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Nutrient Management Module No. 3

Nitrogen Cycling,Testing and Fertilizer

Recommendationsby Clain Jones and Jeff Jacobsen

IntroductionThis is the third in a series of Extension materials designed

to provide pertinent information on a variety of nutrient

management, water management, and water quality issues to

Extension agents, Certified Crop Advisers (CCAs), consultants, and

producers. We have included a series of questions at the back ofthis module that will make the learning active as well as offer the

potential for CEU credits for CCAs. In addition, we have included a

resource section of other Extension materials, books, web sites, and

professionals in the field.

Objectives1. Describe the 9 major nitrogen processes that occur in soil

2. 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

conditions

5. Understand how a soil nitrate test result is used to estimate Nfertilizer requirements

6. Calculate N fertilizer application rates

3CCA

2 NM

CEU

Nu

trient

Mana

geme

nt

aself-studycoursefromtheMSU

ExtensionServiceCo

ntinuingEducation

Series

4449-3

June 2005

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2/162 Module 3 Nitrogen Cycling, Testing and Fertilizer Recommendations

BackgroundNutrient Management Module 2

discussed the 14 mineral nutrients thatare required by plants for growth andreproduction. Of these, nitrogen (N) isgenerally taken up in larger amountsthan the others and is the most common,and most important, limiting nutrientfor agricultural crops. Not only does Naffect yield, but it also affects the quality(protein or sugar content) of crops suchas wheat, barley, and sugar beets. Inaddition, N also interacts with most ofthe other macronutrients. To understandhow cropping systems, N fertilizerforms, application rates, and timing of Nfertilization affect crop yield and quality,it is important to first understand thevarious transformations that N undergoeswithin the soil.

Nitrogen CyclingOf all the mineral nutrients, N

has the most complex nutrient cycle,largely because N can exist as a gas (bothammonia and nitrogen gas), whereas theother 13 mineral nutrients do not existas gases under normal soil conditions. To

help 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,a type of organic N, may also be plantavailable.

Nitrogen cycling consists of ninemajor processes: 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 abouteach of these processes, think about howeach will affect the amount of nitrateand ammonium in different soil systemsbecause these two forms are available toplants, and therefore, can directly affectcrop yield.

PLANT UPTAKEAnnual crop uptake of N can vary from

approximately 50 to 200 lb/ac per year,depending on crop and yield (Table 2).N uptake can be estimated by dividing agrowers yield by the yield shown in thetable, and then multiplying this amount bythe N uptake. A more accurate approachis to multiply plant tissue N content (as

a fraction) by dry yield (in lb/ac). Itsuseful to compare actual uptake ratesto N fertilizer rates, because N fertilizer

Table 1. Definitions of each N form.

NITROGENFORM

Nitrogen gas

Ammonia gas

Ammonium

Nitrate

Nitrite

Organic N

MOLECULARFORMULA

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

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3/163Module 3 Nitrogen Cycling, Testing and Fertilizer Recommendations

rates that are much higher (two-fold ormore) than N uptake suggest an excessiveloss of N and a possible need to refineN application rates or managementpractices. The amount of N uptake will belargely controlled by the concentrationof available N in the soil, a quantity thatis controlled by the processes described

below.

EXCHANGECation exchange was described in

Nutrient Management Module 2. Briefly,exchange indicates that ions (chargedmolecules) are attracted to a soil surface.Because clays generally have negativecharges, and ammonium (NH

4+) has a

positive charge, it will be attracted to, and

held 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, ammoniumcan move away from the soil surface asammonium levels decrease in soil solutiondue to the process known as diffusion(see NM Module 2). Hence, there is anexchange of ammonium between soil and

soil solution. More ammonium is heldby high pH (neutral to alkaline) soils,and conversely, 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 thefewer positive charges on clay surfaces.Therefore, nitrate and nitrite both haverelatively high mobility, meaning they

can move easily through the soil and donot undergo 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 harvested

portions of selected agricultural crops.

CROP

Alfalfa

Barley

Brome

Corn silage

OatsOrchard grass

Potatoes

Sugar Beets

Timothy

Wheat

ASSUMEDYIELD

PERACRE

2.5 t

50 bu

1.5 t

20 t

60 bu1.5 t

300 cwt.

25 t

1.5 t

40 bu

N UPTAKE (lb/ac)

150

80

66

167

7075

162

210

56

70

Adapted from CFA (1995).

Figure 1. The Nitrogen Cycle.

