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Nutrient Management Module No. 6

SecondaryMacronutrients: Cycling, Testingand Fertilizer Recommendationsby Nathan Korb, Soil Scientist, Clain Jones, Soil Chemist,and Jeff Jacobsen, Extension Soil Scientist

IntroductionThis module is the sixth in a series of Extension materials designed

to provide Extension agents, Certified Crop Advisers (CCAs),consultants, and producers with pertinent information on nutrientmanagement issues. To make the learning ‘active’, and to possiblyprovide credits to Certified Crop Advisers, a quiz accompanies thismodule. In addition, realizing that there are many other goodinformation sources including previously developed Extensionmaterials, books, web sites, and professionals in the field, we haveprovided a list of additional resources and contacts for those wantingmore in-depth information about sulfur, calcium, and magnesium.

ObjectivesAfter reading this module, the reader should:1. Understand the major processes that determine the availability of

the secondary nutrients, sulfur, calcium, and magnesium, in thesoil

2. Know the factors that affect each of these nutrient cyclingprocesses

3. Recognize how different crops and cropping systems affect nutrientavailability

4. Understand how to calculate S fertilizer requirements

6 CCACCACCACCACCA2 NM2 NM2 NM2 NM2 NMCEUCEUCEUCEUCEU

Nutrient Management

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

4449-6July 2002

2 Module 6: Secondary Macronutrients: Cycling, Testing and Fertilizer Recommendations

BackgroundSulfur (S), calcium (Ca), and magnesium (Mg)

are considered secondary macronutrientsbecause they are less commonly yield-limiting than the primary macronutrients(N, P, and K), yet are required by crops inrelatively large amounts. Although mostsoils in Montana and Wyoming containadequate secondary nutrients for cropproduction, S deficiencies are on the riseboth in the region and throughout theworld (Rasmussen and Kresge, 1986).Three global trends are responsible forincreasing S deficiencies:

1) the shift in modern fertilizers tomore concentrated, higher-analysis formscontaining little to no S (a historical by-product of the manufacturing process);

2) the reduction of sulfur dioxideemissions from burning coal and oil,which decreases atmospheric S additions;and

3) the steady increase in crop S uptakeand removal due to high-yielding varietiesand more productive management.Effective management of secondarynutrients requires an understanding ofthe processes that determine theiravailability to crops and the methods tomanage soils with inadequate secondarynutrient levels. Because S is a greaterconcern for most producers in Montana andWyoming than Ca and Mg, it will be discussedfirst and in greater detail in this module.

Table 1. Description of each S form.

SULFUR FORM MOLECULAR FORMULA NOTES

Sulfate SO4-2 Plant available form, anion found in solution and weakly sorbed to soil

Sulfides S-2 Reduced S, common in saturated soils

Elemental S S0 Uncommon in significant amounts, oxidizes to plant available S

Mineral S CaSO4, FeS

2Can be a source or sink (loss) of S in soil

Organic S Organic S Typically the largest S reserve, slowly supplies S to soil solution

Atmospheric S SO2, H

2S, —COS Oxidized to sulfate in soil and plants

Volatile S Organic S Microbial volatilization releases S from soil

Reduction

OxidationS0/S2-

SoilSolution

SO42-

Plant Uptake

AtmosphereDeposition

OrganicS

Weathering/Dissolution

Precipitation

Leaching

Fe/Al oxides

Sorption

Desorption

S minerals

Volatilization

Mineralization

Imm

obilization

H2SSO2

Figure 1. Sulfur Cycle.

3Module 6: Secondary Macronutrients: Cycling, Testing and Fertilizer Recommendations

Sulfur CyclingSulfur undergoes numerous

transformations in the soil, involvingbiological, chemical, and atmosphericprocesses, similar to the nitrogen cycle(NM Module 3, Nitrogen Cycling, Testingand Fertilizer Recommendations). Themajor forms of S are listed in Table 1. Onlya small fraction of the total S in the soil isreadily available to crops. The majorprocesses governing S availability in thesoil are plant uptake, mineralization,immobilization, exchange, volatilization,precipitation, oxidation, reduction,mineral weathering, and leaching (Figure1). Soil properties such as water content,pH, temperature, and aeration influencethese processes and consequently affectthe amount of S available to satisfy croprequirements. Management of S isimproved by an understanding of how thisnutrient cycles in the soil and under whatconditions deficiencies are likely to occur.

Table 2. S removal amountsin harvested portions ofselected agricultural crops.

CROP ASSUMED YIELD S REMOVALPER ACRE (LB/AC)

Alfalfa 2.5 t 13

Barley 50 bu 10

Brome 1.5 t 6

Canola 2t 8

Corn silage 20 t 19

Oats 60 bu 12

Orchard grass 1.5 t 9

Potatoes 300 cwt. 14

Sugar Beets 25 t 38

Timothy 1.5 t 6

Wheat 40 bu 10

From Jones, 1998.

