Figure13.9Cross-sectionofpartsoftwofields.Totheleftofthepost,thesoilisundisturbed.Totheright,thetopsoilwasremovedandsaved,andthenthedesiredsubsoilwasremoved.Theoriginaltopsoilwasreplaced,therebypermittingcropproduction.

ReclamationandRemediationProcedures

ConfinementofObjectionableSubstancesContainmentofwaste-waterandbiosolidsatwastewatertreatmentplantsisnecessarytopreventillegalandecologicallydamagingspillsfromflowingintoadjacentwaters.Fertileagriculturalsoilitselfbecomesapollutingwastematerialifallowedtowashintostreamsandlakes.Inthissense,thereasonforpreventingsoilerosionistoconfinethesoilbykeepingitinplace.Brineatoilwellsisnowcommonlypumpedintodeeplayersintheground,whereitcanbeconfined,insteadofbeingpouredontheland.

Agriculturalchemicals,especiallypesticides,shouldbeconfinedabove-ground,wheretheymaybesafelyusedordisposed,toavoidcontaminationofthegroundwaterandtheresultingpollutionofdrinkingwater.

Aswithbiosolids,flyashfromcoal-burningelectricpowerplants,iffreeofheavymetalcontaminants,maybespreadonfarmlandasafertilizerorsoilconditioner.Ifheavymetalcontaminationispresent,thosesubstancesmustfirstberemovedorthematerialsmustbedisposedofinanapprovedhazardouswastehandlingfacilityorlandfill.

RemovalofHarmfulSubstancesfromSoilExcesssaltinsoilcanbeleacheddownwardifthereisanadequatesupplyoffreshwaterandproperdrainageundergroundorthroughconduitsandditchestodisposeofthesaltyeffluent.Itisimportantthatthesoilsbesufficientlypermeabletoallowthemovementofwaterthroughthem.

Wherechunksofironsulfide(pyriteandmarcasiteminerals)inminetailingsyieldacideffluent,removalandproperdisposalofthesulfideandtheneutralizationofacidwithlime

maybeimplemented.

Whenthesoilbeneathundergroundstoragetankshasbeencontaminatedwithpetroleumproductssuchasgasoline,thesoilsurroundingthetankmayberemediatedbyusingseveraltechniques.Thesoilismostoftenexcavatedandhauledawayfromthesiteandspreadonagriculturallandorplacedinalandfill.Manyofthecontaminantsevaporateoraredegradedbysoilmicroorganisms.Atsomesites,thesoilistreatedinplacebyenhancingthesoilenvironmentfordegradationofcontaminants(bioremediation).Thistechniqueinvolvesinjectingairintothesoil,whichdrivessomeofthecontaminantstothesurfacewheretheyevaporate,whileprovidinganenvironmentmoresuitableforcertaincontaminant-degradingmicroorganisms.Researchersarecurrentlytryingtoidentifysoilmicroorganismsthateffectivelydegradecontaminantsandcouldbeintroducedintoanareawherethesoiliscontaminated.Ifthegroundwaterbeneaththesesitesiscontaminated,itmustalsoberemediated.

LandFormingShapingthelandbyconstructionofgrasseddiversionterracesandwaterwayscanspreadrunoffandconductwatersafelyfromslopingfarmland.Whererunoffiscaughtinholdingponds,thewatercanbeusedforirrigationorcansimplybeallowedtopercolatetothewatertable.Properlymaintainedterracescansuccessfullysubdividelongslopesintoasequenceofshortones,therebyreducingbothrunoffandsoilerosion.

Strip-minedlandmaybesmoothedtoslopesthatarenosteeperthantheoriginalones.Correctstockpilingoftopsoilandsubsoilduringtheinitialphaseofminingmakesitpossibletorestorethetopsoilcover.

Someofthesedimentthatisdredgedeachyearfromstreams,canals,andcatchbasinsforthebenefitofnavigationandaquaticlifemaybeusefulonagriculturalfields.Carefulattentionmustbegiventotexture,organicmattercontent,pH,andotherphysicalandchemicalcharacteristicsofthedredgedmaterials(spoils).

Someabandonedquarriesarefilledwithsoilinsuchawayastomakethemusefulforcroppingorotherpurposes.Inlandscapeswithafairlyhighwatertable,standingwaterisusedasacoverfordisturbedland.Roadsideexcavationsmaybecomerecreationallakesandpondsorsourcesofirrigationwater.

InterimProtectionoftheSoilSurfaceIndisturbedland,thesoilsurfacenormallyneedsprotection.Atsomeconstructionsiteswheredisturbanceofsoilcontinuesformonthsorevenyears,mulchesofstraworsheetsofspecialfabricshavebeenusedtocoverthebaresoiluntilfinalcoverbybuildings,pavements,andlawnshasbeencompleted.

EstablishmentofVegetativeCoverToreestablishvegetation,itisjustasdesirabletohaveagoodseedbedpreparedatthe

disturbedsoilsiteasitisinafieldtobeplantedtocrops.Inmanycases,thismaybedifficultbecauseofthenatureofthesoilmaterial.Inmostcases,soilamendmentsandfertilizersmayneedtobeapplied.Disturbedsoilsmaybehighinsomeessentialelements,butnitrogenisusuallyquitelow.

Reestablishmentofnativespeciesofvegetativecoverbyseedingandirrigationhasbeensuccessfulonlandthatwasstrip-minedforcoalintheFourCornersareaofthesouthwesternUnitedStates.InOhioandIllinois,agriculturalcropsarebeinggrowntodayonsomeprosperousfarmsthatwereinactivatedforseveralyearsbystrip-miningoperationsthatendedwithreconstructionofthelandscapeanditssoils.

Researchersarecurrentlytryingtodevelopmetal-scavengingplantscalledhyperaccumulatorsthatremovemetalsfromcontaminatedsoil,especiallyinminedareas,andstorethemintheirleavesandstems.Theleavesandstemsareharvestedandthemetalsrecycled.