Nitrogen FixationNH

3(g)

Exchange

Plant

Uptake

Leaching

Denitrification

Volatiliz

ation

PlantU

ptak

e

Min

eraliza

tion

Immobili

zatio

n

Nitrification

NO3-

NO2

Organic Nitrogen

NH

NH4+

ClayParticle

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NITRIFICATIONSoil ammonium (NH

4+)

can quickly (hours toweeks) be converted intonitrite (NO

2-) and then into

nitrate (NO3-). This process,

known as nitrification, onlyoccurs in the presence of

oxygen, so generally it willbe slow or non-existent inwater logged, anaerobicsoils. Notice that the N inNH

4+ loses H+, lowering

pH, and gains oxygen (O)during nitrification. 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 nitriteis toxic to both plants andanimals.

Nitrification occursmost rapidly at pH levelsbetween 6.5 and 8.5, atsoil temperatures betweenabout 75 and 95oF, and atmoisture contents above thewilting point. Nitrificationoccurs rapidly in mostagricultural soils, becausethey are generally well

aerated, near neutral pH, and have warmtemperatures. Interestingly, anhydrousammonia fertilizer undergoes nitrificationsomewhat slower than other ammoniafertilizer materials because the high pHand ammonia in the band (explainedlater) inhibit the nitrifying bacteria. Aspointed out above, it would be desirableif nitrification occurred more slowly,because nitrate can be leached out of thesoil profile, whereas ammonium generallystays in the soil profile and is readilyavailable for plant uptake and utilization.

DENITRIFICATIONDenitrification refers to the process

where nitrate (NO3-) becomes nitrogen gas

(N2(g)

). It is the opposite of nitrificationin that oxygen is removed rather thanadded. Denitrification requires the absenceof oxygen, or anaerobic conditions.Poorly drained soils can result in a 4-

5% nitrate loss per day, possibly causingsubstantial yield losses (Hoeft et al., 2000).Similar to nitrification, microorganismsare responsible for denitrification, andtherefore it occurs faster in warm,moist soils. Recall from the discussionon nitrification that nitrate can onlyform in the 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 micrositeshave 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 soilsthat have readily decomposable organicmatter because denitrifying organismsrely on organic matter for energy. Plantshave been found to increase denitrificationrates likely because of the release of readilyavailable organic matter from roots androot tissue. Denitrification increases withtemperature between 40 and 80oF, and isrelatively constant above 80oF (Havlin et

Q&A #1It sounds like

it would be

beneficial to

stop or slow

nitrification to

prevent leaching

losses. Are there

any products that

do this?

There are two labeledcompounds (nitrapyrinand dicyandiamide)designed to inhibitnitrification as of theyear 2000 in the U.S.(Hoeft et al., 2000).However, they are notwidely used in Montanaor Wyoming, and researchon the effectiveness ofnitrification inhibitors ismixed (Prasad and Power,1997).

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al., 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.

VOLATILIZATIONAmmonia (NH

3) volatilization refers

to the loss of ammonia as a gas into theatmosphere, and can be a source of Nloss. The process is increased at high pHbecause NH

4+ will more easily convert

to NH3

at high pH. Therefore, thosefertilizers that increase pH further (ureaand anhydrous ammonia) may increaseammonia volatilization. This is less of

an issue in well-buffered soils, becausethe fertilizer cannot increase the pH assubstantially as in poorly buffered soils.Buffering refers to the soils 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, incorporatingammonia-based fertilizers into the soil will

also 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 two

reasons that researchers and producers inthis region have not noticed substantiallosses of surface applied urea (Jackson,Jacobsen unpub. data).

Applying anhydrous ammonia invery dry 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 loam

soil (Prasad and Power, 1997). Applyingammonia-based fertilizer immediatelybefore a rainstorm can help push itfurther into the soil profile where it isless available for volatilization. The bestmethods to decrease volatilization areto incorporate fertilizers, apply duringcalm and cool periods, and if possible, usesplit applications to decrease applicationrates (Table 3). Keep in mind from theabove that ammonium converts to nitrate

(nitrification) in hours to weeks, andonce N becomes nitrate, it can no longervolatilize.

MINERALIZATIONAs microorganisms decompose organic

matter, ammonium is released in a processcalled mineralization. The amount of Nconverted from organic forms to availableforms by mineralization ranges from

approximately 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 increasesoil OM (with no-till, minimum till, ororganic additions) can help supply arelatively constant amount of availableN to the soil. As a rule of thumb, 20-30

Table 3. Optimum

conditions to minimize

ammonia volatilization

losses.