S Plant UptakePlants require significant

S, usually amountscomparable to P, but less thanN or K (Havlin et al. 1999).Most crops absorb between 10-100 lb S/ac from the soil in theform of sulfate (SO4

-2),although only 10-40 lb S/ac areremoved from the system withharvest (Table 2). Sulfate is ananion and exists primarily insoil solution because thenegative charge of the soiloffers few positively chargedsorption sites for S to occupy.Crops can uptake adequateamounts of S in soils withsoluble SO4

-2 concentrationsabove the critical level of 5 –10 ppm in the top six inches ofsoil via mass flow (see NMModule 2, Plant Nutrition andSoil Fertility). Soluble Susually represents less than10% of total S in the soil and isreplenished by mineralization of organicmatter, weathering of mineral S, oxidationof reduced S, and desorption.

Q&A #1How do modern,high-analysisfertilizers affect Sinputs?For a time, many N,P, andK fertilizers included S asa manufacturing by-product. The result wasthat some S was added tothe soil every year with N,P, and K. Since then,higher analysis N andother fertilizers haveevolved which are morerefined and contain littleto no S, such as urea andammonium nitrate.

Figure 2. Effect of varying levels of S on thedistribution of N in tops of wheat plants (Modifiedfrom Steward and Porter, 1969).

100%

80%

60%

40%

20%

S Level (ppm)

% o

f tot

al N

0%

0 10 20

Nitrate-NAmide-NAmino Acid-NProtein-N


Plants require S as aconstituent of three aminoacids which are essential toprotein synthesis andrepresent approximately90% of the S content inplants. S is also necessary inthe formation ofchlorophyll, vitamins,enzymes, and aromatic oils,which give crops such asmustards their flavor andodor. Efficient N utilizationrequires adequate S becauseboth are needed to formproteins in the plant.Insufficient S inhibitsprotein synthesis, causing Nto accumulate in the plantas nitrate, amides, andamino acids rather thanprotein (Figure 2). A plantN:S ratio of about 15:1 isideal for effective proteinsynthesis. Ratios higherthan this suggest thatprotein synthesis is beinglimited by lackof S, whilesmaller ratiosimply surplus S,which isretained as

soluble S within the plant(Stewart and Porter, 1969). Inthis way, N and S interactionsare positively related andshould be managed together.No response to S additions willoccur if N is limiting plantgrowth. Similarly, an optimal Slevel maximizes the effect of Nfertilization on yield (Figure 3),although actual N uptake inthe plant does not change(Rasmussen et al., 1975).

Research has shown that Splays an important role in cropqualities such as the breadmaking quality of wheat and

Q&A #2What is ‘efficientuse of N’ in aplant?Plants require N and S as‘building blocks’ foramino acids and proteins.If S is in short supply,protein synthesis slowsand N cannot be used,even if it is in abundantsupply. The N thenaccumulates in the plantin ‘building block’ forms(nitrates, amides, etc.)and plant needs are notsatisfied. In croppingsystems where Nadditions seem to havelittle effect, S may beinhibiting N utilizationand therefore Sfertilization can releasecrop potential.

3600

2700

1800

900

0 lbs/ac

40 lbs/ac

20 lbs/ac

0

See

d yi

eld,

lb/a

c

Available N level (lb N/ac)0 50 100 150 200 250 300

Figure 3. Increased effect of N fertilizer withS additions in dryland canola seed yield(Modified from Jackson, 2000).

the protein content of forages and grains.Breadmaking varieties of wheat haveapproximately 10% more S in grain thannon-breadmaking varieties, although totalplant S uptake is similar (Rendig, 1986).Two important factors related tobreadmaking are loaf volume and doughextensibility. Both of these factors aredirectly related to S concentration in grain,which in turn is dependent upon availableS in the soil (Unger et al., 2002). As aconstituent of amino acids, sufficient S isessential for high protein content inforages and grains.

Sulfur dioxide (SO2) and hydrogensulfide (H2S) from the atmosphere can alsorepresent significant sources of S for plantsin locations down-wind from coal-burningplants, metal smelters, geo-thermal areas,and urban areas. These compounds arebeneficial only in low concentrations andare absorbed by plant leaves throughstomata. Atmospheric additions to cropsand soils range significantly, but areusually between 3 to 11 lb S/ac (Foth andEllis, 1997).


Crop residues contain considerableamounts of S because this nutrient isincorporated into the plant material and isnot easily released. As is the case withother nutrients, the plant materialremoved at harvest can represent asignificant loss of S from the soil. In foragecrops, such as alfalfa with its high Suptake, most of the biomass is removed,resulting in a major loss of S. On the otherhand, in canola, only about 15% of thetotal plant S is removed with the harvestedseeds, so most of the S remains in theresidue (Jackson, 2000). Grazing foragesonsite can significantly reduce S lossesbecause livestock release most S back tothe soil. In fact, as much as 90% of Sconsumed by sheep is released back to thesoil (Tisdale et al., 1986).

