Nutrient Management Module No. 3 3 Nitrogen Cycling...

Nutrient Management Module No. 3

Nitrogen Cycling,Testing and FertilizerRecommendationsby Clain Jones, Soil Chemist, andJeff Jacobsen, MSU Extension Soil Scientist

IntroductionThis is the third in a series of Extension materials designed to

provide pertinent information on a variety of nutrientmanagement, water management, and water quality issues toExtension agents, Certified Crop Advisers (CCAs), consultants, andproducers. We have included a series of questions at the back ofthis module that will make the learning “active” as well as offer thepotential for CEU credits for CCAs. In addition, we have included aresource section of other Extension materials, books, web sites,and professionals in the field.

Objectives1. Describe the 9 major nitrogen processes that occur in soil2. Describe the major factors that affect each of the nitrogen

processes3. Recognize how different crops and cropping systems affect N

availability4. Understand optimum nitrate sampling depths for different

conditions5. Understand how a soil nitrate test result is used to estimate N

fertilizer requirements6. Calculate N fertilizer application rates

3CCACCACCACCACCA2 NM2 NM2 NM2 NM2 NMCEUCEUCEUCEUCEU

Nutrient Management

a self-study course from the MSU Extension Service Continuing Education Series

4449-3Dec. 2001

2 Module 3 • Nitrogen Cycling, Testing and Fertilizer Recommendations

BackgroundNutrient Management Module 2

discussed the 14 mineral nutrients that arerequired by plants for growth andreproduction. Of these, nitrogen (N) isgenerally taken up in larger amounts thanthe others and is the most common, andmost important, limiting nutrient foragricultural crops. Not only does N affectyield, but it also affects the quality (proteinor sugar content) of crops such as wheat,barley, and sugar beets. In addition, N alsointeracts with most of the othermacronutrients. To understand howcropping systems, N fertilizer forms,application rates, and timing of Nfertilization affect crop yield and quality, itis important to first understand thevarious transformations that N undergoeswithin the soil.

Nitrogen CyclingOf all the mineral nutrients, N has the

most complex nutrient cycle, largelybecause N can exist as a gas (bothammonia and nitrogen gas), whereas theother 13 mineral nutrients do not exist asgases under normal soil conditions. Tohelp understand the various componentsof the N cycle, definitions and molecularformulas of the numerous N forms are

provided in Table 1. Available N isgenerally considered to be the sum ofammonium and nitrate, although urea, atype of organic N, may also be plantavailable.

Nitrogen cycling consists of nine majorprocesses: plant uptake, exchange,nitrification, denitrification, volatilization,mineralization, immobilization, N2fixation, and leaching (Figure 1). Each ofthese processes, and the effect that eachhas on plant available N (and hence yield),is described below. As you read about eachof these processes, think about how eachwill affect the amount of nitrate andammonium in different soil systemsbecause these two forms are available toplants, and therefore, can directly affectcrop yield.

PLANT UPTAKE

Annual crop uptake of N can vary fromapproximately 50 to 200 lb/ac per year,depending on crop and yield (Table 2). Nuptake can be estimated by dividing agrower’s yield by the yield shown in thetable, and then multiplying this amount bythe N uptake. A more accurate approach isto multiply plant tissue N content (as afraction) by dry yield (in lb/ac). It’s usefulto compare actual uptake rates to Nfertilizer rates, because N fertilizer rates

Table 1. Definitions of each N form.

NITROGEN FORM

Nitrogen gas

Ammonia gas

Ammonium

Nitrate

Nitrite

Organic N

MOLECULAR FORMULA

N2 (g)

NH3 (g)

NH4+

NO3-

NO2-

-

NOTES

Represents about 80% of the air we breathe

Generally cheapest form of N, toxic at high concentrations

Plant available, attracted to exchange sites on clay particles

Very mobile, requires more energy by plant than ammonium

Mobile, generally low concentrations, toxic to young mammals

Slowly supplies available N to soil solution

3Module 3 • Nitrogen Cycling, Testing and Fertilizer Recommendations

that are much higher (two-fold or more)than N uptake suggest an excessive loss ofN and a possible need to refine Napplication rates or management practices.The amount of N uptake will be largelycontrolled by the concentration ofavailable N in the soil, a quantity that iscontrolled by the processes describedbelow.

EXCHANGE

Cation exchange was described inNutrient Management Module 2. Briefly,‘exchange’ indicates that ions (chargedmolecules) are attracted to a soil surface.Because clays generally have negativecharges, and ammonium (NH4

+) has apositive charge, it will be attracted to, andheld weakly on clay particles. The generalterm for this process is ‘sorption,’ whichprevents ammonium from moving veryrapidly through the soil. Although it mayseem that the ammonium would not beavailable for plant uptake, ammonium canmove away from the soil surface asammonium levels decrease in soil solutiondue to the process known as diffusion (seeNM Module 2). Hence, there is anexchange of ammonium between soil andsoil solution. More ammonium is held byhigh pH (neutral to alkaline) soils, andconversely, ammonium moves morereadily in low pH (acidic) soils.