Shelterbeltsoftreesandshrubsillustratediscontinuousestablishmentofvegetationfortheprotectionofadjacentcroplandfromwinderosion.Thetreesreducethewindspeed,therebyreducingtheamountofsoildetachmentandoftencreatingamicroclimatethatreducesevaporationandincreasesyields.

Roadsidepitsfromwhichconstructionmaterialshavebeenremovedmaybestabilizedbycut-and-filloperationsandsubsequentrevegetationwithtreesand/orherbaceouscover.Toavoidtheneedforreclamation,everyeffortshouldbemadetoavoidcontaminationofthesoilandtomaintainitinacceptableform.Todothisrequiresknowledgeanddedicationbythosewhousetheland.

Economicconsiderationshavesometimesledtomisuseoftheland;thus,governmentregulationshavebeennecessarytoprotecttherightsofcitizensandtoensureproductiveandbeautifullandforfuturegenerations.

GlossaryAASHTO

AmericanAssociationofStateHighwayandTransportationOfficials.TheAASHTOsystemofsoilclassificationisusedbyengineersforthedesignandconstructionoftransportationlines(roads,raillines,andairportrunways).

AcceleratederosionSoilerosionincreasedbyhumanactivitybeyondthenormalorgeologicalrate.

AcidAsubstancewithhydrogenionsavailableforchemicalactivity.

AcidrockArocksuchasgranitethatcontainsconsiderableamountsofsilicaandrelativelylittlecalcium,magnesium,andiron.

AcidsoilSoilwithapHvalueoflessthan7.0,whichisneutral.

ActinomycetesThreadlikebacteria.Somefixatmosphericnitrogensymbioticallywithnonlegumeplants.

Aeration,soilTheprocessbywhichairinthesoilisreplacedbyairintheatmosphere.

AggregateSeePed.

AhorizonThenaturalsurfacelayerofmineralsoilthatisoftenreferredtoastopsoil.

AlbichorizonAlight-coloredhorizonjustbelowthesurfacefromwhichclaysandhumushavebeenleached.

AlfisolAnorderoffertiledeciduousforestsoilsthathasanaccumulationofclayintheBhorizon.

AlkalisoilAsoilcontainingsufficientsodiumtointerferewiththegrowthofcrops(sameassodicsoils).ThepHisnormally8.3orhigher.

AlkalinesoilAsoilwithapHvalueofmorethan7.0,whichisneutral.

AlluvialfanDepositfromastreamasitentersaplainorlargerstream.

AlluvialsoilAsoilformedfromalluvium.

AlluviumDepositsmadebyriversandstreams.

AluminosilicatemineralsMineralscomposedlargelyofoxygencombinedwithsiliconbutwithsomeofthesiliconreplacedbyaluminum.

AmendmentAnysubstanceaddedtosoilthatalterssoilproperties,suchasgypsum,lime,fertilizer,andsawdust.

AmmonificationThereleaseofammoniabythemicrobialdecompositionofprotein.

AmphiboleAgroupofdark,basicmineralscommonlyfoundinigneousandmetamor-phicrocks.

AndisolThesoilorderrepresentingsoilsthatdevelopedfromvolcanicdeposits.

AnionAnegativelychargedion.

AnionexchangecapacityThesumtotalofexchangeableanionsthatthesoilcanadsorb.Expressedascentimolesofchargeperkilogram(cmolc/kg)ofsoilmaterial.

AnthropichorizonAdarksurfacehorizonenrichedwithphosphorusduetohumanactivity.

ApatiteAcalciumphosphatemineralusedasasourceofphosphateforfertilizer.

AquiferAnundergroundlayerofpermeablematerialthatstoresandcansupplywater.

ArgillichorizonBhorizonofsoilthatcontainsmoreilluviatedclaythantheoverlyingAorEhorizon.Someclaycoatingsarepresentonsurfacesofblockypeds,havingmoveddownfromabove.

AridisolAsoilthatdevelopedinanaridicenvironment.

AtmosphereThelayerofgassurroundingtheearth:nearly80%nitrogen,about20%oxygen,and0.03%carbondioxide.

AtterberglimitsLiquidandplasticlimitsasmeasuredbystandardtestprocedurestodetermineasoil'ssuitabilityasabuildingmaterial.

AvailableplantnutrientSeeNutrients.

AvailablesoilwaterSeeSoilwater.

AzotobacterFree-livingbacteriathatconvertatmosphericnitrogenintoorganicnitrogeninthesoil.

BasaltAdark(igneous)extrusiverockthatishighiniron,magnesium,andcalcium.

BasesCommonparlanceforionsofcalcium,magnesium,andsodiuminthesoil.

BasicAnotherwordforalkaline,butoftenwithemphasisonthepresenceofcalciumandmagnesium.

BasicrockArockwithahighcontentofcalcium,magnesium,andpossiblyironandarelativelylowcontentofsilica.

BeddingPreparingaseriesofparallelridges(beds)usuallynowiderthanthatoftwocroprows,separatedbyshallowtrenchesusuallylessthanthewidthbetweencroprows.

BedrockThesolidrockunderlyingunconsolidatedsurfacematerials.

BhorizonSubsoilhorizonfoundbelowanAand/oranEhorizon.

BiodegradableCapableofbeingdecomposedintosimplerproductsbylivingorganisms.

BioremediationAnyofseveraltechniquesforoptimizingthephysical,chemical,andbiologicalconditionsinthesoiltopromotethedegradationand/ordetoxificationofpollutants.

BiosolidsAtermforsewagesludge.Aby-productofwastewatertreatmentthatcontainssolidshavingappreciableamountsoforganicmaterialandnutrientsbutmayalsocontainheavymetalsandotherpotentialcontaminants.

BogsoilApeatormucksuchasaHistosol.

BorealNorthernclimatethatiscoolinsummerandcoldinwinter;permafrostiscommon.