Mineralization andImmobilization

Organic S compounds held in plant,animal, and microbial residues collect inthe soil organic matter (OM) and representthe largest S pool in well-developedgrassland soils (mollisols), whichpredominate much of Montana’s andWyoming’s cultivated agricultural lands(Figure 4). Over 90% of the total soil S inthis region exists in the organic form(Havlin et al., 1999), except in soils wheremineral S accumulations of gypsum aresignificant. Microorganisms decomposethe OM and release plant available Sthrough the process of mineralization,which occurs similarly in N cycling (seeNM Module 3). About 1 to 3% of organic Sis mineralized each year, contributing 4-13lb/ac of soluble S (SO

4-2) for plant use

annually. The amount of S made availableto plants annually via mineralizationdepends on the amount of OM in the soiland the concentration of S in OM;therefore, taking steps to maintain orincrease soil OM (with no-till, minimumtill, or organic additions) can help supply arelatively constant amount of available Sto the soil.

Immobilization refers tothe process of convertinginorganic S (SO4

-2) toorganic S, and is essentiallythe reverse ofmineralization.Microorganisms removeavailable S from solutionand convert it into proteinsand other organiccompounds. Although thisprocess removes S from theavailable pool, the S is stillin a reserve pool that couldeventually become availableto plants via mineralization.

Since mineralizationand immobilization areprimarily biologicalprocesses, any factorsaffecting microbial growthwill influence both of theseS transformations.Important factors includesoil temperature, watercontent, pH, C:S ratio,aeration, and residuecomposition. The highestmineralization andimmobilization rates willoccur under aerated, warm,and moist conditions withnear neutral pH levelsbecause these conditionsare optimal for microbialactivity.

The N:S ratio is relatively stable inOM, remaining near 8:1 (Foth and Ellis,1997), but the C:S ratio is more varied,and strongly affects the relative amountsof mineralization and immobilization. Ifresidues and organic matter lack sufficientamounts of S, microbes will pull theneeded S from the soil, and immobilize itin organics, as is the case when a residuelike sawdust is added to soil. Residues withC:S ratios higher than about 400:1 resultin net immobilization of S from the soil,while C:S ratios lower than 200:1 result innet mineralization of S into the soil

Figure 4. Typicaldistribution of organic andinorganic S in a mollisol(Brady, 1999).

0

20

40

600 100 200 300 400

Soil sulfur (ppm)

Mollisols

Inorganic S

Organic S

Soi

l dep

th (

in.)


(Freney, 1986). Between ratios of 200:1 and400:1, a combination of the two processesoccurs. The C:S ratios of a number oforganic residues are listed in Table 3. Eachprogressive decomposition cycle of soilresidues results in C expelled as carbondioxide, thereby lowering the C:S ratio. Inthis way, even residues with high C:S ratioscan eventually supply the soil withavailable S.

Sorption, Precipitation, andWeathering

Inorganic S occurs in solid phases inthe soil as sorbed S or S minerals. Sorptionof SO

4-2 increases as anion exchange

capacity (AEC) of the soil increases (seeNM Module 2). Soils dominated bypositively charged Fe/Al oxide clays willhave a high AEC and therefore sorbsignificant sulfate. In this region, soilstypically have low AEC because of theirstrong net negative charge and high pHlevels, so sorption of sulfate is minimal. Asa general rule, SO

4-2 sorption is

insignificant above pH 6.5.Inorganic S minerals, such as gypsum

(CaSO4), represent an important S ‘pool’ in

Montana and Wyoming soils. Commonly,gypsum accumulates in the subsoil,forming a S-rich layer often in closeproximity to calcium carbonate (CaCO3)

layers. Although the surface layers of thesesoils may have low levels of plant availableS, the deeper mineral S maintains anadequate supply of S within the rootingdepth of most crops (Beaton and Soper,1986). Early in the growing season, subsoilS may not be available to crops because itis out of reach of the growing roots. Laterin the growing season, when soil water isusually moving upward through the profileand plant roots have grown deeper,adequate S is generally accessible to crops.

S occurs in numerous primary andsecondary minerals, which release eithersulfate or sulfide (S-2) as they weather.Gypsum is widely distributed in arid andsemi-arid soils where precipitation is toolow to leach the mineral out of the profile.Shales and claystones in the region containgypsum and release plant available S asthey weather (Veseth and Montagne, 1980).Ore deposits in Montana and Wyomingoften contain metal sulfides, such as iron,zinc, and manganese sulfides. As thesulfides (S-2) are exposed to oxygen andwater they oxidize to sulfates and cansignificantly lower the pH of thesurrounding soil or water, as is the casewith acid mine tailings. In agriculturalsettings, the slow release of sulfideminerals into the soil can be beneficial, butexcess acidity may be a concern. ElementalS (S0) also oxidizes to sulfate in thepresence of water and air and is commonlyapplied to high pH soils both as an Sfertilizer and as a means of lowering the pHfor plant growth. The resulting acidity isgenerally short lived in well-buffered soilsand requires continued application to bemaintained. Gypsum added to soil asfertilizer will neither raise nor lower thepH of the soil because the S is alreadycompletely oxidized in the form of sulfate.Sulfides and elemental sulfur are found inwaterlogged soils and wetlands, andgenerally exist in low concentrations inmost agricultural soils.

Table 3. C:S ratio of various residues.