The two negatively charged N forms(nitrate and nitrite) will be repelled fromnegative charges on the clay surface, andare not attracted very strongly to the fewerpositive charges on clay surfaces.Therefore, nitrate and nitrite both haverelatively high mobility, meaning they canmove easily through the soil and do notundergo much exchange. In addition,nitrate requires more energy by the cropafter it is taken up because the nitratemust be converted to ammonium in theplant before it is made into proteins.Unfortunately, in well-aerated agriculturalsoils, ammonium readily converts tonitrate in a process called nitrification,which is described on the next page.

Table 2. N uptake amounts in harvestedportions of selected agricultural crops.

CROP

Alfalfa

Barley

Brome

Corn silage

Oats

Orchard grass

Potatoes

Sugar Beets

Timothy

Wheat

ASSUMED YIELD

PER ACRE

2.5 t

50 bu

1.5 t

20 t

60 bu

1.5 t

300 cwt.

25 t

1.5 t

40 bu

N UPTAKE (lb/ac)150

80

66

167

70

75

162

210

56

70

Adapted from CFA (1995).

Figure 1. The Nitrogen Cycle.

Nitrogen FixationNH3(g)

Exchange

Pla

nt U

ptak

e

Leac

hing

Den

itrifi

catio

n

Vol

atili

zatio

n

Plant U

ptake

Minera

lizati

on

Immob

ilizati

on

Nitrification

NO3 -

NO2

Organic Nitrogen

NH3

NH4+

ClayParticle


NITRIFICATION

Soil ammonium (NH4+)

can quickly (hours to weeks)be converted into nitrite(NO2

-) and then into nitrate(NO

3-). This process, known

as nitrification, only occursin the presence of oxygen, sogenerally it will be slow ornon-existent in water logged,anaerobic soils. Notice thatthe N in NH

4+ loses H+,

lowering pH, and gainsoxygen (O) duringnitrification. Themicroorganisms that convertammonium to nitrite andthen to nitrate are callednitrifiers or nitrifyingbacteria. The second part ofthis process, the conversionof nitrite to nitrate, occursvery rapidly in all butsaturated soils. This isfortunate because nitrite istoxic to both plants andanimals.

Nitrification occurs mostrapidly at pH levels between6.5 and 8.5, at soiltemperatures between about75 and 95oF, and at moisturecontents above the wiltingpoint. Nitrification occursrapidly in most agriculturalsoils, because they are

generally well aerated, near neutral pH,and have warm temperatures.Interestingly, anhydrous ammoniafertilizer undergoes nitrification somewhatslower than other ammonia fertilizermaterials because the high pH andammonia in the band (explained later)inhibit the nitrifying bacteria. As pointedout above, it would be desirable ifnitrification occurred more slowly, becausenitrate can be leached out of the soilprofile, whereas ammonium generallystays in the soil profile and is readilyavailable for plant uptake and utilization.

DENITRIFICATION

Denitrification refers to the processwhere nitrate (NO

3-) becomes nitrogen gas

(N2(g)). It is the opposite of nitrification inthat oxygen is removed rather than added.Denitrification requires the absence ofoxygen, or ‘anaerobic’ conditions. Poorlydrained soils can result in a 4-5% nitrateloss per day, possibly causing substantialyield losses (Hoeft et al., 2000). Similar tonitrification, microorganisms areresponsible for denitrification, andtherefore it occurs faster in warm, moistsoils. Recall from the discussion onnitrification that nitrate can only form inthe presence of oxygen, whereasdenitrification requires that nitrate bepresent and there be no oxygen. Therefore,denitrification losses of N are mostsignificant when soils alternate betweenaerobic conditions, which allow nitrate toaccumulate, and anaerobic conditions. Infine textured soils, this could occur in aflood-irrigated field. It can also occur infields with shallow groundwater tables,especially during irrigation cycles oroscillating dry and wet periods.

Interestingly, denitrification has beenfound to occur in soils containing 5%oxygen (air contains about 20% oxygen).How is that possible if denitrifyingorganisms require anaerobic conditions?The answer is that there are small poreswithin the soil that can be saturated andanaerobic. These anaerobic ‘microsites’have been found to result in substantiallevels of denitrification even in surfacesoils (Havlin et al., 1999), although theamount of denitrification that occurs inMontana and Wyoming soils is not known.

Denitrification is increased in soils thathave readily decomposable organic matterbecause denitrifying organisms rely onorganic matter for energy. Plants havebeen found to increase denitrification rateslikely because of the release of readilyavailable organic matter from roots androot tissue. Denitrification increases withtemperature between 40 and 80oF, and is

Q&A #1It sounds like itwould bebeneficial to stopor slownitrification toprevent leachinglosses. Are thereany products thatdo this?