BufferingcapacityCapacityofasoiltoresistchange,suchasachangeinthepH.

BulkdensityThemassofadrysoilsampleperunitbulkvolume(voidsandall)ascomparedwiththemassofanequalvolumeofwater.

CalcareoussoilAsoilcontainingenoughfreecalcium(usuallyalsomagnesium)carbonatetoshoweffervescencewithacid.

CalcichorizonAsubsurfacehorizonenrichedwithcalciumandothersecondarycarbonates.

CalicheAlayernearthesurface,moreorlesscementedbysecondarycarbonatesofcalciumormagnesiumprecipitatedfromsoilsolution.

CambichorizonAweaklydevelopeddiagnosticsubsurfacehorizonalteredbyphysicalalterations,chemicaltransformation,oracombinationofthese.

CapillaryactionTheactionbywhichthesurfaceofaliquidiselevatedordepresseddependingontherelativeattractionofthemoleculesoftheliquidforeachotherorasolidwithwhichitisincontact.

CapillarywaterWaterheldintheverysmallporesofthesoil.

Carbon-nitrogenratio(C:N)Theratiooftheweightoforganiccarbontotheweightoftotalnitrogen(mineralplusorganicforms)insoilororganicmaterial.

CarbonsequestrationThetyingupofatmosphericCO2inplanttissue.

CarboxylgroupAgroupingofcarbon,hydrogen,andoxygen(COOH)thatispresentinorganicacidsinhumusandothermaterials.

CatenaAsequenceofsoilsfromthetopofahilltothefootslope.

CationApositivelychargedion.

CationexchangeTheexchangebetweenacationinsolutionwithoneonthesurfaceofasoilparticle.

CationexchangecapacityThetotalofexchangeablecationsthatasoilcanadsorb,commonlyreportedincentimolesofchargeperkilogram(cmolc/kg)ofsoilmaterial.

ChannelizationThedeepeningand/orstraighteningofnaturaldrainagechannels.

ChernozemBlack,organicmatter-richsoilscommoninthesubhumidsteppesofNorthAmericaandAsiathatarecommonlyclassifiedasMollisols.

ChiselAtillagetoolthatusesnarrowshankstobreakupthesoilalongplanesofweaknesscommonlytoadepthof15cm.Ifuseddeeperthan15cm,itisusuallycalledsubsoiling.

ChlorosisLackofchlorophyllinaplantthatresultsinalightgreentoyellowcoloroftheplanttissue.

ChorizonThesoilhorizonthatisundevelopedtheparentmaterial.

ClayMineralmaterialcomposedofparticleslessthan0.002mmindiameter.

CoarseearthThepartofmineralsoilthatistoocoarsetopassthrougha2-mmsieve.

ColloidalparticlesClayandorganicparticlesthataresosmalltheytendtoremainsuspendedinstandingwater.

ColluvialdepositSoilorrockmaterialgatheredatthefootofaslope,primarilythroughtheforceofgravity.

CompostOrganicresiduessometimesmixedwithsoilthathavebeenpiled,moistened,andallowedtoundergobiologicaldecomposition.

ConductionHeattransferduetothemovementofkineticenergybetweenadjacentatomsinasubstancebroughtaboutbyatemperaturegradient.

ConvectionHeattransferthroughthemovementofafluid(airorwater).

CrustoftheearthTheouter12-mile-(19-km-)thicklayerofthelithosphere.

CrustAsomewhatdense,hard,orbrittlesoillayeratthelandsurface.

CrystallinerocksIgneousandmetamorphicrocks.

CyanobacteriaFree-livingbacteriathatconvertatmosphericnitrogenintoorganicnitrogenincruststhatwereoncecalledblue-greenalgae.

DendriticpatternAtreelikepatternthatmaybeobservedaterosionsites.

DenitrificationThemicrobialconversionofnitratetothegaseousNformwithsubsequentreleasetotheatmosphere.

Density

Themass(commonlyexpressedasweight)perunitvolumeofasubstance.Diagnosticsoilhorizons

Thesoilhorizonsusedforsoilclassification.Divalentcation

Acationhavingtwopositivecharges,suchascalcium(Ca2+).Dolomite

Calcium-magnesiumcarbonatecommonlycalledlimestone.Drainage

Watermovementthroughsoil.Naturaldrainageoccurswhenwaterdrainsoutoftherootzonetodeeperlayersortogroundwater.Surfacechannelsorsubsurfacedrains(tiles)canbeusedtoartificiallydrainsoils.

DriftSeeGlacialdrift.

DrylandfarmingThepracticeofcropproductioninlow-rainfallareaswithoutirrigation.

DuripanAdiagnosticsubsurfacehorizoncementedmainlywithsilica.

EluviationTheremovalofsoilmaterialinsuspension(orsolution)fromalayerorlayersofasoil.

EntisolAveryweaklydevelopedsoil;anepipedonunderlainbyparentmaterial.

Eolian(aeolian)depositMaterialsdepositedbywind.SeealsoLoess.

EpipedonAdiagnosticsurfacesoilhorizon.

ErosionTheprocessbywhichsoiliswashed,blown,orotherwisemovedbynaturalagentsfromoneplaceonthelandscapetoanother.

EscarpmentAclifforverysteepslopethatisfairlycontinuousandfoundattheedgeofanareawithmoregentleslopes;oftentheresultofdifferentialerosionratesofpreviouslyoverlyingstrata.

EukaryotesOrganismswhosecellshavenucleimaybeplantoranimal.

EvaporationThechangeofthestateofwaterfromaliquidtoagas,ashappensatthesurfaceofbaresoil.

EvapotranspirationThetransferofwatervaportotheairbyacombinationofevaporationandtranspiration.

ExchangecomplexSurfaceofclayandhumushavingprimarilynegativelychargedsitesinmostsoils.

ExchangeablecationsCationssuchasthoseofcalcium,magnesium,andpotassiumthatareheldlooselyenoughonsurfacesofcolloidalsoilparticlesthattheycanexchangeplaceswithcationsinthesoilsolutionnearby.