OPTIMUMLow

Calm

15-20%

Fine

Incorporated

Low

FACTORTemperature

Wind

Moisture

Soil texture

Fertilizer

placement

N application

rates

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lb N/ac is mineralized per 1% OM.However, because mineralizationrequires microorganisms, it is highlyaffected by soil conditions. For example,mineralization occurs optimally inaerobic, moist, and warm soil, with nearneutral pH levels.

The amount of mineralization is

also dependent on the type of organicmatter present. Fresh manure or cropresidue will break down faster thanhumus that is the result of years ofdecomposition. In addition, the ratiobetween total soil carbon (C) and totalsoil N affects how quickly this processoccurs, because micro-organisms, likeplants, need N to live. For example,when the C:N ratio (i.e., total C dividedby total N) is less than about 30:1, net

mineralization generallyoccurs (Pierzynski et al.,2000). At C:N ratios lessthan 20:1, ammonia tendsto accumulate, whichcan volatilize and causeodor. For those who applyorganic materials, such asmanure, sludge, or straw, itis especially important to tryto optimize mineralization

to avoid depleting available Nin the soil (if too high a C:Nis used) 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 generallymore odorous, yet will breakdown more quickly than

those materials with high C:N ratios. Different organicamendments can be mixed toobtain a desired C:N ratio.

IMMOBILIZATIONN immobilization refers to the process

where inorganic N (NO3- or NH

4+) is

biologically converted to organic N, and isessentially the reverse of mineralization.Microorganisms immobilize N by taking

it up and converting it into proteinsand cell walls. By definition, plants alsoimmobilize N, but immobilization morecommonly refers to the process wheremicro-organisms remove availableN from solution. As you may expect,immobilization occurs more easily athigh C:N ratios (above 30:1) becausemicroorganisms scavenge any availableN in the soil as they help break down therelatively N-free organic material (Figure

2). Plant growth can be substantiallystunted following the addition of a highC:N material unless N fertilizer is addedto offset the depletion of available N. Itcan take from four to eight weeks foravailable N levels to begin to climb afteraddition of a high C:N crop residue oramendment (Havlin et al., 1999), althoughthe time is dependent on all the factors

Table 4. Carbon to nitrogen

(C:N) ratios of various organic

materials.

ORGANIC MATERIALRaw 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 crop

residues have C:N

ratios much higher

than soil organicmatter. What makes

the C:N of crop

residues go down as

they decay?

Microorganismsconvert organic carbon toCO

2gas, which goes into

the air, but N stays in thesoil. Therefore, C levelsdecrease, and N levels stayabout the same, causing theratio of C:N to decrease.

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listed previously that affect microbialhealth such as soil water, available N,and temperature. 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 controlled

by 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:N organic materials, or to supplementsuch additions with N fertilizers. Alsorecall from Figure 2 that immobilizedN will eventually become mineralizedas microorganisms 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 FIXATIONNitrogen gas (N2(g)) can be converted

into available forms of N through theprocess known as nitrogen fixation.

There are three major N fixationprocesses: ammonia fertilizer production,lightning, and biological fixation.Ammonia fertilizers 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, biologicalN

2fixation is estimated at 145 to 200

million tons per year, compared toapproximately 90 million tons per year ofworld fertilizer use (Havlin et al., 1999). Incrop production in the U.S., the amount ofbiological N

2fixation is approximately 1/3 of

the amount of fertilizer N applied (Havlinet al., 1999). Symbiotic N

2fixation 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 thenodule continue to multiply and fix N

2

Figure 2. Available N changes following

addition of high C:N organic material.

Figure 3. Bacteria nodules on bean roots.

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Q&A #3What do bacteria

get from the plant

in exchange for

providing N to theplant?

The bacteria receivecarbon from the plant,which it needs for energyand growth. The loss ofcarbon from the plant canbe considerable and is whythe plant does not assistwith fixation in a high

available N environment.In addition, the noduleprovides a controlled, lowoxygen, environment thatallows the N

2fixation to

occur.

that is in the soil. Nodulesare generally pink to salmoncolor when the organismsare actively fixing N2(g)due to a compound calledleghemoglobin, whichcontains iron, and is similarto hemoglobin in humanblood.