There are two labeledcompounds (nitrapyrinand dicyandiamide)designed to inhibitnitrification as of the year2000 in the U.S. (Hoeft etal., 2000). However, theyare not widely used inMontana or Wyoming, andresearch on theeffectiveness ofnitrification inhibitors ismixed (Prasad and Power,1997).


relatively constant above 80oF (Havlin etal., 1999). It is inhibited at pH levels below5.6, but is relatively constant from pH 5.6to 8, which encompasses the vast majorityof Montana and Wyoming soil pH levels.Using practices that prevent waterloggedsoils is probably the best way to ensurethat denitrification losses are minimal.

VOLATILIZATION

Ammonia (NH3) volatilization refers tothe loss of ammonia as a gas into theatmosphere, and can be a source of N loss.The process is increased at high pHbecause NH

4+ will more easily convert to

NH3 at high pH. Therefore, those fertilizersthat increase pH further (urea andanhydrous ammonia) may increaseammonia volatilization. This is less of anissue in well-buffered soils, because thefertilizer cannot increase the pH assubstantially as in poorly buffered soils.Buffering refers to the soil’s ability toresist change; for example, clay soils aregenerally better buffered than sandy soils,and calcareous soils are generally highlybuffered. Because ammonia needs to be incontact with air to volatilize, incorporating

ammonia-based fertilizers into the soil willalso substantially decrease volatilizationpotential and increase yields. Volatilizationincreases with increasing wind, increasingtemperature (up to about 110oF), soilcoarseness (likely due to better gas flow),and N fertilizer application rates. Cooltemperatures and generally well-bufferedsoils in Montana and Wyoming may be tworeasons that researchers and producers inthis region have not noticed substantiallosses of surface applied urea (Jackson,Jacobsen unpub. data).

Applying anhydrous ammonia in verydry or very wet soils can increasevolatilization, because the soil will notquickly seal behind the injector knife,allowing the vapor to escape. Volatilizationhas been observed to occur the slowestbetween 15 and 20% moisture in a loamsoil (Prasad and Power, 1997). Applyingammonia-based fertilizer immediatelybefore a rainstorm can help push it furtherinto the soil profile where it is lessavailable for volatilization. The bestmethods to decrease volatilization are toincorporate fertilizers, apply during calmand cool periods, and if possible, use splitapplications to decrease application rates(Table 3). Keep in mind from the abovethat ammonium converts to nitrate(nitrification) in hours to weeks, and onceN becomes nitrate, it can no longervolatilize.

MINERALIZATION

As microorganisms decompose organicmatter, ammonium is released in a processcalled mineralization. The amount of Nconverted from organic forms to availableforms by mineralization ranges fromapproximately 13 to 62 lb/acre per year(Pierzynski et al., 2000). Mineralizationamounts are higher in soils with higheramounts of organic matter; therefore,taking steps to maintain or increase soilOM (with no-till, minimum till, or organicadditions) can help supply a relativelyconstant amount of available N to the soil.

Table 3. Optimumconditions to minimizeammonia volatilizationlosses.

OPTIMUM

Low

Calm

15-20%

Fine

Incorporated

Low

FACTOR

Temperature

Wind

Moisture

Soil texture

Fertilizerplacement

N applicationrates


As a rule of thumb, 20-30 lb N/ac ismineralized per 1% OM. However,because mineralization requiresmicroorganisms, it is highly affected bysoil conditions. For example,mineralization occurs optimally inaerobic, moist, and warm soil, with nearneutral pH levels.

The amount of mineralization is alsodependent on the type of organic matterpresent. Fresh manure or crop residuewill break down faster than humus thatis the result of years of decomposition.In addition, the ratio between total soilcarbon (C) and total soil N affects howquickly this process occurs, becausemicro-organisms, like plants, need N tolive. For example, when the C:N ratio(i.e., total C divided by total N) is less

than about 30:1, netmineralization generallyoccurs (Pierzynski et al.,2000). At C:N ratios lessthan 20:1, ammonia tendsto accumulate, which canvolatilize and cause odor. Forthose who apply organicmaterials, such as manure,sludge, or straw, it isespecially important to try tooptimize mineralization toavoid depleting available N inthe soil (if too high a C:N isused) and to possibly avoidexcessive odor, if this is apotential concern. C:N ratiosof various organic materialsare shown in Table 4. Notethat the materials with lowC:N ratios are generally moreodorous, yet will break downmore quickly than thosematerials with high C:Nratios. Different organicamendments can be mixed toobtain a desired C:N ratio.

