phosphorus cycling, testing and fertilizer recommendations

Nutrient Management Module No. 4

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

IntroductionThis module is the fourth in a series of Extension materials

designed to provide Extension agents, Certified Crop Advisers(CCAs), consultants, and producers with pertinent information onnutrient management issues. To make the learning ‘active,’ and topossibly provide credits to Certified Crop Advisers, we haveincluded questions at the back of this module. In addition,realizing that there are many other good information sourcesincluding previously developed Extension materials, books, websites, and professionals in the field, we have provided a list ofadditional resources and contacts for those wanting more in-depthinformation about phosphorus.

Objectives1.Understand the various soil forms of phosphorus

2.Recognize how soil and climate properties affect phosphoruscycling in soil

3.Recognize how cropping systems affect phosphorus management

4.Be able to make a phosphorus fertilizer recommendation basedon a soil test report

5.Recognize the advantages and disadvantages of differentphosphorus fertilizers

4CCACCACCACCACCA2 NM2 NM2 NM2 NM2 NMCEUCEUCEUCEUCEU

Nutrient Management

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

4449-4July 2002

2 Module 4 • Phosphorus Cycling, Testing and Fertilizer Recommendations

BackgroundPhosphorus (P) fertilizer is second

only to nitrogen (N) in the amountapplied annually to cropland in Montana.It is required for many plant functions,including energy transfer and proteinsynthesis. Unlike N fertilizer, which ismade from virtually unlimited amountsof N

2 gas in the atmosphere, commercial

P fertilizer is mined from a finite supplyof P ore deposits. Compared to most partsof the country, the amount of P inMontana and Wyoming soils is low. Manysoils in these states show significant cropresponses to P fertilization.

Combined with growingenvironmental concerns and regulations,it is useful for agricultural professionalsto more fully understand the effect thatvarious management practices have on Pavailability and movement. This isaccomplished in this module by

1) describing the factors that affect Pcycling in soils;

2) explaining the usefulness of P soiltests;

3) demonstrating how to make P fertilizerrecommendations based on soil testresults; and

4) discussing pros and cons of various Pfertilizer management practices.

Phosphorus CyclingPhosphorus can exist in the soil as

phosphate (HPO4-2 or H

2PO

4-), sorbed P,

organic P, or in P minerals (Table 1).Phosphate is the only form that plants cantake up, yet in most agricultural soils thereis less than 1 mg/L (1 ppm) of phosphate insolution, which represents much less than1% of the total soil P. Organic P, which is Pbound in organic matter, has been found torepresent between about 25% and 65% oftotal P in surface soils, with mineral P(such as calcium phosphate minerals) andsorbed P, representing the remainder(Brady, 1984).

Organic P decreases quickly with soildepth, paralleling decreases in organicmatter. The processes that control theamount of available P in the soil are: plantuptake, sorption, desorption,mineralization, immobilization,precipitation, dissolution, and erosion(Figure 1).

PLANT UPTAKE

Despite low concentrations ofphosphate in soil solution, plants can takeup substantial amounts of P due to Pdesorption and dissolution (discussedbelow), followed by P diffusion to the plantroot (Foth and Ellis, 1997). By taking uplarge amounts of P, a strong ‘diffusiongradient’ is created (see Nutrient

Management Module 2,Plant Nutrition and SoilFertility) which movesP toward the plant rootat much higher ratesthan water is movingvia transpiration. Puptake for selectedagricultural cropsranges from about 20 to60 lb P2O5/ac per year,although these amountsare dependent on yield(Table 2). Commercial Pfertilizers have

Table 1. Definitions of each P form.

PHOSPHORUS FORM

Phosphate

Sorbed P

Mineral P

Calcium phosphates

Aluminum phosphate

Iron phosphate

Organic P

EXAMPLE MOLECULAR FORMULA

HPO4

-2, H2PO

4-

—

CaHPO4

AlPO4

FePO4

—

NOTES

Form that plants can use

Can slowly become available

Relatively insoluble

Slowly supplies available P toplants and microorganisms

3Module 4 • Phosphorus Cycling, Testing and Fertilizer Recommendations

historically been expressed as the oxideform (P

2O

5) rather than the elemental

form (P), therefore P values are stillexpressed as P2O5. Using the ratio of theirmolecular weights, %P

2O

5 can be

converted to %P by multiplying by 0.44(%P = %P2O5 x 0.44). To estimate P2O5uptake for a specific field, one can divideactual yield by the yield in Table 2 andthen multiply this result by the P2O5uptake given. This is an estimate of Pfertilizer that is being removed by thecrop, and may help in determining Pfertilization recommendations.

A more exact method to estimate Premoval is to have P analyzed in the planttissue that will be removed from the field,and multiply the result by dry matter yieldremoved. The amount of P uptake isaffected by the amount of available P in thesoil; a quantity that is controlled by otherprocesses involving P that are discussedbelow.

SORPTION/DESORPTION

Sorption refers to the binding of P tosoil particles. Because phosphate has anegative charge, it is attracted to, andbinds strongly to positively chargedminerals, such as aluminum (Al) and iron(Fe) hydroxides and oxides. Recall that rustis a type of iron hydroxide. Like other soilparticles, these minerals become morepositively charged at lower pH; therefore,more phosphate is sorbed at lower pH(Figure 2). Finer textured soils generallycan sorb more P because they have moresurface area. In the calcareous soils ofMontana and Wyoming, sorption is likelynot as important as precipitation(discussed later) at controlling available Plevels.