ORGANIC MATERIAL C:S RATIO

Municipal wastewater 20:1

Soil organic matter ~100:1

Horse manure 494:1

Poultry manure 518:1

Cow manure 719:1

Corn stalk 2029:1

Sawdust 11011:1

From Tabatabai and Chae, 1991


S LossesS is lost from the soil by two processes

other than crop uptake: leaching andvolatilization. Leaching is the physicalremoval of plant available S by watermoving through the profile, whereasvolatilization represents a biological orchemical transformation and subsequentrelease of S into the atmosphere. The areaswith the highest risk for sulfate leachingare associated with high precipitation,irrigated conditions, coarse textured,shallow soils, and soils with low AEC.Irrigation following SO4

-2 fertilizerapplication can move SO

4-2 through a

profile and eventually out of reach of plantroots (Figure 5). Sulfate leachingrepresents an economic loss, because oncethe SO4

-2 has left the soil system, it is nolonger available for crop uptake. In semi-arid climates, sulfate often collects in thesubsoil, as described earlier, because thereis insufficient water to flush the anionbelow the rooting zone of plants. Leachinglosses are generally less in high pH soilsbecause sulfate will precipitate with Ca orMg. It should be noted that in some soils,water moving upward through the soilprofile via capillary action late in thegrowing season can carry S towards theplant roots and the soil surface. In suchinstances, excess Ca and Mg sulfate saltscan accumulate to harmful levels in thesurface and limit plant growth. Carefulwater management, especially inirrigated cropping systems canprevent many of the problemsassociated with leaching of S or theover-accumulation of S salts.

Volatilization of sulfurcompounds represents a relativelysmall loss in most agriculturalsoils. Generally, the amount of Svolatilized from the soil is less than0.05% of total S in the soil, andfrom live plants the release isbetween 0.03% and 6% of total S inthe plant (Havlin et al., 1999). Mostvolatilization in the soil occursduring biological decomposition of

organic residues. Biological volatilizationcan occur under both aerobic andanaerobic conditions, but is lesssignificant in well-drained, well-aeratedsoils (Tisdale et al., 1986).

Probably the most significant volatileloss of S occurs when agricultural areasare subject to fire. Although sulfateconcentrations may increase due tochemical mineralization (burning) oforganic S, burning dry grass has resultedin losses of 75% of total S in vegetative

0 10 20 30 40 50 60 70

0

2

4

6

8

10

12

14

16

18

Dep

th in

soi

l (in

.)

Distribution of S (%)

1" water

8" water

Figure 5. Distribution of S added to the soil surfaceas a function of irrigation amounts (Modified fromHavlin et al., 1999).

Table 4. Sufficiency range for selected crops.

CROP %S PLANT N:SAlfalfa 0.25-0.50 12:1

Barley 0.15-0.40 16:1

Canola 0.50-0.90 11:1

Corn 0.25-0.80 12:1

Grass 0.17-0.30 14:1

Wheat 0.15-0.40 16:1

Jones, 1998; Grant and Bailey, 1993; Beaton and Soper, 1986.


cover (Tisdale et al., 1986). The conversionof organic S to inorganic S by burning mayalso make the nutrient more susceptible toleaching.

Sampling for SSampling soil, plant, or irrigation

water for S can be useful in determining Sfertilizer needs. Each of these is discussedbelow.

The primary goal of soil testing for S isto determine S fertilizer requirements for aspecific crop. The mechanics of soil testingand sampling have been previouslydescribed (NM Module 1, Soil Samplingand Laboratory Selection). Sampling for S

has been a subject of debate because thestandard soil test for extractable SO

4-S has

proven somewhat unreliable in predictingyield responses in Montana and Wyoming(Gavlak, 1982; Jackson, 2000). Although ahigh test level (SO4-S >5-10 ppm) in thetop 6 inches of soil will nearly guaranteeadequate S, soils with low and moderate Stest levels do not always respond to Sadditions. A low testing soil may stillsupply a crop with adequate S because ofample S below the testing depth (i.e., agypsum layer), significant organic Smineralized during the growing season, orhigh S levels in shallow groundwaterespecially for deep-rooting crops such asalfalfa. Other soil tests for S includemeasuring the organic S content toestimate mineralization during thegrowing season and measuring the soil N:Sratio. Soil S tests help determinepotentially deficient soils, but plant tissuetests have proven more effective inidentifying actual S deficiencies (Spencerand Freney, 1980).

Plant tissue testing is used to measurethe amount of nutrients in the plant.Tissue tests can analyze the total percent Sin dry matter or the plant N:S. Total %Slevels in crops and plant parts vary andsufficient ranges are listed in Table 4.Results below these sufficient rangessuggest deficiency and the need for Sadditions. The plant N:S ratio is commonlyused as an indicator of S status in crops(Table 3). For example, the critical ratio for

Calculation Box 1Calculation: S supplied by irrigation (lb/ac) = Irrigation depth (ft.) x SO4-S (ppm) x 2.7 (The 2.7 factor is because there are 2.7 million pounds in 1 acre-ft of water)

Example: The irrigation water contains 16 ppm SO4-S. One foot of irrigation water isapplied per year (i.e. 1 acre-ft/acre).How much S is supplied to the crop?