IMMOBILIZATION

N immobilization refers to the processwhere inorganic N (NO

3- or NH

4+) is

biologically converted to organic N, and isessentially the reverse of mineralization.Microorganisms immobilize N by taking itup and converting it into proteins and cellwalls. By definition, plants also immobilizeN, but immobilization more commonlyrefers to the process where micro-organisms remove available N fromsolution. As you may expect,immobilization occurs more easily at highC:N ratios (above 30:1) becausemicroorganisms scavenge any available Nin the soil as they help break down therelatively N-free organic material (Figure2). Plant growth can be substantiallystunted following the addition of a highC:N material unless N fertilizer is added tooffset the depletion of available N. It cantake from four to eight weeks for availableN levels to begin to climb after addition ofa high C:N crop residue or amendment(Havlin et al., 1999), although the time is

Table 4. Carbon to nitrogen(C:N) ratios of variousorganic materials.

ORGANIC MATERIAL

Raw municipal wastewater

Treated municipal sludge

Soil organic matter

Sweet clover

Poultry manure

Steer manure

Rye

Corn roots

Corn/sorghum stover

Straw

Sawdust

C:N5

10

10

12

16

20

36

48

60

80

400

Q&A #2It looks like cropresidues have C:Nratios muchhigher than soilorganic matter.What makes theC:N of cropresidues go downas they decay?

Microorganismsconvert organic carbon toCO

2 gas, which goes into

the air, but N stays in thesoil. Therefore, C levelsdecrease, and N levels stayabout the same, causingthe ratio of C:N todecrease.


dependent on all the factors listedpreviously that affect microbial healthsuch as soil water, available N, andtemperature. Note that available Nconcentrations can eventually be greaterthan initial N concentrations, even withthe addition of a high C:N organic materialsuch as tilled-in grain stubble.

Because immobilization is controlledby microorganism growth, it occurs mostreadily in warm, moist soils with nearneutral pH levels. The best way to avoidsubstantial amounts of N immobilizationis to avoid large applications of high C:Norganic materials, or to supplement suchadditions with N fertilizers. Also recallfrom Figure 2 that immobilized N willeventually become mineralized asmicroorganisms die and decompose,increasing available N levels. Therefore,immobilization is not necessarily anegative outcome, especially if it can betimed with a period when a field is fallow,thereby capturing the N in a form that isnot leachable.

NITROGEN FIXATION

Nitrogen gas (N2(g)) can be convertedinto available forms of N through theprocess known as ‘nitrogen fixation.’ Thereare three major N fixation processes:ammonia fertilizer production, lightning,and biological fixation. Ammoniafertilizers require natural gas, steam,oxygen, and a catalyst to fix N2(g).Therefore, ammonia fertilizer prices areheavily dependent on the price of naturalgas. Lightning also fixes N, although theamount of available N that reaches theearth from the atmosphere is generallyless than 5 lb/acre per year (Brady, 1984).

Some organisms are able to convertatmospheric N2(g), which representsapproximately 80% of the air we breathe,into ammonium. Worldwide, biological N

2fixation is estimated at 145 to 200 milliontons per year, compared to approximately90 million tons per year of world fertilizeruse (Havlin et al., 1999). In crop

production in the U.S., the amount ofbiological N2 fixation is approximately 1/3 ofthe amount of fertilizer N applied (Havlinet al., 1999). ‘Symbiotic’ N

2 fixation occurs

when a bacterium, such as Rhizobium,‘infects’ a root hair of a legume, such asalfalfa. The root hair wraps around thebacterium, creating a nodule on the root(Figure 3). The bacteria trapped inside the

Figure 2. Available N changes followingaddition of high C:N organic material.

Figure 3. Bacteria nodules on bean roots.


Q&A #3What do bacteriaget from the plantin exchange forproviding N to theplant?

The bacteria receivecarbon from the plant,which it needs for energyand growth. The loss ofcarbon from the plant canbe considerable and iswhy the plant does notassist with fixation in ahigh available Nenvironment. In addition,the nodule provides acontrolled, low oxygen,environment that allowsthe N2 fixation to occur.

nodule continue to multiplyand fix N2 that is in the soil.Nodules are generally pink tosalmon color when theorganisms are actively fixingN2(g) due to a compoundcalled ‘leghemoglobin,’ whichcontains iron, and is similarto hemoglobin in humanblood.

Symbiotic N2 fixation is

affected by many factors,including nutrient content,inoculation, soil pH,moisture, and plant health.Symbiotic N2 fixation isslowed by a lack of calcium,phosphorus, cobalt, boron,iron, copper, or molybdenum.In addition, high levels ofavailable N can greatlydiminish N fixation becausethe plant stops releasing achemical that attracts thebacteria to the roots, and theplant does not allow nodulesto form. Specifically, in amixed grass-legume stand, Nfertilization with up to 100 lbN/ac significantly decreased

legume yield, significantly increased grassyield, and had no effect on total yield above33 lb N/ac (Tueller, 1988). Essentially, highlevels of N fertilization favor grass overlegumes, decreasing the amount of N thatthe legumes supply to the stand, andconverting the stand to primarily grass.Therefore, fertilizing pure legumes orlegume-grass stands with more than 30 to40 lb N/ac is generally not recommended,although in some grass-alfalfa stands,additional N can be a benefit for thesecond and third cuttings. Keep in mindthat phosphorus fertilizer requirementsare generally met by the addition ofammonia phosphate fertilizer materials, sosome N is often supplied with P. For moreinformation on the effects of excess nitrateon legumes, see Nitrate Poisoning ofLegumes (MT9801AG-see Appendix forordering information).