Sorption is decreased, and henceavailable P levels are increased, when thesoil solution contains high levels of otheranions such as bicarbonate, carbonate,silicate, sulfate, or molybdate that competefor sorption ‘sites.’ In addition, dissolvedorganic compounds associated withorganic matter can increase P availability

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

P2O5 UPTAKE

(LB/AC)28

30

19

23

24

25

60

50

21

25

Table 2. P uptake amounts in harvestedportions for selected agricultural crops.

From Pierzynski et al. (2000) and CFA (1995).

Figure 1. Phosphorus cycle.

Pla

nt U

ptak

e

Dissolution

Precipitation

PMinerals

Sorption Desorption

HPO4-2

Organic P

Erosion

Min

eral

izat

ion

Imm

obiliz

atio

n

Fe or AlOxide

++

+


by competing for phosphatesorption sites or coating Fe/Aloxides. In addition, as more Pfertilizer is added, P sorptionsites can begin to becomesaturated, which increases therecovery, or P use efficiency, ofadded P. Although Fe/Al oxidesdo not represent a largefraction of Montana andWyoming soils, they are someof the only minerals that carrya substantial amount ofpositive charge at pH levelstypical in this region, and thuscan sorb large amounts ofdissolved P, which isnegatively charged. Sorptionof P generally increases withincreased temperatures in P

fertilized soils, but there is no correlationwith temperature in soils that have not hadP added (Havlin et al., 1999). P desorption(the opposite of sorption) generallyincreases as solution P decreases, or underflooded conditions due to dissolution of Fehydroxides and oxides, that release sorbed P.

MINERALIZATION/IMMOBILIZATION

P mineralization is the process whereorganic P becomes converted to phosphateas organic P decomposes, andimmobilization is the process whereavailable P becomes tied up in

microorganism cells. In the United States,annual amounts of P mineralization havebeen found to range from 4 to 22 lb P2O5/ac, representing a significant portion ofcrop P uptake. These values are averagesfrom 30-80 year studies and would beexpected to currently be less due to lowerorganic matter levels in most cultivatedsoils. Mineralization occurs most readilywhen the C:P ratio is less than 200:1, andimmobilization occurs when that ratio isgreater than 300:1 (Havlin et al., 1999). Forcomparison purposes, the C:P ratio for beefcattle manure is approximately 100:1 (St.Jean, 1997), suggesting that decomposingmanure should mineralize (release) Prather than immobilize P. Mineralizationand immobilization of P are affected bytemperature, moisture, aeration, and pH insimilar ways as N mineralization andimmobilization, because they involve thesame microbial processes (see NutrientManagement Module 3, Nitrogen Cycling,Testing and Fertilizer Recommendations).

PRECIPITATION/DISSOLUTION

Available P concentrations are largelycontrolled by the solubility of P mineralsthat are dominated by calcium phosphates(Ca-P) in neutral to high pH soils typical ofMontana and Wyoming soils, and by Al and

Q&A #1Is ‘sorbed P’ partof a mineral?

Actually, no. P that issorbed is bound only tothe outside of minerals,but does not become partof the mineral itself. Theprocess you’re describingis called precipitation andis described later.

100908070605040302010

0

4 5 6 7 8 9 10pH

Rel

ativ

e ph

osph

ate

sopr

tion

%

Figure 2. Phosphate sorption to aniron oxide. Adapted from Havlin etal. (1999).

CA-P FORM

Monocalcium P (MCP)

Dicalcium P (DCP)

Octacalcium P (OCP)

Tricalcium P (TCP)

Hydroxyapatite (HA)

Fluorapatite (FA)

Table 3. Selected calcium phosphates.TIME TO FORM

FOLLOWING FERTILIZATION

minutes

days to weeks

2-5 months

8-10 months

1-2 years

1-2 years

SOLUBILITY

High

Low


Fe phosphates (Al-P and Fe-P) at pH levelsbelow about 6.5. There are numerousforms of calcium phosphates in soil,ranging from the very solublemonocalcium phosphate (MCP) to the veryinsoluble fluorapatite (Table 3).

After fertilizing with P in a neutral orhigh pH soil, MCP will form first, followedby the other calcium phosphates in orderfrom high to low solubility. The time foreach mineral to form is highly dependenton temperature; for example, OCPformation has been found to be four-foldfaster at 68o F compared to 50o F. If a soilhas a mixture of the various Ca-P solids,the more soluble forms will dissolve morereadily as phosphate levels in solutiondecrease during the growing season.

Research has found that DCP will keepsoluble P levels at approximately 1.5 mg/Lor ppm (at pH 7.5), whereas TCP willmaintain soluble P levels at about 0.03 mg/L (Lindsay, 1979). Some research hasfound that P limits wheat growth atsoluble P levels below 0.03 mg/L (Havlin etal., 1999); therefore, a soil could have highlevels of TCP, HA, or FA, and still limitplant growth due to lack of available P.Accessing these more insoluble forms of Pthat are at fairly high levels in most MTand WY soils would be advantageous,although this would require eitherlowering Ca or pH levels, which isvery difficult in calcareous soils.Decreasing pH would take largequantities of acid, or acid-producing substances such aselemental S, Fe+3, or manure.

