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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
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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 UPTAKEAnnual crop uptake of N can vary from
approximately 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
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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.
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 (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.
CROPAlfalfa
Barley
Brome
Corn silage
Oats
Orchard grass
Potatoes
Sugar Beets
Timothy
Wheat
ASSUMED YIELDPER 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
Plan
t Upta
ke
Mine
raliz
ation
Immo
biliza
tion
Nitrification
NO3 -
NO2
Organic Nitrogen
NH3
NH4+
ClayParticle
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4 Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
NITRIFICATIONSoil 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.
DENITRIFICATIONDenitrification refers to the process
where nitrate (NO3-) 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).
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5Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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.
VOLATILIZATIONAmmonia (NH3) volatilization refers to
the 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.
MINERALIZATIONAs microorganisms decompose organic
matter, 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.
OPTIMUMLow
Calm
15-20%
Fine
Incorporated
Low
FACTORTemperature
Wind
Moisture
Soil texture
Fertilizerplacement
N applicationrates
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6 Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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.
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
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 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 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.
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7Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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 FIXATIONNitrogen gas (N2(g)) can be converted
into 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.
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8 Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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.
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9Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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.
GAINSRelease fromexchange sites
Mineralization
Biological fixation
Fertilizer
Precipitation
Irrigation water
Manure
LOSSESSorption 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 (
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10 Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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.
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11Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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 POTENTIALbu/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
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12 Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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 GRADE34-0-0
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 SOURCEAmmonium 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.
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13Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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.
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14 Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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15Module 3 • Nitrogen Cycling, Testing and Fertilizer
Recommendations
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
BOOKSWestern 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 MATERIALSFertilizer Guidelines (EB104), single
copy
is 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.
PERSONNELEngel, 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 RESOURCEShttp://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].