Each leguminous plant (called the‘host’ plant) has a different strain ofbacteria that fixes N2. Therefore, thatparticular type of bacteria either needs tobe in the soil, or added with the seed, astep called ‘inoculation.’ For example, thebacteria species that inoculates alfalfa willnot work with beans, and vice versa. Thepositive effects of inoculating legumes onplant health can be dramatic (Figure 4).

N2 fixation is inhibited by pH levels

below 6 for alfalfa and 5 for red clover.Legume roots and N2 fixing bacteria canboth be injured by high levels ofaluminum and manganese, which areelevated at low pH levels. Therefore,liming low pH soils can help increase N

2fixation in legumes. N2 fixation is alsoincreased when photosynthetic activity isincreased, likely because the N

2 fixing

organisms obtain more carbon whenphotosynthesis levels are high (Q&A #3).Therefore, adequate moisture and warmtemperatures will generally increase N2fixation.

Not only does N2 fixation supply N to

the microorganism and plant, but it canalso increase available N levels in the soilfor years following a legume crop. This is

Figure 4. Effect of inoculation onnodulation and bean health. Plant on leftwas not inoculated, causing N deficiency.


addition, because of health concerns withnitrate in groundwater, nitrate is regulatedby the U.S. Environmental ProtectionAgency (USEPA). In Montana, there areseveral counties where average nitratelevels in groundwater approach the USEPAdrinking water standard of 10 ppm (mg/L)as N in drinking water, and the majority ofcounties located east of the continentaldivide had at least one well exceeding this

Table 5. Available N gainsand losses in the soil.

GAINS

Release fromexchange sites

Mineralization

Biological fixation

Fertilizer

Precipitation

Irrigation water

Manure

LOSSES

Sorption toexchange sites

Immobilization

Denitrification

Volatilization

Plant uptake

Leaching

why rotating legumes with grains can bean attractive cropping strategy. Forexample, in a study of dryland wheat-legume rotations, wheat yield was 38 bu/acfor a wheat-field pea rotation compared to32 bu/ac for continuous wheat (Miller etal., 1998). In addition, wheat grownfollowing peas had a protein level of 13%compared to 12.1% for continuous wheat.Barley also shows increased yield followinga pea crop, with especially significant yieldincreases at low fertilizer N rates (Figure5). The difference in yields between barleygrown in wheat versus canola stubble isattributed to differences in pest pressure.

In addition to the symbiotic N2 fixationdiscussed above, there are also bacteriathat fix N2 that are not attached to roots.Generally, these ‘free-living’ bacteria arenot believed to add more than about 5 lbN/ac to most agricultural soils (Havlin etal., 1999).

Leaching and UpwardMovement

An available N ‘mass balance,’ orsummary of inputs and outputs, should bestarting to form, meaning we’ve looked atN gains (release from exchange sites,mineralization, and biological N2 fixation)and N losses (plant uptake, sorption toexchange sites, denitrification,volatilization, and immobilization) to theavailable N pool. In addition, N fertilizer,irrigation, manure, and precipitation (<5lb/ac-year), represent other inputs to thepool (Table 5). The final potential loss fromthe soil system is nitrate leaching.

Nitrate is highly mobile as discussedpreviously and in Nutrient ManagementModule 2. The areas with the highest riskfor nitrate leaching are associated withhigh precipitation, irrigated conditions, orcoarse textured, shallow soils. Nitrateleaching, like denitrification andvolatilization, represents an economic loss,because once the N has left the soil system,it is not available for crop uptake. In

Figure 5. Yield of barley grown in pea, canola, andwheat stubble. Modified from Beckie and Brandt(1997).


standard (Bauder et al., 1993). Factors thatwere correlated with high groundwater

nitrate concentrations inMontana included coarse soiltextures, low slopes, drylandcrop production, tilledcropland, and summer fallow.The lack of N and wateruptake during summer fallowperiods likely increasesnitrate leaching. Practicesthat increase crop uptake,and decrease excessivepercolation, should minimizethe amount of nitrateleaching.

Nitrate can also moveupward, especially in semi-arid and arid regions. Upwardmovement of nitrate andother soluble ions occurswhen evaporation exceedsprecipitation, causing waterto move upwards. The easethat nitrate moves eitherupward or downward affectssoil sampling methods fornitrate as described below.