As may be expected, soils withhigher levels of calcium carbonate(lime) will tie up more P due toprecipitation of Ca-phosphates,and thus lower the soil test P(Figure 3). Note that the samelarge addition of P (114 lb P2O5/ac)increased the Olsen soil test resultby about 13 ppm at 1% lime, butonly 8 ppm at 12.5% lime.Therefore, higher amounts of Pfertilizers are needed in soils with

high amounts of lime to offset the effect ofcalcium phosphate precipitation. Limeconcentrations may be especially high insome surface soils that have been erodedor leveled for irrigation purposes,exposing subsurface calcareous horizons.

Al phosphates and Fe phosphates arethe predominant P minerals in soils withpH levels below about 6.5 (Havlin et al.,1999). The solubility of these mineralsdecreases at lower pH, directly opposite ofthe solubility for calcium phosphates.Therefore, P is most available around pH6.5, because at lower pH levels, P

Figure 3. The effect of soil lime concentration onOlsen P test level. Adapted from Westermann(1992).

Figure 4. The effect of soil pH on P retention andavailability. From Havlin et al. (1999).

0 lb P2O5/ac

114 lb P2O5/ac

0 5 10 15Lime %

35

30

25

20

15

10

5

0

Ols

en P

(ppm

)

pH 6.5 for optimumavailability

Insoluble Fe/Alphosphates.Adsoprtion

to oxides and clay

Very High

High

Medium

Low

Insoluble Caphosphates.Adsorptionto CaCo3

EX

TE

NT

OF

RE

TE

NT

ION

3 4 5 6 7 8 9


retention is high due to Al-Pand Fe-P precipitation, and athigher pH levels, Ca-Pminerals precipitate (Figure4). Because most soils inMontana and Wyoming havepH levels closer to 8.0, weexpect P availability to berelatively low. In fact, basedon results from 2.5 millionsoil samples collected in 47states from Fall 2000-Spring2001, Montana had thesecond highest percentage(78%) of soil samples testingeither medium or lower insoil test P (PPI, 2001).Wyoming was 12th with 58%testing medium or lower.Alabama had the highestpercentage (79%), likely dueto low soil pH levels thatcause P to precipitate with Aland Fe. Although Figure 4only focuses on the effect ofpH on P availability, warmthand moisture also increase Pavailability.

In a study looking at thelong term effects (>20 years)of P fertilization andcropping on a dryland wheat-fallow system in Montana,total P in the upper 6 incheswas found to beapproximately 15% higher inthe fertilized soil than in thecontrol, whereas available P(Olsen P) was nearly 50%lower (Jones et al., 2002).This demonstrates that

fertilization can increase total P withoutincreasing available P, likely due toprecipitation of relatively insoluble Ca-Pminerals.

EROSION AND RUNOFF

Soil erosion represents a loss of P fromagricultural fields and can occur fromwater or wind because P is generally boundso tightly to the soil. Although low rainfalland relatively flat topography inagricultural valleys of Montana andWyoming prevent high amounts of erosionfrom water, total annual erosion from mostof Eastern Montana is quite high (8 to 16tons/acre), largely due to wind (USDA,1994). Assuming a typical total Pconcentration of 500 ppm, this equates to18 lb P2O5/ac per year, a substantial loss inthe overall P budget. Some eroded soilfrom upwind or upstream may be depositedto replace a portion of that lost, althoughrarely is the redistribution of eroded soiluniform as much of it is deposited at theedge of fields or in ditches. Some factorscontributing to high amounts of soilerosion include: 1) long slopes in fieldsfarmed without runoff diversions, 2) rowsplanted up and down moderate or steepslopes, 3) inadequate crop residue, 4) lackof buffer strips, 5) poor stands, 6) lack ofwindbreaks, 7) intensive tillage, and/or 8)overirrigation.

Dissolved P in runoff can representanother loss of P from agricultural fields.However, the concentration of dissolved Pin runoff is generally low due to the highamount of sorption and precipitation of Pminerals. One exception to this generalrule is animal manure stockpiles ormanure application sites, where P isconcentrated and sorption sites in themanure and surrounding soil may beapproaching saturation. For example,soluble P has been found to approach 80mg/L (ppm) in runoff from pasturesfertilized with poultry manure (Pierzynskiet al., 2000).

In another study, dairy manureapplications of 100 lb P

2O

5/ac increased P

in runoff by up to six-fold (Sharpley andTunney, 2000). With relatively newenvironmental regulations and guidelinesinvolving Total Maximum Daily Loads(TMDLs) and Nutrient Management Plans

Q&A #2Much more P isapplied to acertain field eachyear than removedby a particularcrop, yet the P testlevels stay low.Where’s the Pgoing?

The added P is likelyprecipitating intoinsoluble P minerals, or isbeing sorbed strongly tothe soil. With large annualapplications of P, the soiltest P should eventuallyincrease over time,although applying the Pfertilizer with the seed willincrease the odds that thecrop can use it. Inaddition, adoptingpractices that increaseorganic matter shouldproduce more mineralizedP, and hence increase soiltest P.