FaunaAnimalspresentatasiteorinaregion.

FeldsparThemostcommonprimarymineralintheearth'scrust.

Fertility,soilThestatusofasoilwithrespecttoitsabilitytosupplythenutrientsessentialtoplantgrowth.

FertilizerAnyorganicorinorganicmaterialofnaturalorsyntheticoriginthatisaddedtoasoiltosupplyoneormoreelementsessentialforthegrowthofplants.

FieldcapacityThepercentageofwaterbyweightthatisheldinthesoilbycapillaryactionafterfreedrainagebytheforceofgravityhaspracticallyceased.

FineearthThatportionofasoilthatisfinerthan2mmindiameter,includingmineralsand,silt,andclay.

FloraTheplantspresentatasiteorinaregion.

FragipanVerydensesubsoillayersthatarebrittlewhendry,butnotwhenwet,primarilycementedbysilica.

FurrowdikingCreatingadikeandasmalldepressioninfurrows(normally6-12ft[1.8-3.6m]apart)toholdwaterandreducerunoff.

GabbroAdark-coloredcrystallineigneousrock.

GelisolsAnorderofsoilscharacterizedbyhavingpermafrost.

GenesisofsoilSoilformation.

GeomorphologyThestudyoflandformsonthesurfaceoftheearth.

Gilgai

SeeVertisol.Glacialdrift

Depositsmadebyglaciersandtheirmeltwaters,includingtillandoutwash.Glacialtill

Unsorteddebrisleftbyaglacier.Glacier

Alargebodyofslowlymovingice.Globalpositioningsystem(GPS)

Aglobalnavigationsystemthatusessatellitestotriangulatepositionsonearth.Gradient

Ameasureofsoilinfeet(meters)ofriseorfallper100ft(31m)ofhorizontaldistance.Granite

Alight-colored,crystallineigneousrockcontainingconsiderablequartz(about25%).Greenmanure

Agrowingcropthatisplowedunderandmixedwiththesoiltoenrichitwithorganicmatter;alsousedtopromoteplantuptakeofmobilenutrientsbetweenseasons.

GroundwaterWaterbeneaththeearth'ssurfaceinsaturatedsoilorporousrockstrata.

GullyerosionRemovalbywaterofsufficientsoiltoformchannelslargeenoughtopreventmachineryfromcrossing

HardpanAsoillayerthatactsasabarriertothemovementofwaterandplantroots.

HeatoffusionTheamountofheatnecessarytochangeasubstancefromitssolidtoitsliquidphase.

HeatofvaporizationTheamountofheatnecessarytochangeasubstancefromitsliquidtoitsvaporphase.

HectareAnareaoflandequalto10,000sqmor2.47acres.

HisticepipedonAnorganicsurfacehorizonthatischaracterizedbysaturationforprolongedperiodsunlessartificiallydrained.

HistosolAsoilorderofpeatsandmucksthatarethickerthan18in.(46cm).

HorizonAsoillayerthatformedparalleltothelandsurfaceduringthenaturaldevelopmentofthesoilbody.

Humus

Thedark,ratherstablepartofsoilorganicmatterthatremainsafterthemajorportionofanimalandplantresidueshavedecomposedanddisappearedintheformofwaterandgasessuchascarbondioxide.

HydrationAmineralweatheringprocesswhereinwatermoleculescombinewithsoilconstituents.

HydrologiccycleAcyclicpathwaythatdescribeswatermovementonearth.Includessuchprocessesasevaporation,sublimation,precipitation,runoff,percolation,andgroundwaterflow.

HydrolysisAchemicaldecompositionreactioninwhichachemicalbondissplitandtheelementscontainedinwaterareadded.

HydrousmicaAcommonlayersilicateclaythatisintermediateinresponsivenesstowettinganddryingbetweensmectiteandkaolinite.

HyphaeIndividualthreadsofmycelia.

IgneousrockArockthatformedbythecoolingandsolidificationofliquidpartsofthelithosphere.

IlliteAhydrousmicatypeofsilicateclay.

IlluviationTheprocessofdepositionofsoilmaterialremovedfromonehorizontoanotherinthesoil;usuallyfromanuppertoalowerhorizoninthesoilprofile.

ImmobilizationTheconversionofinorganicionsfromthesoilintoorganicmoleculesinlivingtissue.Changesavailablenutrientsintounavailableforms.

InceptisolAnorderofmoderatelydevelopedsoils.Itconsistsofanepipedonunderlainbyacambichorizonandparentmaterial.

InfiltrationThedownwardentryofwaterintosoil.

InfiltrationrateTherateatwhichwatercanenterthesoilunderspecifiedconditions,includingthepresenceofanexcessofwater.

InterveinalBetweentheveins.

IonAnelectricallychargedatom,withasurplusordeficiencyofelectrons.

Ironstone

SeePlinthite.Kaolinite

A1:1layersilicateclaywithalowdegreeofresponsivenesstowettinganddryingandlownutrient-holdingcapacity.

LandformAnaturalfeatureoftheearth'ssurface,suchasahilloraplain.

LatentheatTheamountofenergyusedtochangethephaseofasubstance.

LateriteAsurfaceformationrichinaluminumandironthatdevelopedthroughtheweatheringoftheunderlyingparentrock.Ithasbeenusedextensivelyinbuildingthroughoutthetropics.

LatticeAthree-dimensionalgridoflinesconnectingpointsthatrepresentthecentersofatomsorionsinamineralclay.

LayersilicateclaySeeSilicateclay.

LeachingTheremovalofmaterialsinsolutionbydownwardmovementofwaterthroughsoil.

LimeGroundlimestone,eithercalciteordolomite.

LimestoneArockrichincalciumcarbonatederivedfromshellsofseaorganismsandthemineralcalcite,includingdolomite,whichisrichincalciumandmagnesiumcarbonate.