Soil Sampling for NitrateSoil sampling methods and laboratory

selection were described in detail inNutrient Management Module 1. This

section briefly describes specialconsiderations for the sampling andtesting of soil N. Generally, only soilnitrate, and not ammonium, is sampled insoils because ammonium is converted soquickly to nitrate via nitrification inagricultural soils that ammonium levelsare generally much lower than nitratelevels. Because nitrate is very mobile insoils, and can move upward as pointed outabove, sampling just the upper 6 inches isgenerally not a good indicator of the totalamount of nitrate available to the plant-root system. Nitrate N should be sampledto 3 feet where possible, and up to 4 feetfor deep rooted crops such as sugar beetsand wheat, if a truck-mounted probe isavailable. Generally, the top 6 inch sampleand the 6- to 24-inch sample will beanalyzed for nitrate N. Samples greaterthan 24 inches can be composited and alsosubmitted.

The laboratory will generally calculatethe total nitrate in lb/ac, although if thedata is reported in ppm, the conversion tolb/ac can be performed as shown(Calculation Box 1). The factor of 2 in theequation is derived from the assumptionthat an acre-furrow slice (6 inch slice) ofsoil weighs 2 million pounds. This numberis somewhat higher in soils with aboveaverage ‘bulk densities,’ which is the casewith compacted soils, and is somewhatlower with soils high in organic matter or

Calculation Box 1

CALCULATION: NITRATE-N (lb/acre) = NITRATE-N CONCENTRATION (ppm) X 2 X SAMPLE THICKNESS/6"

Example: 0-6 inch 8 ppm NO3-N (or nitrate-N, meaning nitrate expressed as N in ppm)6-24 inch 4 ppm NO3-N

N in 0-6 inch increment = 8 x 2 x 6"/6" = 16 lb/acreN in 6-24 inch increment = 4 x 2 x 18"/6" = 24 lb/acreN total in 0-24 inch profile = 40 lb/acre

Q&A #4Why is nitrate ingroundwater aconcern?Nitrate can cause adisease referred to asmethemoglobinemia, orblue-baby disease. Infants,as well as young livestock,have a different type ofhemoglobin than adults.If infants ingest water,food, or milk withexcess nitrate and nitrite,oxygen is pulled fromtheir bloodstream,depriving them ofnecessary oxygen.


that have been recently plowed. The bulkdensity is simply the dry weight of the soildivided by the volume, and is usuallyexpressed in pounds per cubic foot (lb/ft3).

Sampling deeper than 24 inches is notgenerally possible with a hand probe, but ifa truck-mounted probe is available, deepersamples can provide useful information.For example, if a bulk of the soil nitrate isbelow 2 feet, and it’s believed that much ofthis will be available to the crop, thefertilizer N recommendation can bedecreased. Soils can be broken into evenmore sections, especially when sampleddeeper than 2 feet. This provides theprofessional making fertilizerrecommendations with more informationthat will help to fine-tune therecommendation. For example, if the bulkof the soil nitrate was near the bottom ofthe soil profile, the soil was coarse andmoist, and heavy precipitation had fallensince the time of sampling, it’s possiblethat much of the deep nitrate leached outof the profile and should not be includedin the calculations of soil profile nitrate.Conversely, in a dry year, some nitrate maynot become available if roots cannotpenetrate some dry sections of the soil. Ineither case, N fertilizer recommendationsmay be increased somewhat. Growersshould sample following periods ofdrought to assess the soil nitrate levels

since these levels tend to accumulateduring periods of below average yields. Thefollowing section introduces the science,and art, of making accurate N fertilizerrecommendations.

N Fertilizer RecommendationsThere are a number of strategies for

determining N recommendationsincluding historical amounts, budgetinventories of gains and losses, and usingyield-response curves. The strategies usedby the different laboratories that serve

Table 6. Spring wheat Nfertilizer guidelines forMontana.

YIELD POTENTIAL

bu/acre

30

40

50

60

70

80

SOIL NO3-N +FERTILIZER N

lb N/acre

84

112

140

168

196

224

Calculation Box 2

CALCULATE THE N FERTILIZER REQUIREMENT FOR SPRING WHEAT THAT HAS A YIELD POTENTIAL OF 50 bu/ac.ASSUME SOIL N = 40 lb/ac AS SHOWN IN CALCULATION #1

Recommended Soil NO3-N + Fertilizer N = 140 lb/ac (from Table 6)Fertilizer N = 140 lb/ac – Soil NO

3-N

Fertilizer N = 140 lb/ac – 40 lb/acFertilizer N = 100 lb/ac

Fertilizer needed = Fertilizer N/fraction of N in fertilizerUrea needed = (100 lb/ac)/0.46 = 217 lb/ac