(NMPs), it is important to realize that thegeneral perception that P binds strongly tosoil is true only to a certain soil P level orapplication amount. So, how do we knowwhere this point is without collecting awater sample leaving a field? A largeamount of research has correlated soil testP levels with dissolved P in soil solution,drainage water, or runoff. One of thesestudies found that P in solution began tosignificantly increase only when Olsen Plevels were above 60 ppm (Heckrath et al.,1995). Therefore, maintaining Olsen Plevels between the critical level and thisthreshold should optimize yield withoutlosing substantial quantities of P in runoff.

Preventing surface loss of P fromerosion or runoff can be attained throughno-till, conservation tillage, windbreaks,improved soil fertility, minimal residueburning, winter cover crops, runon/runoffcontrol, and buffer strips, among others.Your local Natural Resource ConservationService (NRCS) office can offer numeroussolutions for decreasing soil erosion andrunoff, and hence loss of P.

LEACHING

Leaching was not shown on the P cycle(Figure 1) because it does not occur veryreadily in most soils. One 20-plus yearstudy in Wisconsin found that P fromcommercial fertilizers had not movedmore than 5 cm below the plow layer, andP from manure applications had movedless than 20 cm below the plow layer(Meyer et al., 2001). The higher movementof P associated with manure is likely dueto the effects that organic materials haveon sorption, as described previously. Insandy soils, commercial fertilizerapplications have been found to increaseavailable P concentrations up to 3 feetbelow the surface, but to have no effect onavailable P at 4.5 feet below the surface(Pierzynski et al., 2000). A three-year studyin Wyoming on a sandy clay loam, foundthat a very high irrigation rate (3 feet perseason) leached P to approximately 4.5 feet

(Peterschmidtet al., 1979).Therefore, Pleaching isprobably onlya concern inMontana andWyoming oncoarse soilsthat are eitherfrequentlyflood-irrigatedor have hadlong-term,high ratemanure applications.

P BUDGET

We have now looked at each of theinputs and each of the outputs to theavailable P pool. A summary of thisavailable P budget is shown in Table 4. Atotal P budget can also be constructed fora specific field based on total P inputs andoutputs. This exercise can be helpful indetermining if soil P levels are increasingor decreasing. Unlike the N budget, wheregains and losses from the atmosphere arevirtually impossible to measure, a Pbudget is fairly easy to construct. The onlymajor total P inputs are fertilizer andanimal manure, and the only major total Poutput is crop removal, which can beestimated from Table 2 or calculateddirectly from tissue P concentration anddry matter yield. Cropland with initiallylow levels of available P will likely showincreased total P levels as P fertilizer isapplied to increase yields. Conversely, insoils with moderate to high levels ofavailable P, the soil may be “mined” of Puntil yields are negatively impacted.Manure-applied fields will almost alwaysshow an increase in total P, due to highconcentrations of P in manure. Anaccumulation of P may not be a problem ifrunoff and erosion from the area areminimal, although very high levels of Phave been found to cause zinc deficiencies

Table 4. Available P gainsand losses in the soil.

GAINS

Fertilizer/Manure

Desorption

Mineralization

Dissolution

LOSSES

Plant uptake

Sorption

Immobilization

Precipitation

Erosion


in some crops (Havlin et al.,1999). By comparing annualP application rates with soil Ptest results (discussed below)for a certain field, the effectof fertilization amounts on Pavailability can bedetermined.

P Soil TestingThe primary goal of soil

testing for P is to determineP fertilizer requirements fora specific crop. Themechanics of soil testinghave been previouslydescribed (NutrientManagement Module 1). Soilsamples for P analysis aregenerally collected from theupper 6 inches because Pfrom fertilizers will stay inthis upper layer due to strongsorption and precipitation. Inaddition, P fertilizerrecommendations aregenerally based on resultsfrom this upper 6 inches.Unlike N, simply measuringdissolved P will not give anadequate estimate of Pavailability. Therefore,extractions have beendeveloped which aredesigned to remove themore readily availablesorbed and mineral Pfractions (Table 5). The

Bray-1 and Mehlich-3 tests weredeveloped for acidic soils, and work bydissolving the more soluble Al-P and Fe-P minerals that control phosphateconcentrations in acidic soils. The OlsenP test was developed for neutral to highpH soils, and relies on the bicarbonateextractant to dissolve the more solubleCa-P minerals and release some sorbed P.

Another promising soil testtechnique is using anion exchangeresins, either encapsulated in a synthetic

mesh or in a probe. The advantage of aresin over an extractant is that the resinbetter imitates a plant root by decreasingsolution P, promoting P desorption anddissolution. Research in Montana on P-responsive calcareous soils has found thatResin-P is more responsive than Olsen-P tochanges in P availability following Pfertilization (Yang et al., 2002). Thedisadvantage is that there has beeninsufficient calibration of the resin Presults against yield to enable a fertilizerrecommendation to be made.

