Basic Concepts in Soil Fertility

Jonathan DeenikAssistant Specialist, Soil Fertility

Department of Tropical Plant and Soil Sciences

Outline• Soil as a Nutrient Reservoir• Organic Matter• Soil Acidity• Liming• N and P• Soils of Hawaii

Soil Plant Relationships

Havlin et al., 2005. Soil Fertility and Fertilizers

Idealized Soil Composition

25% H2O

25% Air

45% Mineral

5% Organic Matter

50% Solids 50% Pores

Clay is Where the Action is!

•Clay Properties:Microscopic size (<0.002 mm)Extremely high surface area

- water retention- chemical reactions- biological activity

Clay surfaces carry charge (-/+)Site of chemical weathering

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

In Hawaii Type of Clay is Critical2:1 layer silicates

1:1 layer silicates

oxides

Clay Minerals

2:1 Clays (high activity)

Montmorillonite (smectite)

Properties:

• Expanding• Permanent charge (-)• High CEC (80-120 cmolc kg-1)• High surface area• Sticky

http://webmineral.com/specimens/picshow.php?id=1285

Clay Minerals

Clays

Properties:

• Non-expanding• pH dependent charge• low CEC (1-10 cmolc kg-1)• Relatively low surface area• Non-sticky

Kaolinite

http://soil.gsfc.nasa.gov/forengeo/thnsec.htm

Al Oxides (Gibbsite)

Source:http://www.icmab.es/multimetox/docs_lectures/lectures_html/Gale_J/img033.JPG

Stable at low pHZero point of charge (pH = 5.9-6.7)Al only dissolves under very acidic conditions

Al and Clay Minerals

Clay Minerals

Al & Fe Oxides

Properties:• Non-expanding• pH dependent charge• Very little CEC • High anion retention• Non-sticky

Surface ChargePermanent Charge• Isomorphic substitution• Net negative• 2:1 clays

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

Surface ChargepH-Dependent Charge• Net Negative or net positive• Charge determined by H+/OH-

• Oxide-rich soils have + charge at low pH

Protonation De-protonation

Weathering Intensity and Clay Type

Smectite Kaolinite Oxides

Fox et al., 1991

Organic Matter

Humus

http://www.sct.embrapa.br/diacampo/2004/releases.htm

Importance of Organic Matter in Soils

Physical- Improves aggregation (glue)- Improves water holding capacity (surface area)

Chemical- Increases nutrient availability (N cycling, P and

micronutrient solubility)- Increases CEC (200 cmolc kg-1)- Buffers the soil against pH changes

Biological- Increases microbial diversity- N fixation (rhizobia), P availability (mychoriza)- Assists in pathogen suppression

Clays and OM Buffer Soil System

Buffering Capacity:1.Resistance to change

2.The ability of the soil to re-supply nutrients the soil solution

Nutrientin

solution

Plant uptake

Solid phase nutrient(adsorbed/absorbed)

Total soil nutrient(unavailable)

Buffering capacity depends on:• clay content and type• CEC• organic matter

Soil Acidity

Natural Sources of Acidity:

Carbonic acid and organic acidsOrganic matterPrecipitation and cation leachingNitrificationN ImmobilizationAmmonium volatilizationCation uptakeDeprotonation of pH-dependent charge

Human Induced Acidity:

Acid rainUreaAmmonium fertilizersMono and diammonium phosphateElemental S

Soil Acidity40 years of N application

Source: Schwab et al., 1990 SSSAJ

(NH2)2CO + 4O2 2NO3- + 2H+ + CO2 + H2O

Adverse Effects of Soil AcidityAluminum toxicityManganese toxicityNutrient deficienciesDecreased microbial activity

Source: Hue et al., 1998

Source: Bohn et al., 2001

Al precipitated as Al(OH)3 down to pH 4.5

Aluminum Toxicity

At pH below 5.0 Al toxicity a problemUltisols most likely to have Al toxicity under acid conditionsSoils acidified by pineapple production may be problematic especially if Sicontent is high (>20%)Liming (CaCO3/CaSO4) and/or organic matter inputs alleviate Al toxicity