Symbiotic N2

fixation

is affected by manyfactors, including nutrientcontent, inoculation, soilpH, moisture, and planthealth. Symbiotic N

2

fixation is slowed by a lackof calcium, phosphorus,cobalt, boron, iron, copper,or molybdenum. In addition,high levels of available Ncan greatly diminish N

fixation because the plantstops releasing a chemicalthat attracts the bacteriato the roots, and the plantdoes 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,high levels of N fertilization favor grassover legumes, decreasing the amount of Nthat the legumes supply to the stand, andconverting the stand to primarily grass.Therefore, fertilizing pure legumes or

legume-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,so some N is often supplied with P. Formore information on the effects of excessnitrate on legumes, see Nitrate Poisoning

of Legumes (MT9801AG-see Appendix forordering information).

Each leguminous plant (called thehost plant) has a different strain ofbacteria that fixes N

2. Therefore, that

particular 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 on

plant health can be dramatic (Figure 4).N

2fixation is inhibited by pH levels

below 6 for alfalfa and 5 for red clover.Legume roots and N

2fixing bacteria

can both be injured by high levels ofaluminum and manganese, which areelevated at low pH levels. Therefore, liminglow pH soils can help increase N

2fixation

in legumes. N2

fixation is also increasedwhen photosynthetic activity is increased,likely because the N

2fixing organisms

obtain more carbon when photosynthesislevels are high (Q&A#3). Therefore,adequate moisture and warm temperatureswill generally increase N

2fixation.

Not only does N2

fixation supply N tothe microorganism and plant, but it canalso increase available N levels in the soilfor years following a legume crop. Thisis why rotating legumes with grains can

Figure 4. Effect of inoculation on nodulation

and bean health. Plant on left was not

inoculated, causing N deficiency.

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with nitrate in groundwater, nitrate isregulated by the U.S. EnvironmentalProtection Agency (USEPA). In Montana,there are several counties where averagenitrate levels in groundwater approachthe USEPA drinking water standard of 10ppm (mg/L) as N in drinking water, andthe majority of counties located east of thecontinental divide had at least one well

Table 5. Available N gains

and losses in the soil.

GAINS

Release from

exchange sites

Mineralization

Biological fixation

Fertilizer

Precipitation

Irrigation water

Manure

LOSSES

Sorption to

exchange sites

Immobilization

Denitrification

Volatilization

Plant uptake

Leaching

be an attractive cropping strategy. Forexample, in a study of dryland wheat-legume rotations, wheat yield was 38 bu/acfor a wheat-field pea rotation comparedto 32 bu/ac for continuous wheat (Milleret al., 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 N

2that are not attached to roots.

Generally, these free-living bacteria arenot believed to add more than about 5 lb

N/ac to most agricultural soils (Havlin etal., 1999).

Leaching and Upward

MovementAn available N mass balance, or

summary of inputs and outputs, shouldbe starting to form, meaning wevelooked at N gains (release from exchange

sites, mineralization, and biological N2fixation) and N losses (plant uptake,sorption to exchange sites, denitrification,volatilization, and immobilization) to theavailable N pool. In addition, N fertilizer,irrigation, manure, and precipitation (

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exceeding this standard (Bauder et al.,1993). Factors that were correlated with

high groundwater nitrateconcentrations in Montanaincluded coarse soil textures,low slopes, dryland cropproduction, tilled cropland,and summer fallow. The lackof N and water uptake duringsummer fallow periods likelyincreases nitrate leaching.Practices that increasecrop uptake, and decreaseexcessive percolation, shouldminimize the amount ofnitrate leaching.

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 detailin Nutrient Management Module 1.

This section briefly describes specialconsiderations for the sampling andtesting of soil N. Generally, only soilnitrate, and not ammonium, is sampledin soils because ammonium is convertedso quickly to nitrate via nitrification inagricultural soils that ammonium levelsare generally much lower than nitrate

levels. Because nitrate is very mobile insoils, and can move upward as pointed outabove, sampling just the upper 6 inchesis generally not a good indicator of thetotal amount of nitrate available to theplant-root system. Nitrate N should besampled to 3 feet where possible, and upto 4 feet for deep rooted crops such assugar beets and wheat, if a truck-mountedprobe is available. Generally, the top 6 inchsample and the 6- to 24-inch sample will

be analyzed 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 conversionto lb/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 number

is 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 XSAMPLETHICKNESS/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 a

concern?

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.