A large amount of research has beenconducted to determine relationshipsbetween P soil test results and relativeyields. The general relationship betweensoil test P levels and yield is shown inFigure 5. Soil test levels are generallybroken into low, medium, or high, andsometimes also into ‘very low’ or ‘very

Q&A #3How do I convertBray-P to Olsen-P?

Some researchers haveused formulas such as:Olsen-P = Bray-P/1.5;however, the constant of1.5 is very dependent onsoil conditions,particularly pH. Forexample, in one study inMontana, it was found thatat pH 8, Olsen-P = Bray-P/0.5, whereas at pH 5.6,Olsen-P = Bray-P/2 (Joneset al., unpub. data).Instead of converting, youshould ask your lab to runOlsen-P if your soil has apH above 7, or usefertilizer-yield responsecurves for Bray-P. Montanafertilizer guidelines basefertilizerrecommendations on a flat‘soil test P’ and make theassumption that high pHsoils are extracted with theOlsen method, and acidicsoils are extracted with theBray method.

Table 5. Selected soil Ptests and extractants.

TEST

Olsen

Bray-1

Mehlich-3

EXTRACTANTS

NaHCO3 at pH 8.5

HCl, NH4F

NH4F, CH3COOH,NH

4Cl/HCl

Critical Level

50 MEDIUM

HIGH

LOWP

erce

ntag

e Y

ield

Soil test P level (ppm)

EnvironmentalThreshold

100

Figure 5. Effect of soil test P levelon crop yield.


high’. At soil test P levels above the‘critical level’ only minimal yield responsescan be expected. The critical level will varywith crop and climate; for example, thecritical level for spring wheat isapproximately 16 ppm (Jackson et al.,1997) and for winter wheat isapproximately 24 ppm (Jackson et al.,1991). In addition, because there are somany factors that affect P availability andyield, fertilizing a soil that has a mediumP test level increases the probability thatthere will a yield response, but does notguarantee a yield response (Figure 6).Conversely, there is still some chance thatfertilizing a soil with a high soil test P levelwill produce a profitable yield increase.Specifically, in some calcareous Montanasoils, significant growth responses to Pfertilization are sometimes observed insmall grains when the Olsen P level isabove the critical level (Jackson et al.,1991; Jackson et al., 1997; Yang andJacobsen, 1990). Therefore, testing yieldresponses to P fertilization on specificfields is recommended to supplement andvalidate soil test results. In addition,starter P with or near the seed will usuallyprovide some faster growth earlier in thespring growing season when soils are cooland wet.

At P levels above the ‘environmentalthreshold’ there may be water qualitydegradation, although there is noagreement on what this threshold shouldbe. The environmental threshold isprimarily a regulatory criteria and variesfrom state to state. It is generally 2 to 4times higher than the critical level. InMontana and Wyoming, there currently isno set threshold, although the federalNRCS Waste Utilization guidelinesrecommend no animal manureapplications if the soil test P level is above150 ppm and manure application ratescannot exceed crop P removal rates if thetest level is above 100 ppm. Idaho uses anenvironmental threshold for Olsen-P of 50ppm on sandy soils and 100 ppm for siltloam soils (Sharpley and Tunney, 2000).

One recognized problem withenvironmental thresholds is that they donot take into account the potential forerosion or runoff from the site, so are nota good predictor of P loss from a field(Sharpley and Tunney, 2000). Due to thisconcern, the USDA has developed a ‘Pindex’ tool that considers erodibility,runoff, soil test P result, P applicationrate, and P application method. The toolrequires verification, but is one possiblemethod for estimating the potential forsurface P loss. For more informationabout determining and using the P indexsee the NRCS website listed in theappendix.

P Fertilizer RecommendationsSoil analytical laboratories often

provide a recommended P applicationamount based on soil P test results.However, producers may alter this amountdepending on fertilizer costs, changes inyield potential, management intensity, ordue to different philosophies (sufficiencyor maintenance/build-see NutrientManagement Module 1). P fertilizerguidelines are useful for determining a P

Figure 6. The effect of soil test level on theprobability of a profitable yield increase fromfertilization. A probability of 0.85 means an 85%chance. From Havlin et al. (1999).

V. LOW LOW MEDIUM HIGH V. HIGH

1.00

0PR

OB

AB

ILIT

Y O

F P

RO

FIT

AB

LE IN

CR

EA

SE

LEVEL OF SOIL FERTILITY

0.85

0.60

0.40

0.15


fertilizer requirement. As an example,fertilizer P recommendations for grass areprovided from the MSU FertilizerGuidelines (EB 104), which uses asufficiency approach (Figure 7). Note thatif the soil test P level is 8 ppm, and thegrass yield goal is 1.5 t/ac, these guidelineswould recommend applying approximately13-14 lb P2O5/ac. By comparison, Premoval amounts for the three grassesshown in Table 2 average approximately 22lb P2O5/ac per year; suggesting that theremaining 8 lb P2O5/ac must be provided bythe soil. If the goal were to maintain or

Calculation Box

CALCULATION: P FERTILIZER TO APPLY = P2O5 RECOMMENDATION/ P2O5 FRACTION IN P FERTILIZER

Example: The fertilizer requirement is 14 lb P2O5/ac. How much MAP (11-52-0) is needed?