Montana and Wyoming are listed in MSUExtension Bulletin 150 (orderinginformation is in the appendix). Keep inmind that fertilizer recommendationssupplied by laboratories can vary based ontheir philosophies and databases, andshould therefore be reviewed carefully (seeNM Module 1). Budget inventoriesgenerally assume an N mineralizationamount (based on organic matter contentof the soil), previous crop contributions,residual (nitrate) N, and yield goal.Montana Fertilizer Guidelines (EB 104) arebased on applied research in the NorthernGreat Plains, and require yield potentialand soil NO3-N to 2 feet (Table 6). Theguidelines are currently being revisedbased on ongoing research, and Table 6reflects revised spring wheat guidelines.The yield potential is generally based onpast yields and can be adjusted based onsoil moisture. Sometimes the yieldpotential is assumed to equal an amount 5to 10% higher than average historicalyields. The higher yields may be realistic if,for example, plans call for seeding withhigher yielding cultivars. The soil NO3-N,or ‘residual nitrate’ is either provided bythe laboratory or calculated as was shownin Calculation Box 1.

An example fertilizer N calculation isshown in Calculation Box 2 (previous

Table 7. Composition of selected N fertilizers.

COMMERCIAL GRADE

34-0-0

16-20-0

10-34-0

21-0-0

82-0-0

18-46-0 to 21-54-0

10-48-0 to 11-55-0

46-0-0

FERTILIZER SOURCE

Ammonium nitrate

Ammonium phosphate-sulfate

Ammonium polyphosphate

Ammonium sulfate

Anhydrous ammonia

Diammonium phosphate

Monoammonium phosphate

Urea

page). Note that the fertilizer guidelinesrecommend approximately 2.8 lb N/bu ofyield potential. This value is sometimesused instead of the tables. Keep in mindthat fertilizer guidelines are 1) guidelinesthat should be adjusted based on yourregion and historical results and 2) oftendesigned to optimize yield, not quality.Recent research has shown that 3.2 lb N/bu is needed at yield potentials between 40and 60 bu/ac to produce winter wheat with14% protein, a protein level that pays apremium (Jackson, 2001).

Once a fertilizer N requirement isdetermined, the amount of fertilizer toapply can be calculated by knowing thefraction, or percentage, of N in thefertilizer to be used (Table 7). For example,urea (CO(NH2)2) has an analysis of 46-0-0,meaning it contains 46% N, 0% P

2O

5, and

0% K2O. Therefore, the fraction of N inurea is 0.46 (46/100), and the amount ofurea needed can be calculated as shown inCalculation Box 2. Additional informationon the pros and cons of various Nfertilizers, application methods, andtiming of fertilizer application will becovered in a future module.

SummaryN can undergo numerous

transformations in the soil that eithermake it more, or less, available to plants.Some of these processes cannot be alteredby producers, but are instead controlled bysoil factors such as soil texture andtemperature. Some of these processes,however, can be affected by differentmanagement practices, such as tillage,irrigation, and residue management. Byunderstanding the various factors thataffect the N cycle, N losses can beminimized and yields optimized.

Soil samples for N should be collectedas deep as possible due to nitrate’s highmobility, and hence availability, in soils. Nfertilization recommendations aregenerally supplied by laboratories, butshould be verified by using publishedfertilizer guidelines and publications.


ReferencesBauder, J.W., K.N. Sinclair, and R.E.

Lund. 1993. Physiographic and LandUse Characteristics associated withnitrate nitrogen in Montanagroundwater. J. Environ. Qual. 22:255-262.

Beckie, H.J., and S.A. Brandt. 1997.Nitrogen contribution of field pea inannual cropping systems. 1. Nitrogenresidual effect. Can. J. Plant Sci.77:311-322.

Brady, N.C. 1984. The Nature andProperties of Soils. 9th Edition.Macmillan Publishing Company NewYork. 750 p.

CFA. 1995. Western Fertilizer Handbook.8th ed. California FertilizerAssociation. Interstate Publishers, Inc.Danville, Illinois. 338 p.

Havlin, J.L., J.D. Beaton, S.L. Tisdale, andW.L. Nelson. 1999. Soil Fertility andFertilizers. 6th Edition. Prentice Hall.Upper Saddle River, NJ. 499 p.

Hoeft R.G., E.D. Nafziger, R.R. Johnson,and S.R. Aldrich. 2000. Modern Cornand Soybean Production. MCSPPublications. Champaign, IL.

Jackson, G.D. 2001. Fertilizing winterwheat with nitrogen for yield andprotein. Fertilizer Fact Sheet 26. MSUExtension Service and AgriculturalExperiment Station, Bozeman, MT.