LimysoilAsoilcontainingappreciableamountsofcarbonatemineralssuchascalciumormagnesiumcarbonateoracombination.

LithosphereThesolid,rigidrockportionoftheearth.

LoessDepositofwindblownsoilparticles,largelysiltsize.

MacronutrientAchemicalelementthatisessentialforplantgrowthandisusedinrelativelylargequantities(usually>50ppminplants).

MagmaLiquidrockfoundatgreatdepthsintheearth'scrustorexposedduringvolcaniceruptions.

ManureExcretaofanimals,withorwithoutbeddingmaterial,normallyaddedtosoiltoimproveitwithrespecttocropproduction.SeealsoGreenmanure.

Masswasting

Themovementdownslopeunderthepullofgravityoflargemassesofsoiland/orrock.Matricforce

Theforceofattractionbetweenwaterandsoilparticlesthatholdscapillarywaterinthesoil.

MatrixSomethingwithinwhichsomethingelseoriginatesordevelops.

MelanichorizonAdark,organicmatter-enrichedsurfacehorizoninsomesoilsformedinvolcanicdeposits.

MetamorphicrockRockformedbyrecrystallizationofigneousorsedimentaryrockundergreatpressureandheatwherebythemineralscontainedintherockbecomereoriented.

MicronutrientAchemicalelementnecessaryforplantgrowthandusedinrelativelysmallamounts(usually<50ppmintheplant).

MicroreliefSlightirregularitiesofalandsurface.

MineralAnaturalinorganiccompoundwithknownphysicalpropertiesandchemicalformula,amixtureofwhichformsarock.

MineralizationThemicrobiologicaldecompositionoforganicmatterintoinorganicproducts.

MitesArthropodsthatareabundantinsoil.

MollicepipedonDark,thick,organicmatter-rich,fertiletopsoilcommonlyfoundinthesubhumidgrasslandsofAsiaandNorthAmerica.

MonovalentcationAcationhavingasinglepositivecharge,suchassodium(Na+).

MontmorilloniteAveryreactive2:1layersilicateclay.Amemberofthesmectitegroupofclays.

MoraineAblanketorridgeofunsorteddebrisleftbyaglacier.

MottlingSpottedareasofcolorinasoil,usuallyassociatedwithperiodicwetconditions.

MucksoilAnorganicsoilthatismorethanhalfdecomposedfewvisibleorganicfibersconstituteaminorportionofthemass.

MulchAlayerofmaterialspreadoverthesoilsurfacetoprotectitandplantrootsfromerosion,

crusting,freezing,anddrying.Munsellcolors

Asystemforuniformlydescribingthecolorofsoil.Mycelium

Athreadlikemassoffungalhyphae.Mycorrhizae

Fungithatlivesymbioticallywiththerootsofhigherplants.Myxomycetes

Slimemolds;morphologicallybetweenbacteriaandfungi.Necrosis

Deathordecayofplantoranimaltissue.Netradiation

Thedifferencebetweenincomingandoutgoingshort-waveandlongwaveradiation.Nitrification

Biologicaloxidationofammoniumtonitriteandnitrate.Nitrobacter

Bacteriathatperformthesecondandfinalstepofnitrificationinthenitrogencycle.Nitrogencycle

Thebiochemicalchangesthattakeplaceinrepetitivesequenceasorganismstakeupandreleasenitrogen.

NitrogenfixationBiologicalconversionofmoleculardinitrogen(N2)toorganiccombinationsutilizableinbiologicalprocesses.

NitrosomonasBacteriathatperformthefirststepofnitrificationinthenitrogencycle.

NutrientsSubstancesessentialforthegrowthofplants,suchasnitrogen,phosphorus,andpotassium.Anavailablenutrientisonethatcanbereadilyabsorbedbytheplant.

OchricepipedonSurfacesoilthatfailstomeetthecriteriaforanyotherepipedon.

OhorizonAthin,surficialorganiclayeronsoil,suchasleaflitter.

OrganicgardeningGardeningwithouttheuseofcommercialfertilizersandpesticides.Emphasisisontheuseofcompostedmaterials,greenmanures,andjudiciousarrangementofplantsandassociatedbeneficialorganisms.

OrganicsoilAsoilthatcontainsahighpercentageoforganiccarbon.

OrtsteinAnaturalsoilpanformedinspodosols(Podzolsoils)bycementationofthesubsoilbyorganicmatterandironoxide.

OsmosisThediffusionofwaterthroughadifferentiallypermeablemembrane,suchasaroothair,fromanareaofhighwaterconcentrationtoanareaoflowerwaterconcentration(orlowsaltconcentrationtohighsaltconcentration)ifpressureandtemperatureareequaloneachsideofthemembrane.

OutwashDepositmadebyflowingmeltwatersfromglaciers.

OxicsoilhorizonArelativelyinfertilesubsurfacediagnostichorizoncommonintropicalareas.

OxidationAmineralweatheringprocesswhereinoxygenionscombinewithmultivalentelementssuchasiron,lossofelectronsfromanatom,molecule,anion,orcation.

OxideclayFineparticlescomposedofoxidesofironandaluminum,commonlyinnoncrystallineoramorphousforms.

OxisolAsoilordercommonintropicalareasthatisrelativelyinfertile.

ParentmaterialConsolidatedorunconsolidatedmaterialfromwhichthesoildevelopsthroughpedogenesis.

ParticlesizedistributionProportionofclay,silt,andsandparticlesinthefineearthofasoil.

PeatAsoilwithrelativelyundecomposedfibrousorganicmaterial.

PedAunitofsoilstructureinasoilhorizonmaybeblocky,platy,orgranular.Asoilaggregateisthesameasasoilped.

PedonAvolumeofsoilasitoccursinnaturethatislargeenoughtoshowthevariationsinthehorizonsinthethirddimensionusuallyanareaof1-10sqm.

PercolationThedownwardmovementofwaterinsoil.

