Chapter Chapter

SOIL AND FERTILIZER KSOIL AND FERTILIZER K

Soil PotassiumSoil Potassium

– Total K in soils averages about 40,000 lb/acreTotal K in soils averages about 40,000 lb/acre– Soil potassium is present in four categorical Soil potassium is present in four categorical

formsformsoccluded (within soil minerals such as feldspar, mica, occluded (within soil minerals such as feldspar, mica, etc), 98% of totaletc), 98% of total

fixed (trapped within the lattice of 2:1 expanding clay fixed (trapped within the lattice of 2:1 expanding clay minerals), 1% of totalminerals), 1% of total

exchangeable. 1% of total (100-1000ppm)exchangeable. 1% of total (100-1000ppm)

solution, 0.1% of total (1-10 ppm)solution, 0.1% of total (1-10 ppm)

An equilibrium exists between each An equilibrium exists between each

Soil KSoil K

K+ K+ K+ K+ K+ = K+ K+ K+ K+ = K+ = K+

K+

Mineral weathering = == ==

feldspar, mica (occluded) hydrous micas (fixed) clays (exchangeable)=solution

Available K. Solution and exchangeable K normally represent "available" K for plants during a growing season

Available soil KAvailable soil KPlant uptake is by diffusion (90%) and mass flow Plant uptake is by diffusion (90%) and mass flow

(10%)(10%)K is immobile in soil (on a scale of 1 to 100, with 100 K is immobile in soil (on a scale of 1 to 100, with 100 being most mobile, NO3- is 99, K+ is 33, and HPO42- is being most mobile, NO3- is 99, K+ is 33, and HPO42- is 1)1)

– Factors affecting amount of available K to plantsFactors affecting amount of available K to plantssoil mineralogysoil mineralogy and climate and climateCECCEC

– clay and organic matter contentclay and organic matter content

K fixation and/or releaseK fixation and/or release– wetting and dryingwetting and drying– freezing and thawingfreezing and thawing

subsoil and rooting depthsubsoil and rooting depthsoil pHsoil pH

– competing exchangeable ionscompeting exchangeable ions

(1) Acid soil

` +3 +3 K+ +3 Cl-+H2O= + 3 K+ +3 Cl- + H2O

Ca2+

H+

Al3+

Ca2+

H+

Al3+

(1) Ca from gypsum or lime.

+ 3 K+ +3 Cl- + H2O= K+ + Ca2+ +3 Cl- + H2O

Ca2

+ H+ Ca2+

Ca2

+ H+

K+

K+

3 K+ cannot compete effectively for the more tightly held Al3+ and H+

3 K+ can compete more effectively for Ca2+ than the more tightly held Al3+ and H+. It is easier to increase exchangeable K by fertilizing a Ca saturated soil than Al3+ and H+ saturated exchange complex.

Factors affecting plant uptakeFactors affecting plant uptake– Any condition that affects root growth effects uptake Any condition that affects root growth effects uptake

(plant response) of available K, all other things being (plant response) of available K, all other things being equal.equal.

compacted soil wet soilcompacted soil wet soilacid soilacid soilshallow soilshallow soilherbicide injuryherbicide injury

– K leaching (only a concern on permeable, low CEC K leaching (only a concern on permeable, low CEC soils)soils)

– K Soil testingK Soil testingExchangeable plus solution K (any extraction solution Exchangeable plus solution K (any extraction solution that will provide a strongly held cation, or a weakly held that will provide a strongly held cation, or a weakly held cation in high concentration)cation in high concentration)Must be correlated and calibratedMust be correlated and calibrated

– calibrated on % sufficiency basis like Pcalibrated on % sufficiency basis like P,,

Fertilizer KFertilizer KMuriate of potash (KCI), 0-0-62Muriate of potash (KCI), 0-0-62

– most commonmost common– mined in Canada and New Mexicomined in Canada and New Mexico– solid, 100% solublesolid, 100% soluble

Application methods are similar to that for P because it Application methods are similar to that for P because it is relatively immobile in soil.is relatively immobile in soil.