S supplied = 1 x 16 x 2.7 = 43 lb/ac

Deficiency Zone

Critical Level

Plant N:S Ratio

5 10 15 20 25 30

90

80

70

100

Per

cent

Yie

ld (

%)

Figure 6. Relationship between relative yield ofwheat and plant N:S ratio (modified from Spencerand Freney, 1980).


wheat is about 16:1, so, assuming adequatenitrogen levels in the soil, ratios greaterthan 16:1 represent an S deficiency in theplant (Figure 6). Tissue tests requireadditional laboratory procedures beyondthe standard soil tests, but they are usefulif S deficiency is a likely concern.Unfortunately, deficiencies indicated bytissue tests taken during the growingseason cannot easily be corrected until thefollowing year. By plotting annual soil ortissue test levels, S fertilization amounts,and crop yield, one can begin to determinehow S fertilization amounts affect yieldand soil test levels for a particular field.This may prove more fruitful than usingpublished guidelines, because it is specificto your region, crop variety, andmanagement practices.

Collecting irrigation water for analysisof SO4-S is useful for adjusting S fertilizerrequirements. For example, the MissouriRiver near Toston contains approximately16 ppm SO4-S. Applying 1 foot of thiswater to a field will supply 43 lb S/ac(Calculation Box 1), which should meetany crop’s S needs based on Table 2. Thiscalculation is based on a high SO4-Sconcentration; rarely will the entire crop Sneeds be met from irrigation water alone.

S Fertilizer RecommendationsAlthough S has not historically been

a nutrient of concern in most croppingsystems in Montana and Wyoming,there are increasing reports andresearch results indicating significantresponses to S fertilizer additions forcrop yield and quality. Responses to Soccur most commonly in crops withhigh S requirements such as alfalfa andcanola, with crops where most of theplant material is removed, in sandysoils, and in soils with less than 5-10ppm SO

4-S in the top six inches. S

increases yield when N levels areadequate, and the two nutrients areclosely related in fertilizer response.Unlike N-P-K, accepted fertilizer

guidelines for S do notexist in Montana andWyoming. Fertilizerrecommendations forcrops in this region canbe determined based onpast responses, soil andtissue tests, and cropuptake.

Because foragescontain high amounts ofS and the plant materialis removed from thefield, the ability of thesoil to maintainadequate S is decreasedand S additions are oftenprofitable. In one studyon dryland perennialforages in Montana,additions of 25 lb S/acincreased yield in one ofthree sites andsignificantly increasedthe protein content atall three sites (Figure 7).These sites all had soil SO4-S test levelsbetween 3 and 4 ppm, which suggestsborderline S deficiency. In a differentstudy, orchardgrass forage yields increased235% on irrigated alluvial soils insouthwestern Montana with additions of26 lb S/ac (Rasmussen and Kresge, 1986).

17.8

17.6

17.4

17.2

17.0

16.8

16.6

16.4

16.2

16.0Geyser Moccasin

Location

Moore

Without SulfurWith Sulfur

Pro

tein

con

tent

(%)

Figure 7. Protein content response to fertilizer indryland perennial forages (modified from Wichman,2001).

Q&A #3How does irrigationaffect S management?

Irrigation water often containsconsiderable dissolved S, and,therefore, represents a regularaddition to some soils. In somesystems, irrigation water alone issufficient in maintaining soil Slevels. Irrigation water can betested for sulfate to estimate Sadditions. Conversely, irrigationcan significantly accelerateleaching in the soil and removeplant available S. Maintainingsoil water at field capacity andavoiding over-irrigation canreduce S loss.


In Alberta, forage crop yield increased 25%to 200% following additions of 18 lb S/ac(Beaton and Soper, 1986). For most soils inMontana and Wyoming, additions of 20 lbS/ac for alfalfa-grass forage crops arerecommended if SO4-S soil test levels areless than 5 ppm in the top 6 inches(Rasmussen and Kresge, 1986). Inextremely deficient, non-irrigated soils,rates as high as 50 lb S/ac are necessary tosatisfy crop requirements.

Canola requires high amounts of S, butbecause less S is removed in the harvestedportion than in forage crops, canolagenerally requires smaller yearly additions.Soils with less than 5 ppm SO4-S should befertilized with 15 lb S/ac with an optimalN-P-K blend (Beaton and Soper, 1986;Jackson, 2000). Adding S to meet cropneeds increases both the seed yield and theoil in the seed for canola (Beaton andSoper, 1986). Carefully maintaining theavailable S pool in canola operations isimportant to preventing yield reductionsbecause this crop can be severely affectedby S deficiency. Raising available S inAlberta soils showed increases from 28% toover 600% for canola (Beaton and Soper,1986).

Wheat and other small grains userelatively less S and may be better suitedfor S deficient soils. Despite their low Srequirements, wheat, barley, and oat crops

have responded significantly to S additionsin certain locations in Montana and Albertawith yield increases ranging from 10 to40% (Rasmussen and Kresge, 1986). Thecritical soil test level for wheat is only 3ppm SO4-S and fertilizer recommendationsfor deficient areas range between 10 and 15lb S/ac.