Recall that the 52 means that this fertilizer is equivalent to 52% P2O

5. Expressed as a fraction, 52%

=0.52 (52%/100%).

MAP needed = (14 lb/ac)/0.52 = 27 lb/acre

build available P levels, then at least 22 lbP

2O

5/ac per year would be needed. See the

Calculation Box for converting betweenP2O5 and P fertilizer amounts. By plottingannual soil test levels, P fertilizationamounts, and yield, one can begin todetermine how P fertilization amountsaffect both yield and soil test levels for aparticular field. This may prove morefruitful than using published guidelines,because it is specific to your area, cropvariety, and management practices.

Phosphorus FertilizerManagement

Proper management of P fertilizerapplications is key to optimizing yield andprotecting water quality. As mentionedpreviously, practices that increase Pavailability include banding, increasingorganic matter through manureapplications or conservation tillagepractices, and applying fertilizers as closeto peak crop uptake as possible. Someadaptations should also occur with annualversus perennial crops. In addition, it waspointed out that preventing erosion andkeeping soil P levels below environmentalthresholds are the best way to decrease Plosses from fields. The followingsummarizes some additional steps that canbe taken to increase P availability whereneeded, and decrease the potential forsurface losses of P.

Figure 7. P fertilizer recommendations based onyield goal and soil test P level for grass (pasture andtame hay). Based on MSU Extension Bulletin 104.

1 1.5 2 2.5 3

Grass yield goal (t/ac)

35

30

25

20

15

10

5

0

Rec

omm

ende

d P

2O5

(lb/a

c)

4 ppm8 ppm

12 ppm16 ppm

P test


FERTILIZER TYPE

The two most frequently used Pcommercial fertilizers in Montana are MAP(monoammonium phosphate) and DAP(diammonium phosphate). MAP may betemporarily more available in high pHsoils, because it lowers pH, whereas DAPraises the pH. Specifically, in a watersolution, dissolved MAP will result in a pHof 3.5, while DAP will cause a pH of 8.5(Havlin et al., 1999). The acidic pHimmediately surrounding a MAP granuleshould temporarily decrease the amount ofP minerals that form, and may dissolvesome P minerals already present in thesoil. This effect is temporary, asneutralizing reactions will quickly returnthe pH to near a pre-fertilization level.Agronomically speaking, at the same rateof P there should be no difference in cropresponse between MAP and DAP for a cropthat is limited by lack of available P.Nitrification of the ammonium in bothMAP and DAP will also cause a slightdecrease in pH (Nutrient ManagementModule 3); a potential benefit in high pHsoils.

Fertilizing with either MAP or DAP ispreferable to fertilizing withtriplesuperphosphate (0-45-0 or TSP)because there is evidence that NH4

+

increases P uptake (Havlin et al., 1999) andboth MAP and DAP are less expensive.However, when fertilizing legumes, suchas alfalfa, with very high rates of P (150 lbP

2O

5/acre and higher), TSP is preferable

because the legume does not need the N,and the additional N may favor grasses andweeds. Liquid ammonium polyphosphate(10-34-0) is quickly converted tophosphate, and thus will undergo similarsorption and precipitation reactions asother phosphate fertilizers (Havlin et al.,1999). Rock phosphate, which iscomprised of apatite and fluorapatite, isused only infrequently due to the lowsolubility of these two minerals, especiallyat high pH. Organic producers frequentlyuse this form of raw, unprocessed P as anutrient source, but at very high rates and

pulverized into as small of pieces aspossible, due to its low available Pconcentration and solubility.

TIMING/PLACEMENT

P sorption and precipitation arerelatively fast processes, happening inminutes to weeks after P fertilizersdissolve. Therefore, P fertilizer should beapplied as close to the time of maximum Puptake as possible. In addition, P bandingis recommended over broadcastapplications because banding essentiallysaturates the soil with P in a small area,allowing easy access by plant roots.Conversely, if P is broadcast andincorporated, P will come in contact withmuch more soil surface area, leading tohigh levels of P sorption and low levels ofavailable phosphate. Still, in establishedperennial forages, where surface broadcastapplication of P fertilizer is the onlyfeasible application method, fertilizationwith 100 lb P

2O

5/ac resulted in a 35% yield

increase in a Montana soil with a lowavailable P level (Wichman, 2001). Keep inmind that mineral P and sorbed P canbecome available if phosphate levels insolution become low enough to promote Pdissolution or desorption; however, thesecan be slow processes (see NutrientManagement Module 2 for morediscussion).

FERTILIZER SOURCE

The source of rock phosphate used tomake P fertilizers has been found tosomewhat affect P uptake and yield, due todifferent levels of P-containing impuritiesthat form during the manufacture of Pfertilizers (Bartos et al., 1992). Thepercentage of a fertilizer that is water-soluble has also been shown to affect yield,although this is accounted for in the P2O5analysis shown on the bag (e.g. 10-48-0).In addition, there are a few concerns thatdust-control coatings applied to Pfertilizers may affect P solubility; however,recent research has demonstrated that


coatings did not significantly decreaseOlsen P levels, P uptake, or above groundbiomass of corn for two different sources ofMAP (Jones and Jacobsen, 2002).