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

Sampling deeper than 24 inchesis not generally possible with a handprobe, but if a truck-mounted probe isavailable, deeper samples can provide

useful information. For example, if abulk of the soil nitrate is below 2 feet,and its believed that much of this willbe available to the crop, the fertilizer Nrecommendation can be decreased. Soilscan be broken into even more sections,especially when sampled deeper than 2feet. This provides the professional makingfertilizer recommendations with moreinformation that will help to fine-tune therecommendation. For example, if the bulk

of the soil nitrate was near the bottom ofthe soil profile, the soil was coarse andmoist, and heavy precipitation had fallensince the time of sampling, its 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 nitratemay not become available if roots cannotpenetrate some dry sections of the soil. Ineither case, N fertilizer recommendations

may be increased somewhat. Growersshould sample following periods ofdrought to assess the soil nitrate levelssince these levels tend to accumulate

during 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 using

yield-response curves. The strategiesused by the different laboratories thatserve Montana and Wyoming are listedin MSU Extension Bulletin 150 (ordering

Table 6. Spring wheat N

fertilizer guidelines for

Montana.

YIELD POTENTIALbu/acre30

40

50

60

70

80

SOIL NO3-N +

FERTILIZER Nlb N/acre99

132

165

198

231

264

Calculation Box 2

CALCULATETHE N FERTILIZERREQUIREMENTFORSPRINGWHEATTHATHASAYIELDPOTENTIALOF 50 bu/ac.

ASSUME SOIL N = 40 lb/acASSHOWNIN CALCULATION #1

Recommended Soil NO3-N + Fertilizer N = 165 lb/ac (from Table 6)

Fertilizer N = 165 lb/ac Soil NO3-N

Fertilizer N = 165 lb/ac 40 lb/acFertilizer N = 125 lb/ac

Fertilizer needed = Fertilizer N/fraction of N in fertilizerUrea needed = (125 lb/ac)/0.46 = 272 lb/ac

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information is in the appendix). Keep inmind that fertilizer recommendationssupplied by laboratories can vary basedon their philosophies and databases, andshould therefore be reviewed carefully(see NM Module 1). Budget inventoriesgenerally assume an N mineralizationamount (based on organic matter content

of the soil), previous crop contributions,residual (nitrate) N, and yield goal.Fertilizer Guidelines for Montana Crops(EB 161) are based on applied research inthe Northern Great Plains, and requireyield potential and soil NO

3-N to 2 feet

(Table 6). The guidelines are currentlybeing revised based on ongoing research,and Table 6 reflects revised spring wheatguidelines. The yield potential is generallybased on past yields and can be adjusted

based on soil moisture. Sometimes theyield potential is assumed to equal anamount 5 to 10% higher than averagehistorical yields. The higher yields maybe realistic if, for example, plans call forseeding with higher yielding cultivars. Thesoil NO

3-N, or residual nitrate is either

provided by the laboratory or calculated aswas shown in Calculation Box 1.

An example fertilizer N calculationis shown in Calculation Box 2 (previous

page). Note that the fertilizer guidelinesrecommend approximately 3.3 lb N/bu ofyield potential for spring wheat. This value

Table 7. Composition of selected N

fertilizers.COMMERCIAL GRADE34-0-0

16-20-010-34-0

21-0-0

82-0-0

18-46-0 to 21-54-0

10-48-0 to 11-55-0

46-0-0

FERTILIZER SOURCEAmmonium nitrate

Ammonium phosphate-sulfateAmmonium polyphosphate

Ammonium sulfate

Anhydrous ammonia

Diammonium phosphate

Monoammonium phosphate

Urea

is sometimes used instead of the tables.Keep in mind that fertilizer guidelinesare 1) guidelines that should be adjustedbased on your region and historicalresults and 2) often designed to optimize

yield, not quality. Recent research hasshown that 3.2 lb N/bu is needed at yieldpotentials between 40 and 60 bu/ac to

produce winter wheat with 14% protein,a protein level that pays a premium(Jackson, 2001).

Once a fertilizer N requirement isdetermined, the amount of fertilizerto apply can be calculated by knowingthe fraction, or percentage, of N in thefertilizer to be used (Table 7). For example,urea (CO(NH

2)

2) has an analysis of 46-0-

0, meaning it contains 46% N, 0% P2O

5,

and 0% K2O. Therefore, the fraction of N

in urea is 0.46 (46/100), and the amountof urea needed can be calculated asshown in Calculation Box 2. Additionalinformation on the pros and cons ofvarious N fertilizers, application methods,and timing 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 controlledby soil factors such as soil texture andtemperature. Some of these processes,however, can be affected by differentmanagement practices, such as tillage,irrigation, and residue management.By understanding the various factorsthat affect the N cycle, N losses can beminimized and yields optimized.