There are numerous forms of Sfertilizers available to producers, manywith very different rates of release. Themost common S fertilizers used inMontana and Wyoming are listed in Table5. The major factors in choosing an Sfertilizer are the S content of the fertilizer,the availability of the fertilizer to crops, theacidifying effect of the material, and thecost. Ammonium sulfate, ammoniumphosphate sulfate, gypsum, and epsom saltare the most commonly used S sourcesbecause they quickly release sulfate forplant use. Of these fertilizers, ammoniumsulfate contains the greatest percent S byweight (24%).

Elemental S (S0), on the other hand,must be oxidized to sulfate before plantscan utilize it. The rate of this processdepends on particle size, incorporation,and the soil conditions discussed earlier,and can be very slow. Dispersible, granularelemental S fertilizers are broadcast toincrease surface area and exposure of S,and thereby accelerate oxidation. Even with

improved distribution of S,this form of S must be appliedwell before the growing seasonif it is expected to supply thecrop with S; otherwise somereadily available S should beincluded. The other significantfactor in applying S0 is theacidifying affect it may have onthe soil. Most soils in Montanaand Wyoming are stronglybuffered at high pH levels andshould not be adverselyaffected by this effect, butsandy soils may be moresusceptible to acidification

Table 5. Commonly used forms of S fertilizers.

FERTILIZER SOURCE FORMULA ANALYSIS

Ammonium sulfate (NH4)

2SO

421-0-0-24

Ammonium phosphate sulfate NH4H

2PO

4 • (NH

4)

2SO

416-20-0-14

Ammonium thiosulfate (ATS) (NH4)

2S

2O

312-0-0-26

Gypsum CaSO4

0-0-0-19

Epsom salt MgSO4

0-0-0-13

Elemental S S0 0-0-0-100

Dispersible, granular S0 S0 + bentonite 0-0-0-90


with continued S0 applications. Usingelemental S generally requires moreplanning than other S fertilizers due to itsslow availability, but an advantage of thissource is its high analysis for S (90-100%).

Calcium and MagnesiumCycling

Ca and Mg occur in the soil as soluble‘divalent’ (‘double-charged’) cations (Ca+2

and Mg+2), on cation exchange sites, and inmineral forms. The major processes in theCa/Mg cycle are plant uptake, exchange,precipitation, weathering, and leaching(Figure 8). Ca/Mg dynamics in the soil arequite similar to K (NM Module 5). Like K,plants absorb the soluble ionic form fromsoil solution, which is then replenished byexchangeable and mineral Ca/Mg. Themost notable difference between thesenutrient cycles is the absence of Ca/Mgclay fixation.

CALCIUM

Ca plays an important role in cellelongation and maintaining membrane

structure in plants. The presence of Ca inroots also regulates plant cation uptake bylimiting excessive sodium (Na) andincreasing beneficial K absorption. Mostsoils, especially those in Montana andWyoming, contain abundant Ca insolution (30-300 ppm) relative to mostcrop requirements (~15 ppm). The supply

Figure 8. Calcium and Magnesium Cycle.

Soil SolutionCa2+/Mg2+

SorptionDesorption

EROSION

Clay

Weathering

Plantuptake

Mg2+

Plantrelease

Ca/Mgminerals

Precipitation

Dissolution

Ca2+

Ca2+

Calculation Box 2Calculation: S fertilizer to apply = S Recommendation/S fraction in fertilizer

Example: The fertilizer recommendation is 20 lb S /ac.How much ammonium sulfate (21-0-0-24) is needed?

Recall that the 24 means that 24% of this fertilizer by weight is S. Expressed as a fraction,24% = 0.24.

Amount of (NH4)2SO4 needed = (20 lb S /ac)/0.24 = 83 lb/acre ammonium sulfateFor comparison, how much dispersible, granular S0 would be needed?

If 83 lb/ac of ammonium sulfate were applied, how much N is being added to the soil?83 lb/ac (.21) = 17 lb N /ac, which could be subtracted from the total N recommendation.


of Ca in soil solution can be ten timeslarger than K and plants require much lessCa, so deficiencies are rare. Mass flowsupplies adequate Ca to plant roots, exceptin low Ca soils, where diffusion becomes animportant process.

Ca is usually the dominant base cationin exchange reactions, accounting formore than 70% of base saturation. Basesaturation represents the percentage of theCEC occupied by base cations (Ca, Mg, K,and Na) and generally increases with pH.Exchangeable Ca exists in equilibrium withthe soil solution, replenishing soluble Calosses by plant uptake or leaching.Leaching can be significant in coarse-textured, acidic soils where substantialwater moves through the profile. In manyMontana and Wyoming calcareous soils,some Ca leaches out of the more acidic,organic-rich topsoil and precipitates in aCa-rich ‘calcic’ horizon in the sub-soil inthe form of calcium carbonate (CaCO

3) or

gypsum (CaSO4). In addition to thedissolution of these secondary deposits, Cais also released through the weathering ofprimary minerals such as feldspars, micas,and limestone; all of which are commonthroughout this region.