PetrocalcichorizonAhighlydevelopedandcementedcalcichorizon,oftencalledcaliche.

pHThedegreeofacidityoralkalinity.Thehydrogenpotentialexpressedbyasetofnegative

logarithmicvalueswherebynumberslessthan7.0signifyacidityandnumbersgreaterthan7.0signifyalkalinity.pH7.0isneutral,thatis,neitheracidnoralkaline.

PlinthiteAnoninduratedmixtureofironandaluminumoxides,commonlywithsomequartzandkaoliniteclay,thathasthecapacitytohardenirreversiblyintoironstoneuponrepeatedwettinganddrying.

PodzolAgreatsoilgroupundertheFAOsystem,nowreplacedbythesoilorderSpodosolintheU.S.systemofSoilTaxonomy.

PollutionTheactofpolluting,thatis,tocontaminate,makeuncleanorimpure.

Polypedon(soilbody)Contiguous,similarsoilpedonsconstitutingaunitoflandtothedepthofsoildevelopment.

PorosityThevolumepercentageofthetotalbulknotoccupiedbysolidparticles.

PrairiesoilMineralsoilswithorganicmatter-richmollicepipedonsresultingfromtheannualnecrosisofgrassroots,commonlyclassifiedasMollisols.

PrecipitationRainfallandsnowfallplusminoramountsofdewfallandfogdrip.

PrimarymineralAmineralformednaturallybycrystallizationfrommoltenrock.Feldsparisthemostcommonprimarymineralintheearth'scrust.

ProductivityThecapacityofasoiltoproduceplantmaterial.Thisisexpressedinyieldperunitofareaperunitoftime.

ProfileAtwo-dimensional,verticalcross-sectionofasoilthroughallhorizons.

ProkaryotesSimpleorganismswhosecellslacknucleinotclearlyplantsoranimals.

QuartzAresistantprimarymineralconsistingofsilicondioxidecommoninmostrocks.

QuartziteMetamorphozedsandstone.

RadiationHeattransferbyelectromagneticwaves.

ReductionAnanerobicmineralweatheringprocessthatresultsinthegainofelectronstotheatom,ion,ormolecule.

RegolithUnconsolidatedmaterialabovesolidbedrock.

ReliefTheelevation,ordifferencesinelevationconsideredcollectively,ofalandsurfaceonabroadscale.

RhizobiaBacteriathatconvertatmosphericnitrogenintoorganicnitrogeninrootnodulesoflegumes.

RhizosphereAzoneofthesoilwhereplantrootsareinabundanceandsoilmicrobesareespeciallyactive.

RhyoliteAfine-grainedextrusiveequivalentofgranite.

RillerosionWaterremovalofsoilinchannelssmallenoughtobefilledinbynormaltillage.

RunoffWaterfromprecipitationorsnowmeltthatflowsoverthesoiltosurfacewaterbodies.

SalichorizonAsubsurfacediagnostichorizonenrichedwithsolublesalts.

SaltationTheleaping,jumping,orbouncingofsoilparticlesalongthesurfaceofthesoilduringwinderosion.

SandstoneAsedimentaryrock,usuallyofquartz,boundtogetherbyacementingmaterialsuchassilicaorironoxide.

SaproliteAmassofweatheredrock.

SaturationTheconditionofasoilwhenallporesarefilledwithwater.

SecondarymineralAmineralformedbyweatheringfromaprimarymineral.Forexample,kaoliniteisasecondarymineralformedfromfeldspar.Allcarbonatessuchascalciteanddolomitearesecondaryminerals.

SedimentaryrockArockcomposedofsediment,thatis,depositsmadebywater,wind,ice,andgravity.

SensibleheatTheamountofenergyusedtowarmtheairabovethesoilsurface.

ShaleAsedimentaryrockmadeofclay,silt,andveryfinesand.

Sheeterosion

Waterremovalofathinlayerofsoilfromthesoilsurface.Resultsinthehighesterosionlossesonanareabasisanddifficulttovisuallydetect.

SilicateclayClaycomposedofmineralsmadeuplargelyofcrystallinelayersofsilicaandalumina.

SlickensidePolishedsurfacescausedbyblocksofsoilslidingpasteachotherduringtheformationofVertisols.

SmectiteAgroupofveryreactivesilicateclaysofwhichmontmorilloniteisaprominentmember.

Sodiumadsorptionratio(SAR)Theratioofsolublesodiumtosolublecalciumandmagnesium.SARisusedtoassessthequalityofirrigationwaterandtheriskofsoildispersion.

SoilDefinition1:Theunconsolidatedmineralororganicmaterialontheimmediatesurfaceoftheearththatservesasanaturalmediumforthegrowthoflandplants.---Definition2:Theunconsolidatedmineralororganicmaterialonthesurfaceoftheearththathasbeensubjectedtoandshowstheeffectsofgeneticandenvironmentalfactorsofclimate(includingwaterandtemperatureeffects)andmacro-andmicroorganisms,conditionedbyrelief,actingonparentmaterialoveraperiodoftime.

SoilabsorptionsystemAsystemofpipesburiedinthesoilthroughwhicheffluentfromseptictanksinfiltratesintothesoil.

SoilbodyAcomponentorunitofthenaturalterrain(landscape).

SoilclassificationGroupingofsoilsbaseduponcommoncharacteristicssuchasdiagnosticsurfaceandsubsurfacehorizons.

Soilformation(genesis)Thepedogenicprocessesbywhichparentmaterialistransformedintoabodyofsoil.

SoilheatstorageTheamountofenergystoredasheatinthesurfacesoillayers.

SoillandscapeThesoilportionofthelandscape.

SoilmanagementThesumtotalofalltillageoperations,croppingpractices,fertilizer,lime,andothertreatmentsconductedonorappliedtoasoilfortheproductionofplants.

SoilmoistureregimesAsystemfordescribingthepredictableavailablemoistureinthecroprootzoneofthesoilthroughouttheyear.