Soils with Potential Mn Toxicity

Average MnO2 content of soils = 0.1%Oxisols exisiting at low to moderate elevation (200-750 ft) with moderate rainfall (20-60 in/yr)Molokai, Lahaina, Wahiawa (1.5% MnO2) series

Kaolinitic Mollisols and Inceptisols in dry environmentsKeahua (0.4%), Ewa, Paia (1.7%), Hoolehua (1.5%),

kahana series

Manganese ToxicityMn toxicity increases as pH drops below 5.5.Reducing conditions (saturated soils) increase Mn toxicityFresh organic inputs increase Mn toxicityManage Mn toxicity with lime, water management, and careful attention to organic inputs

To raise pH- Reduce existing/potential toxicities- Increases P availability- pH range 5.5 - 7.0- Liming can be expensive because soils

are buffered (clay content and OM)

• To supply Ca- Highly weathered soils are almost always

deficient in Ca

Liming is Important

Correcting Soil Acidity is Costly

Brady & Weil, 2004

Correcting Soil Acidity is Costly

Al3+ + H2O Al(OH)2+ + H+

Liming Reactions:

CaCO3 + H2O Ca2+ + HCO3- + OH-

OH- + H+ H2O

HCO3- + H+ H2CO3

Al3+ + 3OH- Al(OH)3

Liming

Tons CaCO3 per AcreUchida & Hue, 2000

Nitrogen

http://www.bettersoils.com.au/module2/images/27.gif

N Mineralization

Mineralization: Decomposition of soil organic matter by soil microbes releasing inorganic N in the process.

Heterotrophs use organic molecules as source of energy• Bacteria – neutral to alkaline environments• Fungi - both neutral and acidic environments

Release of N from the organic matter• Soil Organic Matter ~5% N• 1 to 4% organic N mineralized each year• Added organic N sources

(C:N ratio < 20 = mineralization

ImmobilizationImmobilization• Conversion of mineral N to organic N by microbes

Organisms that decompose organic matter as an energy source require nitrogen

Organic materials with a low N content (C:N > 30) cannot supply the needs of these organisms thus they use soil N in competition with the crop.

Freshly immobilized N = 5-15% of soil N

Ammonium N

Ammonium N = NH4+

• Cation, therefore adsorbed on CEC

• Won't leach or denitrify

• Can be fixed in certain clay minerals – micaceous clay

• Plant uptake

• Rapidly converted to NO3-N under most conditions

• Volatilization at high pH

NH4+ + OH- → NH3↑ + H2O

High pH Gas

Nitrate N• Anion, therefore not adsorbed on CEC• Most common mineral form of N in most soils• Most common form taken up by plants• Very susceptible to leaching and denitrification

losses2NO3

- → N2O & N2 + 3O2Anaerobic gases

No oxygen - wet soil

Energy source for bacteria - organic matter

Warm temperatures

Favored by higher pH

Phosphorus

http://biology.kenyon.edu/courses/biol112/Biol112WebPage/Syllabus/Topics/Week%2013/PhosphorusCycle.jpg

P Fixation

Factors Affecting P Fixation

Soil typeAndisol>Oxisol≈Ultisol>Inceptisol>Mollisol≈Vertisol

Honokaa Kapaa MakaweliWaimea Wahiawa Waialua Kula Alaeloa Keahua

Lualualei

Soil pH- acid soils P fixed by Al/Fe oxides- alkaline soils P fixed by CaDecrease P fixation by liming and/or adding OM

Soil Fertility Depends on:

Clay contentClay mineralogy

- 2:1- 1:1- Oxides

Organic Matter

Fertile SoilsSlightly acid to neutral pHHigh CECHigh organic matterVertisols - LualualeiMollisols - WaialuaMedial Andisols -Waimea

Infertile SoilsAcid to strongly acid pHLow CECHigh P fixationToxicitiesOxisols - KapaaUltisols - HaikuHydrus Andisols - Hilo