Low Ca content in soil often causesacidity problems before actual Ca nutrientdeficiency becomes an issue. Both the roleof Ca as a base cation and its frequentoccurrence with carbonates (CO3

-2) andbicarbonates (HCO

3-) buffer soil at high pH

levels. Where acidity is a problem, limingsoils with CaCO3, CaO, or Ca(OH)2 is acommon practice. The role of pH innutrient cycling and how it can bemanaged will be discussed in depth in alater NM Module.

Because of its strong divalent charge,Ca acts as an ionic ‘glue’, attracting clayparticles and promoting aggregationthrough a process called ‘flocculation’.Soils with high levels of sodium (Na),referred to as ‘sodic’ soils, promote‘dispersion’ which is the opposite offlocculation. When monovalent (‘single-

charged’) Na ions dominate the claysurfaces in the soil, the weak positivecharge of the ion is not strong enough toovercome the negative charges of clayparticles, which then repel each other. Theresult of dispersion is a structureless, gel-like soil with insufficient aeration,permeability, and water-holding capacityfor optimum plant growth. Additions of Cain the form of gypsum are frequentlyprescribed for reclaiming sodic soilsbecause it counters the effects of Na andpromotes the aggregation critical for soilproductivity. Gypsum is preferable toCaCO3 because gypsum is more soluble.Low Na irrigation water or rainwater canthen be used to leach Na out of the soilprofile.

MAGNESIUM

Mg plays a critical role in nearly allparts of plant metabolism and proteinsynthesis, and is an essential constituent ofchlorophyll. Plants require less Mg thanCa, but deficiencies are more commonbecause less Mg exists in the soil solution.Mineral forms of Mg are relatively resistantto weathering and represent a largefraction of total soil Mg. Mineral forms ofMg include biotite, horneblende, olivene,dolomite, and most 2:1 clay minerals.Soluble Mg can also precipitate out ofsolution as MgCO

3 or MgSO

4, frequently

along with CaCO3 in the sub-surface.Although Ca and Mg share the same

exchange processes, Mg sorbs less stronglyto soil colloids and therefore is more proneto leaching, particularly in sandy soils. As acation, Mg competes with Ca+2, K+, andNH4

+ for plant absorption and cationexchange sites. Mg deficiencies occur whenthese other cations dominate soils with lowMg concentrations (<10% of basesaturation). A common Mg deficiencyproblem in cattle is ‘grass tetany’, or‘hypomagnesaemia’, due to insufficient Mgin forage. Livestock can be fed Mg salts inlow Mg areas or soils can be amended with


epsom salt (MgSO4) or dolomite (MgCO3)additions. Because of the low solubility ofdolomite at pH levels above 7, sprayingdissolved or dry epsom salt is most suitableon Montana’s and Wyoming’s high pHsoils.

SummarySecondary nutrients are no less

essential to plant growth than the primarynutrients: N, P, and K. The mineralogy ofMontana and Wyoming soils generallymaintain high levels of available Ca andMg. Because plants require relatively smallamounts of these nutrients and leaching isminor, Ca and Mg deficiencies are rare inthis region; accumulation of Ca and Mgsalts are actually a more commonproblem. Although deficiencies of S arealso relatively infrequent, sustained

cropping with few if any inputs may causeyields to be limited by S. Sulfur plays amajor role in both yield and quality formost crops and considerably improves theeffectiveness of N, P, and K fertilization.Effective management for S includesfertilization and leaving maximumamounts of post-harvest residues on site,ensuring the best use of this limitednutrient ‘pool’. Significant yield andprotein responses to S fertilizers havebeen documented throughout the regionfor a variety of crops. Soil and tissuetesting are useful in diagnosing nutrientdeficiencies and managing for them beforesignificant yield losses occur.Understanding the cycling of secondarynutrients in the soil can also helpproducers predict where deficiencies aremost likely to occur.


ReferencesBeaton, J.D. and R.J. Soper. 1986. Plant Response

to Sulfur in Western Canada. p. 375-403. InM.A. Tabatabai, (ed.) Sulfur in Agriculture.Agron. Monogr. 27. ASA, CSSA, and SSSA.Madison, WI.

Brady, N.C. 1984. The Nature and Properties ofSoils. 9th Edition. Macmillan PublishingCompany New York. 750p.

Foth, H.D. and B.G. Ellis. 1997. Soil Fertility. CRCPress, Boca Raton, Florida. 290 p.

Freney, z.R. 1986. Forms and reactions of organicsulfur compounds in soils. p. 207-232. In M.A.Tabatabai, (ed.) Sulfur in Agriculture. Agron.Monogr. 27. ASA, CSSA, and SSSA. Madison,WI.

Gavlak, R.G. 1982. Effect of nitrogen and sulfurfertilization on forages in the Gallatin Valley ofMontana. M.S. Thesis. Dept. of Plant and SoilScience. Montana State University, Bozeman,MT.