SoilreactionDegreeofacidityoralkalinity(pH).

SoilsequenceAnarrayofsoils(soilbodies)suchasfromthetopofahilltothefootslopeorfromtheyoungestsoilontheyoungestlandformsurfacetotheoldestsoilinthesameregion.

SoilsolutionTheaqueousliquidphaseofthesoilanditssolutes.

SoilstructureTheclusteringofsoilparticlesintounitscalledpedsoraggregates.

SoilsurveyTheidentification,classification,mapping,scientificandpracticalexplanation,evaluation,andinterpretationofthesoilcoverofaterrain.

SoiltemperatureregimesAsystemfordescribingthepredictablesoiltemperaturethroughouttheyear.Ithasimplicationsforcropproduction.

SoiltextureTherelativeproportionsofthethreesoilseparates(sand,silt,andclay).

SoilwaterWatercontainedinsoil.Availablesoilwateristhatwhichcanbeabsorbedbytheplant.

SolumTheAandBhorizonstogetherorthesingleoneofthesethatoverliestheChorizonatasite.

SpodichorizonBhorizoninwhichsandgrainsarecoatedwithaluminumoxide,humus,andusuallyironoxide.Itisbestdevelopedunderacidictreelitterasiscommoninconiferousforests.

SpodosolAsoilordercommoninhumidareasunderacidicforesthorizonsinwhichthespodichorizonisprominent.

StomatesMinuteopeningsinaleafthatpermitgaseousinterchange.

StratificationLayeringofwater-laiddepositssuchassandandgravel.

StubblemulchThestubbleofcropsorcropresiduesleftessentiallyinplaceonthelandasasurfacecoverbeforeandduringthepreparationoftheseedbedandpossiblyduringthegrowingofasucceedingcrop.

SublimationThechangeofstateforwaterfromasolidtoavapor.Thetransformationofsnoworicedirectlytowatervapor.

SubstratumAlayerbeneaththesurfacesoil.

SummerfallowWater-conservingshallowtillagethatpreventsvegetativegrowthduringalternatecroppingseasonsinaridandsemi-aridregions.

SurfacecreepTherollingbywindofcoarsesandparticlesoverthesoilsurface.

SuspensionThemovementbywindofsmallsoilparticles(siltandclay)caughtupbyaircurrents.

SymbioticnitrogenfixationThecloseassociationofcertainbacteriaandlegumestoconvertatmosphericnitrogen(N2)intonitrogenformsusableinbiologicalprocesses.

TectonicactivityDisruptionoftheearth'scrust,resultinginearthquakes,volcanoes,faults,andrelatedevents.

TerraceAbenchlikelandformonvalleybottoms(“highbottoms”).

ThermalconductivityPropertyofasubstancethatindicatesitsabilitytoconductheat.

TillageMechanicalmanipulationofsoilforanypurpose.Inagricultureitisusuallyrestrictedtothemodificationofsoilconditionsforcropproduction.

TilthThephysicalconditionofsoilasrelatedtoitseaseoftillageandfitnessasaseedbed.

TopographyThereliefoftheland,thatis,levelnessorhilliness.

TranspirationThetransferofwatertotheatmospherethroughthestomatesofplantleaves.

UltisolAsoilorderofsoilsinwarmregionforestlandswithanargillicdiagnosticsubsurfacehorizon,lessbasesaturationthanalfisols.

UmbricepipedonDark,deep,surfacesoilhorizonthatismoreacidicthanamollicepipedon.

UnsaturationThestateofasoilinwhichmostofthesmallerporesarefilledwithwaterandthelargeronesareprimarilyfilledwithair.

USC(UnifiedSoilClassification)systemTheUSCsystemofsoilclassificationisusedbyengineersinvolvedinfoundationengineering.

VermiculiteA2:1layersilicateclayderivedfromhydrousmica.

VertisolAsoilorderhighinexpandingsmectiticclaysthatinvertduringrepeatedwettinganddryingcyclescausingawavysurfacemicrotopography(gilgai).

WatershedAnareadrainingultimatelytoaparticularbodyofwater.

WatertableThesurfaceofthegroundwater.

WeatheringThephysicaldisintegrationandchemicaldecompositionofmineralsandrock.

WiltingpointTheconditionofasoilwhenitswatercontentissolowthatplantrootscannolongerobtainadequatewatertosustainlife.

WindbreakVegetationplantedtoprotectdownwindcropsfromdesiccationandbreakageandtoprotectsoilfromwinderosion.ItisalsousedtoprotecthomesteadsontheHighPlainsfromhighwinds.