Miller, P., R. Zentner, B. McConkey, C.Campbell, D. Derksen, C. McDonald,and J. Waddington. 1998. Using pulsecrops to boost wheat protein in theBrown soil zone. p. 313-316. In D.B.Fowler et al. (ed.) Wheat Protein

Production and Marketing. Proc.Wheat Protein Symposium,Saskatoon, Saskatchewan. 9-10 March,1998. University Extension Press,Saskatoon, Saskatchewan, Canada.

Pierzynski, G.M., J.T. Sims, and G.F.Vance. 2000. Soils and EnvironmentalQuality. 2nd Ed. CRC Press. BocaRaton, FL. 459 p.

Prasad, R. and J.E. Power. 1997. SoilFertility Management for SustainableAgriculture. CRC Press, Boca Raton,FL. 356 p.

Tueller, P.T. 1988. Vegetation ScienceApplications for Rangeland Analysisand Management. Kluwer AcademicPublishers. Norwell, MA.


AcknowledgmentsWe would like to extend

our utmost appreciation tothe following volunteerreviewers who providedtheir time and insight inmaking this a betterdocument:

Grant Jackson, WesternTriangle AgriculturalResearch Center, Conrad,MT

Mike Lang, Northern AgService, Malta, MT

John Maki, BeaverheadCounty Extension,Dillon, MT

Paul Shelton, USDA-NRCS,Casper, WY

Suzi Taylor, MSUCommunicationsServices. Design andlayout.

APPENDIX

BOOKS

Western Fertilizer Handbook. 8th Edition.1995. Soil Improvement Committee.California Fertilizer Association.Thomson Publications. 351 p. (http://www.agbook.com/westernfertilizerhb.htm) $35including shipping.

Plant Nutrition Manual. J. Benton Jones,Jr. 1998. CRC Press, Boca Raton,Florida. 149 p. Approximately $50.

Soil Fertility. Foth and Ellis. 1997. CRCPress, Boca Raton, Florida. 290 p.

Soil Fertility and Fertilizers, 6th Edition.J.L. Havlin et al. 1999. Upper SaddleRiver, N.J.: Prentice Hall. 499 p.Approximately $100.

EXTENSION MATERIALS

Fertilizer Guidelines (EB104), single copyis free.

Soil, Plant and Water AnalyticalLaboratories for Montana Agriculture(EB 150), single copy is free.

Obtain the above Extension materials(add $1 for shipping) from:

MSU Extension PublicationsP.O. Box 172040Bozeman, MT 59717-2040

See Web Resources below for onlineordering information.

University of Wyoming FertilizerRecommendations (B1045),$3.Ordering information: http://www.uwyo.edu/ces/PUBS/Mp7r2002.PDF, Phone: (307) 766-2115.

PERSONNEL

Engel, Rick. Associate Professor.Montana State University, Bozeman.(406) 994-5295. [email protected]”[email protected]

Jackson, Grant. Associate Professor.Western Triangle AgriculturalResearch Center, Conrad. (406) 278-7707. [email protected]

Jacobsen, Jeff. Extension Soil Scientist.Montana State University, Bozeman.(406) 994-4605. [email protected]

Jones, Clain. Soil Chemist. MontanaState University, Bozeman. (406) 994-6076. [email protected]

Westcott, Mal. Western AgriculturalResearch Center, Corvalis. Phone:(406) [email protected]

WEB RESOURCES

http://www.montana.edu/publications

Montana State University Publicationsordering information on Extensionmaterials.

http://scarab.msu.montana.edu/Agnotesold/agnotes11b_toc.htm

MSU weekly Agronomy Notes by Dr.Jim Bauder on a range of issues,including fertilizer management.Currently there are 23 notes onFertilizer Management, and over 300Agronomy notes total answeringquestions from producers, Extensionagents, and consultants.

http://landresources.montana.edu/FertilizerFacts/

28 Fertilizer Facts summarizingfertilizer findings andrecommendations based on fieldresearch conducted in Montana byMontana State University personnel.

http://www.cals.cornell.edu/dept/flori/growon/field.html#beginning

Contains general nitrogen cyclediagram. Includes information onmineralization, nitrification,immobilization, and sources ofnitrogen loss. Source: CornellCooperative Extension.

The programs of the MSU Extension Service are available to all people regardless of race, creed, color, sex, disability or national origin.Issued in furtherance of cooperative extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperationwith the U.S. Department of Agriculture, David A. Bryant, Vice Provost and Director, Extension Service, Montana State University,Bozeman, MT 59717.

Copyright © 2001 MSU Extension ServiceWe encourage the use of this document for non-profit educational purposes. This document may be reprinted if no endorsement of a commercialproduct, service or company is stated or implied, and if appropriate credit is given to the author and the MSU Extension Service. To use these docu-ments in electronic formats, permission must be sought from the Ag/Extension Communications Coordinator, Communications Services, 416 CulbertsonHall, Montana State University-Bozeman, Bozeman, MT 59717; (406) 994-2721; E-mail - [email protected].

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