Grant C.A. and L.D. Bailey. 1993. Fertilitymanagement in canola production. Can. J. PlantSci. 73:651-670

Havlin, J.L., J.D. Beaton, S.L. Tisdale, and W.L.Nelson. 1999. Soil Fertility and Fertilizers. 6th

Edition. Prentice Hall. Upper Saddle River, NJ.499 p.

Jackson, G.D. 2000. Effects of nitrogen and sulfuron canola yield and nutrient uptake. Agron. J.92:644-649.

Jones, J.B. 1998. Plant Nutrition Manual. CRCPress. Boca Raton, Florida. 149p.

Rasmussen, P.E. and P.O. Kresge. 1986. PlantResponse to Sulfur in the Western United States.p.357-374. In M.A. Tabatabai, (ed.) Sulfur in

Agriculture. Agron. Monogr. 27. ASA, CSSA, andSSSA. Madison, WI.

Rasmussen, P.E., R.E. Ramig, R.R. Allmaras, andC.M. Smith. 1975. Nitrogen-sulfur relations insoft white winter wheat. Agron. J. 67:224-229.

Rendig, V.V. 1986. Sulfur and crop quality. p. 635-650. In M.A. Tabatabai, (ed.) Sulfur inAgriculture. Agron. Monogr. 27. ASA, CSSA, andSSSA. Madison, WI.

Spencer, K. and J.R. Freney. Assessing the sulfurstatus of field-grown wheat by plant analysis.Agron. J. 72:469-472.

Tabatabai, M.A. and Y.M. Chae. 1991. Mineralizationof sulfur in soils amended with organic wastes.J. Environ. Qual. 20:684-690.

Tisdale, S.L., R.B. Reneau, and J.S. Platou. 1986.Atlas of Sulfur Deficiencies. p. 295-322. In M.A.Tabatabai, (ed.) Sulfur in Agriculture. Agron.Monogr. 27. ASA, CSSA, and SSSA. Madison,WI.

Unger, C., D. Flaten, C. Grant, and O. Lukow. 2002.Impact of sulphur fertilizer on spring wheatbreadmaking quality. Proceedings of the GreatPlains Soil Fertility Conference. Vol 9. Denver,Colorado. March 5-6, 2002. www.ppi-ppic.org.

Veseth, R. and C. Montagne. 1980. Geologic ParentMaterials of Montana Soils. Montana Ag. Exper.Stat. Bull. 721.

Wichman, D. 2001. Fertilizer use on drylandperennial forages. Fertilizer Fact #27. MontanaState University Extension Service.

Zhao, F.J., M.J. Hankesford, and S.P. McGrath.1999. Sulphur assimilation and effects on yieldand quality of wheat. J. Cereal Sci. 30:1-17.


AcknowledgmentsWe would like toextend our utmostappreciation to thefollowing volunteerreviewers whoprovided their timeand insight inmaking this a betterdocument:

Jeff Farkell, CentrolAg Consulting, Brady,Montana

Rolan Himes, RockyMountain AgronomyCenter, RivertonWyoming

Grant Jackson,Western TriangleAgricultural ResearchCenter, Conrad,Montana

Sharla Sackman,Prairie CountyExtension Office,Terry, Montana

Dave Wichman,Central AgriculturalResearch Center,Moccasin, Montana

Suzi Taylor, MSUCommunicationsServices. Design andlayout.

Appendix

BOOKS

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

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

Soil Fertility. Foth and Ellis. 1997. CRC Press, BocaRaton, Florida. 290 p.

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

EXTENSION MATERIALS

Fertilizer Guidelines (EB104), Free

Obtain the above Extension publication (add $1 forshipping) from:

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

See Web Resources below for online orderinginformation.

PERSONNEL

Engel, Rick. Associate Professor. Montana StateUniversity, Bozeman. (406) [email protected]

Jackson, Grant. Associate Professor. WesternTriangle Agricultural Research Center, Conrad.(406) 278-7707. [email protected]

Jacobsen, Jeff. Extension Soil Scientist. MontanaState University, Bozeman. (406) [email protected]

Jones, Clain. Soil Chemist. Montana StateUniversity, Bozeman. (406) [email protected]

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

Wichman, Dave. Associate Professor. CentralAgricultural Research Center, Moccasin, (406)423-5421 [email protected]

WEB RESOURCES

www.sulphurinstitute.org/

The Sulphur Institute home pageoffering publications and expertise inworldwide sulfur issues.

http://www.agric.nsw.gov.au/reader/3682#budget

Exploring crop rotations and theirability to utilize S. Source: New SouthWales Agriculture.

http://www.agnr.umd.edu/users/agron/nutrient/Factshee/sulfur/Sulfur.html

University of Maryland site discussingthe S cycle.

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

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

http://www.montana.edu/publications

Montana State University Publicationsordering information for Extensionmaterials

http://Agnotes.org

MSU weekly Agronomy Notes by Dr. JimBauder on range of issues, includingfertilizer management. Currently thereare 23 notes on Fertilizer Management,and over 300 Agronomy notes totalanswering questions from producers,extension agents, and consultants.

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

28 Fertilizer Facts summarizingfertilizer findings and recommendationsbased on field research conducted inMontana by Montana State Universitypersonnel.

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].

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