IndexAASHTO

acidity

acidsoilseealsopH

acre

actinomycetes

aggregates

agriculturallime

agriculturalterraces

agroforestry

albichorizon

alfisols

algae

alkalinity

allophane

alluvium

aluminosilicate

ammonification

ammonia

ammonium

polysulfide

thiosulfate

amphibole

andisols

anion

exchange

anthropicepipedon

ants

apatite

aquifer

argillichorizon

aridisols

arthropods

atmosphere

AtterbergLimits

availablenutrients

azotobacter

bacteria

basalt

bases

basesaturation

bedding

bedrock,II

biodegradable

biologicalcontrol

bioremediation

biosolids

blackalkali

blockystructure

bogsoil

boreal

bufferingcapacity

bulkdensity

C:Nratio

calcichorizon

calcium

carbonate

sulfate

caliche

carbon

cycle

dioxide

sequestration

carbonates

carbonicacid

carboxylgroup

catena

cation

capacity

divalent

exchange

monovalent

channelization

chemicalweathering

chernozem

chisel

chlorosis

clay

aluminosilicates

feldspar

hydrousmica

illite

kaolinite

montmorillonite

silicate

smectite

vermiculite

climate

clods

colloids

colluvium

color

columnarstructure

compaction

composting

conduction

conservationstructures

conservationtillage

consistence

contour

convection

covercrops

cropresidue

cover

incorporation

croprotation

crumbstructure

crust

crystallinerocks

cultivator

cyanobacteria

dams

degradation

denitrification

density

diagnosticsoilhorizons

disk

dispersion

disturbedlands

artificially

characteristics

contaminant

naturally

ditches

diversion

dolomite

drainage

dropstructures

drylandfarming

dunes

duripans

earthworms

ectomycorrhizae

elementalsulfur

eluvial

endomycorrhizae

energy

atmosphere

color

landcover

slope

engineeringsoiluses

classificationsystems

properties

suitability

entisols

eoliansand

epipedon

erosion

accelerated

excessive

geologic

gully

processes

rill

sheet

streambank

water

wind

escarpment

essentialelements

eukaryotes

evaporation

evapotranspiration

fauna

feldspar

ferrihydrite

ferromagnesianminerals

fertilizer

fieldcapacity

fineearth

fixation

flocculation

flora

formicacinera

fragipan

friable

frostheave

fungi

furrowdiking

gabbro

gelisols

glacial

deposits

drift

outwash

till

glaciers

globalpositioningsystem(GPS)

gradient

granite

granularstructure

grasslands

greenmanure

groundwater

gypsum

hardpan

harrow

heatcapacity

offusion

latent

sensible

thermalconductivity

transfer

ofvaporization

hectare

histicepipedon

histosols

humus

hydration

hydrologiccycle

hydrolysis

hydrousmica

hyphae

igneousrocks

illuviation

immobilization

imogolite

inceptisols

infiltration

insecticide

insects

integratedpestmanagement

interveinal

ion

iron

ironoxides

ironstone

irrigation

isomorphoussubstitution

kandichorizon

kaolinite

land

capabilityclasses

forming

landform

laterite

layersilicateclayseeSilicateclay

leachate

leaching

legume

levees

lichen

lime

limestone

lister

lithospohere

loess

lumbricusterrestris

macronutrients

macroorganisms

macropores

magma

magnesium

mammals

managementzone

manganese

manure

masswasting

matrix

maximumfertilizerefficiency

maximumyield

melanicepipedon

metals,heavy

micronurients

microorganisms

micropores

microrelief

mineral

mineralization

mineralsoils

primary

secondary

minimumtillage

mites

moldboardplow

mollieepipedon

mollisols

montmorillonie

moraines

mottling

muck

mulches

mycelium

mycorrhizae

myxomycetes

natrichorizon

necrosis

nematodes

nitrate

nitrite

nitrobacter

nitrogen

ammonification

deficiency

denitrification

fixation

immobilization

inorganic

mineralization

nitrification

organic

nitrogencycle

nitrogenfixation

nonsymbiotic

symbiotic

nitrosomonas

nodules

no-till

nutrient

deficiencysymptoms

ochricepipedon

organicfarming

organicgardening

organicmatter

organicsoil

organisms

ortstein

osmosis

overgrazing

oxichorizon

oxidation

oxisols

oxygen

parentmaterial

particlesizedistribution

peat

ped

pedogenesis

pedology

pedon

percolation

permafrost

pesticides

petrocalcichorizon

pH

phosphorus

physicalweathering

planter

plantnutrients

plantroots

platystructure

plinthite

plow

plowpan

podzol

pollution

polypedon

ponds

porosity

potassium

prairiesoil

precipitation

precisionagriculture

prismaticstructure

productivity

profile

prokaryotes

protectivevegetativecover

protozoa

pyroxene

quartz

quartzite

radiation

raindropenergy

reclamation

reducedtillage

reduction

regolith,II

relief

remediation

remotesensing

residences

residues

rhizobia

rhizosphere

riparianzones

riprap

roads

rock

acid

basic

metamorphic

rockweathering

roots

rotaryhoe

rowcleaners

runoff

salichorizon

saline

saline-sodic

saltation

sand

sandstone

saprolite

saturation

sedimentaryrocks

sensibleheat

shale

shelterbelts

silt

slickenside

smectite

sodium

adsorptionratio

sulfate

soil

absorptionsystem

acid

aeration

alkaline

alluvial

amendment

gypsum

limestone

body

classification

color

Munsell

conservation

contaminants

definitions

degradation

erosion

accelerated

excessive

fineearth

geologic

gully

masswasting

streambank

water

wind

exchangecomplex

family

fertility

fineearth

formation

formingfactors

greatgroups

horizon

landscape

management

mappingunit

mineral

moisture

moistureregimes

aquic

aridic

torric

udic

ustic

xeric

orders

organic

phase

profile

reaction

salinesoil

separates

sequence

series

sodic

solution

stabilizers

subgroups

suborders

surveys

taxonomy

temperature

fluctuation

microorganisms

moisture

solubility

temperatureregimes

cryic

frigid

mesic

thermic

hyperthermic

isohyperthermic

texturaltriangle

texture

tilth

type

water

soilhorizons

A

B

C

E

O

R

solublesalts

solum

solution

spodichorizon

spodosols

springtails

stomates

stratification

streamdegradation

streptomycetes

stripcropping.

structures

stubble-mulching

sublimation

subsoil

substratum

sulfur.

summerfallow

surfacecreep

surfacerunoff

suspension

termites.

terrace

tetrahedron

tillage

minimum

no-till

primary

reduced

secondary

tilth

time

topography

toposequence

topsoil

transpiration

ultisols

umbricepipedon

urbansoils

flooding

hydrology

overlandflow

variability

USCsystem

vegetativecover

vermiculite

vertebrates

vertisols

volcanicash

waste

liquid

solid

treatment

water

capillary

erosion

holdingcapacity

stablesoilaggregates

table

treatment

watershed

waterway

weathering

wetlands

whitealkali

wiltingpoint

windbreaks

winderosion

worms

yieldmaps

zinc

WILEYENDUSERLICENSEAGREEMENTGotowww.wiley.com/go/eulatoaccessWiley'sebookEULA.