SOIL MOISTURE

Soils at field capacity dissolveapproximately 50-80% of P fertilizer in 24hours compared to 20-50% for soils at 2-4% moisture (Havlin et al., 1999). Thesedifferences likely do not have much effecton P availability during peak P uptakeperiods because both moisture contentswould result in complete dissolutionwithin a few days. However, moister soilswill better promote P diffusion to plantroots.

APPLICATION RATE

Due to the soil’s high ability to sorband precipitate P, small P applications maynot substantially increase P availability.Instead, larger applications of P may benecessary to saturate soil binding sites, andincrease P availability. For example, 23months after a P banding experiment wasconducted, the soil P test result one inchaway from the center of the band increasedby only 20% for a 45 lb P

2O

5/ac application,

but by almost 100% for a 75 lb P2O5/ac

application (Havlin et al., 1999). Therefore,there may be economic advantages tohigher, less frequent, P fertilizerapplications, although this has not beendocumented in Montana or Wyoming.

MANURE APPLICATIONS

Applying manure to cropland presentsvery different management options fromthose discussed above. If manure is appliedto meet crop N needs, much more P isapplied than is necessary to meet P needs.Therefore, long-term manure applicationsmay increase soil test P levels well abovecritical levels, and possibly aboveenvironmental thresholds. Manure shouldbe incorporated as soon as possible afterapplication to reduce nutrient runoffpotential. Other practices to decrease P inrunoff include applying manure whenthere is no threat of rain, minimizingapplication amounts when possible, andapplying on low slopes, among others.Application rates were found to affect Plevels in runoff much more than initial soiltest levels when 1 inch of precipitation fell2 weeks after manure application (Table 6).

Manure management has become moreof an issue in the last few decades as cropand animal operations have becomeconcentrated in different regions of thecountry. Environmental concerns relatedto confined animal feeding operations(CAFOs) have been more of a problem instates in the Midwest and East Coast.However, there are fields in Montana andWyoming where the NRCS recommendseither no manure application or limitedapplications because of high soil P testlevels. For example, when the soil test Plevel is between 25 and 100 ppm, the NRCSrecommends that P application rates notexceed crop phosphorus needs if the ‘Pindex’ is high or very high. Otherwise,manure can be applied to meet crop Nneeds. In addition, manure applications arealso based on N crop needs if the soil testlevel is below 25 ppm. The NRCS hashardcopy and online worksheets for

Table 6. Effect of manure applicationand soil test P level, on dissolved P(mg/L) in runoff two weeks aftermanure application.

SOIL TEST

P LEVEL

69

237

NO MANURE

0.25

0.65

100 LB

P2O5/AC

1.35

1.40

200 LB

P2O5/AC

2.42

2.45

From Sharpley and Tunney (2000).


determining manure application rates andcomputing a P index if necessary (see WebResources in Appendix for NRCS websiteaddress).

SummaryProper management of P is essential

due to high crop needs, decreasingsupplies of high quality rock phosphates,and increased environmental concerns andregulations. Phosphorus cycling issomewhat simpler than for N, due to lackof a major gaseous phase. The vastmajority of P in soils is unavailable toplants because it is bound in insoluble Pminerals and/or sorbed strongly. Pavailability is measured with one of threemajor soil tests: Olsen, Bray, or Mehlich.Generally, the Olsen test is recommendedat neutral to high pH levels, and the Brayand Mehlich tests are recommended for

acid soils. The goal of sound Pmanagement is to keep the soil test levelnear or above critical levels for maximumcrop yield, yet below environmentalthresholds.

Due to high amounts of Pprecipitation and sorption in soil, it isdifficult to increase the amount of Pavailable for plant uptake. However, Puptake can be increased substantially byapplying high rates of banded P, usingammonium based phosphate fertilizers,and taking steps to increase soil organicmatter. In Montana and Wyoming, naturallevels of available P are low, decreasing thelikelihood for P transport via surfacerunoff or leaching. However, soils infeedlots and from fields that have hadlong-term manure applications may haveelevated levels of available P, andtherefore, these specific cases deserveextra attention to minimize possible Plosses.


ReferencesBartos, J.M., G.L. Mullins, J.C. Williams,

F.J. Sikora, and J.P. Copeland. 1992.Water-insoluble impurity effects onphosphorus availability inmonoammonium phosphatefertilizers. Soil Sci. Soc. Am. J. 56:972-976.

Bauder, J.W., S. Mahmood, B.E. Schaff,D.J. Sieler, J.S. Jacobsen, and E.O.Skogley. 1997. Effect of phosphorussoil test level on sorghum-sudangrassresponse to phosphorus fertilizer.Agron. J. 89:9-16.

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.

Foth, H.D. and B.G. Ellis. 1997. SoilFertility. 2nd Ed. CRC Press. BocaRaton, Florida. 290 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.

Heckrath, G., P.C. Brookes, P.R. Poulton,and K.W.T Goulding. 1995.Phosphorus leaching from soilscontaining different phosphorusconcentrations in the Broadbalkexperiment. J. Environ. Qual. 24:904-910.

Jackson, G.D., G.D. Kushnak, G.R.Carlson, D.M. Wichman, and J.S.Jacobsen. 1991. Correlation of theOlsen phosphorus soil test: Winterwheat response. Comm. Soil Sci. PlantAnal. 22: 907-918.

