Chapter 8 SOIL AND FERTILIZER K

Soil Potassium

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

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

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

• exchangeable. 1% of total (100-1000ppm)• solution, 0.1% of total (1-10 ppm)

An equilibrium exists between each • Soil 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 KPlant uptake is by diffusion (90%) and mass flow (10%)

• 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 1)

• Factors affecting amount of available K to plants• soil mineralogy and climate• CEC

• clay and organic matter content• K fixation and/or release

• wetting and drying• freezing and thawing

• subsoil and rooting depth• soil pH

• competing 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 uptake• Any condition that affects root growth effects uptake (plant

response) of available K, all other things being equal.• compacted soil wet soil• acid soil• shallow soil• herbicide injury

• K leaching (only a concern on permeable, low CEC soils)• K Soil testing

• Exchangeable plus solution K (any extraction solution that will provide a strongly held cation, or a weakly held cation in high concentration)

• Must be correlated and calibrated• calibrated on % sufficiency basis like P,

Fertilizer K• Muriate of potash (KCI), 0-0-62

• most common• mined in Canada and New Mexico• solid, 100% soluble

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

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

• When both P and K are deficient, the yield loss will be a product of the % sufficiency’s for P and K. For example, if P is 80 % sufficient and K is 70 % sufficient, if neither deficiency is corrected by fertilizing, then the expected yield will be 80 % X 70 % (.80 X .70), or 56 % (0.56 X potential yield).

• Salt Effect: Salt Rate N + K20Corn: <10 lbs Salt/ac with the seedWheat: < 30 lbs Salt/ac with the seed

Soil Testing

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

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

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

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

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

County Oklahoma, 1997

0

50

100

150

200

250

300

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Entry Number

So

il T

est

P

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

0

65

130

195

260

325

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Cell

So

il T

est-

P

Area STP Point STP

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

0

50

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Cell

So

il T

es

t K

Area STK Point STK

K Management

• Nutrient availability for a soil changes with time in relation to management.

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

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

K Management• Approaches to nutrient (fertilizer) management

• Ask the fertilizer dealer “what are farmers using this year?”• Find out what the neighbor is using and fertilize like the

neighbor• Soil test one or two fields and fertilize the rest of the farm

based upon the average• Soil test each field, every year, until you have developed a

confidence in your knowledge of what the field should test, knowing that soil test pH, P and K (immobile chemical properties) should not change much from year-to-year under normal practices.

Soil Testing• How to make soil testing work/not work

• get a good, representative sample• 15 randomly taken, 0-6 inch, cores from the area in question,

placed in a plastic bucket.• avoid small unusual areas (saline or sodic spots, gullies, eroded hill

tops), sampling them separately later.• mix contents of the bucket until all sample cores have been broken

up and the soil is a homogeneous mixture, then fill the sample container.

• make sure sampling depth and time of year are similar if year-to-year comparisons are to be made.

• understand that results are an average of the 15 spots randomly sampled in the field, and do not provide any information about the variation in the field.

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

Soil Testing• use a calibrated test for immobile nutrients, preplant (or

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

• test results must be related to “critical value”, identifying soil test value above which crop response to added fertilizer is not expected.

• test results must be related to the amount of nutrient addition required to correct deficiencies when the soil test value is below the “critical value”.

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

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

• evaluate soil test results over time.

Soil Testing• Soil testing-fertilizer recommendation philosophies

• correct deficiency of current growing season (sufficiency)• correct deficiency of current growing season, plus replace what crop

removed (sufficiency + maintenance)• correct deficiency of current growing season, plus add extra to

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

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

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

N Rich Strip for “other nutrients”