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

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

Lund. 1993. Physiographic andLand Use Characteristics associated

with nitrate nitrogen in Montanagroundwater. J. Environ. Qual. 22:255-262.

Beckie, H.J., and S.A. Brandt. 1997.Nitrogen contribution of field pea in

annual 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 New

York. 750 p.

CFA. 1995. Western Fertilizer Handbook.8th ed. California Fertilizer Association.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, SaskatooSaskatchewan. 9-10 March, 1998.University Extension Press, SaskatooSaskatchewan, Canada.

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

Prasad, R. and J.E. Power. 1997. SoilFertility Management for Sustainabl

Agriculture. CRC Press, Boca Raton,FL. 356 p.

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

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AcknowledgmentsWe would like to extend

our utmost appreciationto the following volunteerreviewers who providedtheir time and insightin making this a betterdocument:

Grant Jackson, Western

Triangle AgriculturalResearch Center, Conrad,MT

Mike Lang, Northern Ag

Service, Malta, MT

John Maki, BeaverheadCounty Extension,Dillon, MT

Paul Shelton, USDA-NRCS,Casper, WY

Suzi Taylor, MSUCommunicationsServices. Design andlayout.

APPENDIX

BOOKSWestern Fertilizer Handbook. 9th

Edition. 2001. Soil ImprovementCommittee. California Fertilizer

Association. Interstate Publishers.351 p. (http://agbook.com/

westernfertilizerhb.asp) $35 including

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:An Introduction to NutrientManagement. Havlin, J.L., S.L. Tisdale,J..C. Beaton and W.L. Nelson. 7thedition, 2005. Pearson Prentice Hall.Upper Saddle River, New Jersey. 515 p.

approx. $100.

EXTENSION MATERIALSFertilizer Guidelines for Montana Crops

(EB161), single copy is free.

Online at: http://www.montana.edu/wwwpb/pubs/eb161.html

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

SeeWeb Resources below for onlineordering information.

University of Wyoming FertilizerRecommendations (B1045),$3.Ordering information: Phone: (307)766-2115.

PERSONNELEngel, Rick.Associate Professor.

Montana State University, Bozeman.(406) 994-5295. [email protected]@montana.edu

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

Jones, Clain. Extension Soil FertilitySpecialist. Montana State University,Bozeman. (406) [email protected]

Westcott, Mal. Western AgriculturalResearch Center, Corvalis. Phone:(406) 961-3025. [email protected]

WEB RESOURCEShttp://www.montana.edu/publications

Montana State University Publicationsordering information on Extensionmaterials.

http://agnotes.org

MSU weekly Agronomy Notes byDr. Jim Bauder on a range of issues,including fertilizer management.Currently there are 23 notes on

Fertilizer Management, and over300 Agronomy notes total answeringquestions from producers, Extensionagents, and consultants.

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

Fertilizer Facts summarizing fertilizerfindings and recommendationsbased on field research conducted inMontana by Montana State Universitypersonnel.
http://agbook.com/westernfertilizerhb.asphttp://agbook.com/westernfertilizerhb.asphttp://www.montana.edu/wwwpb/pubs/eb161.htmlSoilhttp://www.montana.edu/wwwpb/pubs/eb161.htmlSoilhttp://www.montana.edu/wwwpb/pubs/eb161.htmlSoilhttp://www.montana.edu/publicationshttp://agnotes.org/http://landresources.montana.edu/FertilizerFacts/http://landresources.montana.edu/FertilizerFacts/http://landresources.montana.edu/FertilizerFacts/http://agnotes.org/http://www.montana.edu/publicationshttp://www.montana.edu/wwwpb/pubs/eb161.htmlSoilhttp://agbook.com/westernfertilizerhb.asp

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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 coopera-tion with the U.S. Department of Agriculture, Douglas Steele, Vice Provost and Director, Extension Service, Montana State University,

Bozeman, MT 59717.

Copyright 2001, 2005 MSU Extension ServiceWe encourage the use of this document for non-profit educational purposes. This document may be reprinted if no endorsement of a commercial prod-uct, service or company is stated or implied, and if appropriate credit is given to the author and the MSU Extension Service. To use these documentsin electronic formats, permission must be sought from the Ag/Extension Communications Coordinator, Communications Services, 416 Culbertson Hall,

Montana State University-Bozeman, Bozeman, MT 59717; (406) 994-2721; E-mail - [email protected].

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