– exception: for high yielding forage crops, where forage is exception: for high yielding forage crops, where forage is removed (bermudagrass or alfalfa, or turf such as putting removed (bermudagrass or alfalfa, or turf such as putting greens) if soil is sandy, K management should be more like greens) if soil is sandy, K management should be more like that for N, where amount required is more closely related to that for N, where amount required is more closely related to yield.yield.

When both P and K are deficient, the yield loss will be a When both P and K are deficient, the yield loss will be a product of the % sufficiency’s for P and Kproduct of the % sufficiency’s for P and K. For example, . For example, if P is 80 % sufficient and K is 70 % sufficient, if neither if P is 80 % sufficient and K is 70 % sufficient, if neither deficiency is corrected by fertilizing, then the expected deficiency is corrected by fertilizing, then the expected yield will be 80 % X 70 % (.80 X .70), or 56 % (0.56 X yield will be 80 % X 70 % (.80 X .70), or 56 % (0.56 X potential yield).potential yield).Salt Effect: Salt Rate N + KSalt Effect: Salt Rate N + K2200Corn: <10 lbs Salt/ac with the seedCorn: <10 lbs Salt/ac with the seedWheat: < 30 lbs Salt/ac with the seedWheat: < 30 lbs Salt/ac with the seed

Soil Testing Soil Testing

Why soil test?Why soil test?– We cannot sense (smell, taste, feel, see, or hear) the We cannot sense (smell, taste, feel, see, or hear) the

nutrient supplying capacity of the soil, because it is a nutrient supplying capacity of the soil, because it is a chemical property of soils.chemical property of soils.

– Soils are inherently variable from one place to Soils are inherently variable from one place to another in the landscape (spatial variability).another in the landscape (spatial variability).

Gross differences are often separated one from another Gross differences are often separated one from another and managed as individual units or fields. The size and managed as individual units or fields. The size (acres) and shape of these units relative to the size of (acres) and shape of these units relative to the size of field equipment influences whether or not a particular field equipment influences whether or not a particular unit will be managed separately. As agriculture has unit will be managed separately. As agriculture has evolved to the use of larger and larger equipment, field evolved to the use of larger and larger equipment, field size has increased and the separate management of size has increased and the separate management of small, differing areas has decreased. Consequently, small, differing areas has decreased. Consequently, landscape variability that used to exist landscape variability that used to exist amongamong fields fields may now exist may now exist withinwithin a field. a field.

Soil Test P Variability Among First 50 Free Soil Tests for Garfield

County Oklahoma, 1997

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il T

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Soil test P variability among 24 1-acre cells in a field, 2001

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325

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il T

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Area STP Point STP

Soil Test K variability among 24 1-acre cells in a field, 2001

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il T

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Area STK Point STK

K ManagementK Management– Nutrient availability for a soil changes with Nutrient availability for a soil changes with

time in relation to management.time in relation to management.Continued harvest removal of nutrients may result Continued harvest removal of nutrients may result in deficiencies of those that are generally present in deficiencies of those that are generally present in high concentrations in plants and for which the in high concentrations in plants and for which the soil may have limited capacity to provide in plant-soil may have limited capacity to provide in plant-available form (e.g. N and K).available form (e.g. N and K).

Continued fertilizer input of some nutrients may Continued fertilizer input of some nutrients may result in a “build-up” of the nutrient to the point result in a “build-up” of the nutrient to the point that a previous deficiency no longer exists (e.g. P that a previous deficiency no longer exists (e.g. P fertilization of low yielding crops)fertilization of low yielding crops)

K ManagementK ManagementApproaches to nutrient (fertilizer) managementApproaches to nutrient (fertilizer) management

– Ask the fertilizer dealer “what are farmers using this Ask the fertilizer dealer “what are farmers using this year?”year?”