Weathering Intensity and Fertility

Smectite Kaolinite Oxides

Category Series ClassificationHighly WeatheredVery Acid Kapaa Very fine, sesquic, isohyperthermic, Anionic Acrudox

Halii Fine, ferruginous, isothermic Anionic AcroperoxMakapili Very fine, sesquic, isohyperthermic, Anionic AcrudoxPooku Very fine, ferrihydritic, isohyperthermic, Anionic AcrudoxLawai Very fine, ferrihydritic, isohyperthermic, Typic HapludoxMahana Clayey, oxidic, isothermic, Typic AcrohumoxPuu Opae Fine, oxidic, isohyperthermic, Ustoxic PalehumultPuhi Very fine, ferruginous, isohyperthermic, Humic Kandiustox Kalapa Very fine, kaolinitic, isohyperthermic, Typic Palehumultkunuweia Clayey-skeletal, ferritic, isothermic Typic HapludoxHanamaulu Very-fine, ferruginous, isohyperthermic Humic KandiudoxLihue Very-fine, ferruginous, isohyperthermic Rhodic EutrustoxNiu Very fine, kaolinitic, isohyperthermic, Rhodic EustrustoxHihimanu Very fine, parasesquic, isohyperthermic, Oxic Dystrudepts

Moderately WeatheredSlightly acid

Limited Weathering Makaweli Fine, parasesquic, isohyperthermic, Torroxic HaplustollsNeutral to alkaline Pakala Fine loamy, parasesquic, isohyperthermic, Torroxic Haplustolls

Pohakupu Fine, parasesquic, isohyperthermic, Torroxic HaplustollsKoloa Fine, mixed, isohyperthermic, Andic Haplustollskaloko Fine, smectitic, calcareous, isohyperthermic Cumulic EndoaquollsKolokolo Fine, mixed, isohyperthermic, Cumulic haplustollsMokuleia Clayey over sandy sandy skeletal, mixed isohyperthermic, Entic HaplustollsMamala Clay, parasesquic, isohyperthermic, Lithic HaplocambidsKekaha Very-fine, parasesquic, isohyperthermic, Typic HaplocambidsIao Fine, mixed, active, isohyperthermic Cumulic HaplustollsWaikomo Clayey, mixed, isohyperthermic, Lithic HaplustollsNonopahu Fine, mixed, isohyperthermic, Chromic Haplotorrerts Hanalei Very fine, mixed, isohyperthermic, Typic EndoaqueptsKalihi Fine, halloysitic, isohyperthermic, Fluvaquentic Endoaquolls Nohili Very-fine, smectitic, isohyperthermic, Cumulic HaplaquollsKaena Very-fine, smectitic, isohyperthermic, Typic NatraquertsLualualei Fine, smectitic, isohyperthermic, Typic ChromustertWaiawa Clayey smectitic, isohyperthermic, Lithic Vertic Ustropepts

Kauai

Kahului

Paia

Lahaina

Kula

Hana

Kaupo

Kihei

Kaunakakai

Hoolehua

Kalaupapa

HalawaKaluakoi

Kamalu

Kona

Volcano

Pahoa

Summary

Clay mineralogy important determinant of fertilityOrganic matter fundamental to nutrient availabilitySoil fertility in Hawaii closely tied to weathering intensity

Resources

BooksHavlin, J.L., S.L. Tisdale, J.D. Beaton, and W.L. Nelson. 2005. Soil Fertility and Fertilizers. Pearson Education, Inc., Upper Saddle River, NJ

Foth, H. D. and B.G. Ellis. 1997. Soil Fertility. CRC Press, Inc., Boca Raton FL.Brady, N.C. and R.R. Weil. 2004. Elements of the Nature and Properties of Soils. Pearson Education, Inc., Upper Saddle River, NJ

Webhttp://www.extension.iastate.edu/pubs/so.htmhttp://www.montana.edu/wwwpb/pubs/mt4449.htmlExcellent short course in soil fertility