Jackson, G.D., G.D. Kushnak, G.R.Carlson, and D.M. Wichman. 1997.Correlation of the Olsen phosphorussoil test: Spring wheat response.Comm. Soil Sci. Plant Anal. 28:813-822.

Jones, C.A., and J.S. Jacobsen. 2002.Effects of dust control coatings on Pdissolution and uptake. In A.J.Schlegel (ed.) Proceedings of the GreatPlains Soil Fertility Conference. Vol 9.Denver, Colorado. March 5-6, 2002.www.ppi-ppic.org.

Jones, C.A., J. Jacobsen, and S. Lorbeer.2002. Metal concentrations in threeMontana soils following 20 years offertilization and cropping. Comm. SoilSci. Plant Anal. In press.

Lindsay, W.L. 1979. Chemical Equilibriain Soils. John Wiley & Sons, New York.449 p.

Meyer, J.M., J.W. Lyne, M. Avila-Segura,and P. Barak. 2001. Verticalredistribution of phosphorus: miningnew phosphorus data from old fertilityplots. Abstract. Soil Science Society ofAmerica Proceedings. ASA-CSSA-SSSAAnnual Meetings - October 21 - 25,2001. Charlotte, NC.

Peterschmidt, N.A., R.H. Delaney, andM.C. Greene. 1979. Effects ofoverirrigation on growth and qualityof alfalfa. Ag. Jour. 71: 752-754.

Pierzynski, G.M., J.T. Sims, and G.F.Vance. 2000. Soils and EnvironmentalQuality. CRC Press. Boca Raton,Florida. 459 p.

PPI. 2001. Soil test levels in NorthAmerica. PPI/PPIC/FAR TechnicalBulletin 2001-1. www.ppi-ppic.org.

Sharpley, A., and H. Tunney. 2000.Phosphorus research strategies tomeet agricultural and environmentalchallenges in the 21st century. J.Environ. Qual. 29:176-181.

St. Jean, R. 1997. On-farm manurecomposting techniques-understandingnitrogen and carbon conservation.Prepared for Agriculture and Agri-Food Canada, Research Branch,COESA Report No. RES/MAN-003/97.http://res2.agr.ca/london/gpres/download/rep1_3.pdf

USDA. 1994. National ResourceInventory. United States Departmentof Agriculture. Washington. D.C.

Westermann, D.T. 1992. Lime effects onphosphorus availability in a calcareoussoil. Soil Sci. Soc. Am. J. 56:489-494.

Wichman, D. 2001. Fertilizer use ondryland perennial forages. FertilizerFact Sheet 27. Montana StateUniversity Extension Service andAgricultural Experiment Station.Bozeman, MT.

Yang, J.E., and J.S. Jacobsen. 1990. Soilinorganic phosphorus fractions andtheir uptake relationships incalcareous soils. Soil Sci. Soc. Am. J.54:1666-1669.

Yang, J.E., C.A. Jones, J.J. Kim, and J.S.Jacobsen. 2002. Soil inorganic Pfractions and Olsen-P in P-responsivecalcareous soils: Effects of fertilizeramount and incubation time. Comm.Soil Sci. Plant Anal. In press.


AcknowledgmentsWe would like to extend

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

Paul Dixon, YellowstoneCounty Extension Office,Billings, Montana

Jeff Farkell, Centrol Inc.,Brady, Montana

Rick Fasching, NRCS,Bozeman, Montana

Grant Jackson, WesternTriangle AgriculturalResearch Center, Conrad,Montana

Dave Philips, FergusCounty Extension Office,Lewistown, Montana

Suzi Taylor, MSUCommunicationsServices, Bozeman,Montana

OTHER RESOURCES

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), Free

Online at: http://www.montana.edu/wwwpb/pubs/mteb104.pdf

or, obtain the publication from MSUExtension Publications (add $1 forshipping):

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

See Web Resources below for onlineordering information.

PERSONNEL

Engel, Rick. Associate Professor.Montana State University, Bozeman.(406) 994-5295. [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.inform.umd.edu/EdRes/Topic/AgrEnv/ndd/agronomy/PHOSPHORUS_FERTILIZERS.html

Tables of common fertilizers and theamount of phosphorus in them.Source: Pennsylvania State University.

http://www.agricola.umn.edu/anpl3010/3010phosphorus.htm

This site contains general informationregarding the phosphorus cycleincluding an easy to follow diagram.Source: University of Minnesota.

http://www.mt.nrcs.usda.gov/pas/nutrient/nutrient.html

Website for preparing nutrientmanagement plans. Source: NRCS

http://www.ext.nodak.edu/extpubs/plantsci/soilfert/sf882w.htm

Fertilizer recommendation withdifferent soil test results for severalcrops. Source: NDSU

http://www.montana.edu/wwwpub/pubscatalog.html

Montana State University Publicationsordering information for extensionmaterials

http://scarab.msu.montana.edu/AgNotes/AgNotes.exe?

MSU weekly Agronomy Notes by Dr.Jim Bauder on 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.

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 © 2002 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].

phosphorus cycling, testing and fertilizer recommendations

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