– Find out what the neighbor is using and fertilize like Find out what the neighbor is using and fertilize like the neighborthe neighbor

– Soil test one or two fields and fertilize the rest of the Soil test one or two fields and fertilize the rest of the farm based upon the averagefarm based upon the average

– Soil test each field, every year, until you have Soil test each field, every year, until you have developed a confidence in your knowledge of what the developed a confidence in your knowledge of what the field should testfield should test, knowing that soil test pH, P and K , knowing that soil test pH, P and K (immobile chemical properties) should not change (immobile chemical properties) should not change much from year-to-year under normal practices.much from year-to-year under normal practices.

Soil TestingSoil TestingHow to make soil testing work/not workHow to make soil testing work/not work

– get a good, representative sampleget a good, representative sample15 randomly taken, 0-6 inch, cores from the area in 15 randomly taken, 0-6 inch, cores from the area in question, placed in a plastic bucket.question, placed in a plastic bucket.

– avoid small unusual areas (saline or sodic spots, gullies, avoid small unusual areas (saline or sodic spots, gullies, eroded hill tops), sampling them separately later.eroded hill tops), sampling them separately later.

– mix contents of the bucket until all sample cores have been mix contents of the bucket until all sample cores have been broken up and the soil is a homogeneous mixture, then fill broken up and the soil is a homogeneous mixture, then fill the sample container.the sample container.

make sure sampling depth and time of year are similar make sure sampling depth and time of year are similar if year-to-year comparisons are to be made.if year-to-year comparisons are to be made.understand that results are an understand that results are an averageaverage of the 15 spots of the 15 spots randomly sampled in the field, and do not provide any randomly sampled in the field, and do not provide any information about the variation in the field.information about the variation in the field.

– if field is highly variable, then the average made up of 15 if field is highly variable, then the average made up of 15 cores each year will likely vary more than if the field was cores each year will likely vary more than if the field was relatively uniform.relatively uniform.

Soil TestingSoil Testing– use a calibrated test for immobile nutrients, preplant use a calibrated test for immobile nutrients, preplant

(or preseason in the case of perennials) nitrate-N test (or preseason in the case of perennials) nitrate-N test or pre-sidedress nitrate test (corn), PSNT, for N in or pre-sidedress nitrate test (corn), PSNT, for N in conjunction with yield goal .conjunction with yield goal .

test results must be related to “critical value”, test results must be related to “critical value”, identifying soil test value above which crop response to identifying soil test value above which crop response to added fertilizer is added fertilizer is notnot expected. expected.test results must be related to the amount of nutrient test results must be related to the amount of nutrient addition required to correct deficiencies when the soil addition required to correct deficiencies when the soil test value is below the “critical value”.test value is below the “critical value”.

– interpret the test results relative to the degree of interpret the test results relative to the degree of adequacy or deficiency of the nutrient or parameter adequacy or deficiency of the nutrient or parameter (lime or gypsum requirement) measured.(lime or gypsum requirement) measured.

– develop at a fertilizer or soil amendment develop at a fertilizer or soil amendment recommendation that is reasonable.recommendation that is reasonable.

– evaluate soil test results over time.evaluate soil test results over time.

Soil TestingSoil TestingSoil testing-fertilizer recommendation philosophiesSoil testing-fertilizer recommendation philosophies

– correct deficiency of current growing season (sufficiency)correct deficiency of current growing season (sufficiency)– correct deficiency of current growing season, plus replace correct deficiency of current growing season, plus replace

what crop removed (sufficiency + maintenance)what crop removed (sufficiency + maintenance)– correct deficiency of current growing season, plus add extra correct deficiency of current growing season, plus add extra

to “build-up” soil test levels (sufficiency + build up)to “build-up” soil test levels (sufficiency + build up)Field “test” stripsField “test” strips

– Planned treatment “skips” or double applications can be a Planned treatment “skips” or double applications can be a good “in the field” soil test that will be influenced by the field good “in the field” soil test that will be influenced by the field environment and growing conditionsenvironment and growing conditions

– ““wear-bar” strips, like are used to visually show when it is wear-bar” strips, like are used to visually show when it is time to replace worn tires on cars, can be useful, long-term time to replace worn tires on cars, can be useful, long-term field test stripsfield test strips

N Rich Strip for “other nutrients”N Rich Strip for “other nutrients”