Chapter8_K.ppt for soil management and conservation and fertilizer k

Soil

1. Chapter 8
SOIL AND FERTILIZER K

2. 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)

3. 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

4. Available soil K
Plant 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

5. (1) Acid soil

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


Ca
2+

H
+

Al
3+

Ca
2+

H
+

Al
3+
(1) Ca from gypsum or lime.






+ 3 K
+
+3 Cl
-
+ H2O= K
+
+ Ca
2+
+3 Cl
-
+ H2O


Ca
2
+

H
+

Ca
2+

Ca
2
+

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.

6. 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,

7. 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 + K
20
Corn: <10 lbs Salt/ac with the seed
Wheat: < 30 lbs Salt/ac with the seed

8. 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 withina field.

9. Soil Test P Variability Among First 50 Free Soil Tests for Garfield
County Oklahoma, 1997
0
50
100
150
200
250
300
135791113151719212325272931333537394143454749
Entry Number
Soil Test P
Soil test P variability among 24 1-acre cells in a field, 2001
0
65
130
195
260
325
123456789101112131415161718192021222324
Cell
Soil Test-P
Area STP Point STP

10. Soil Test K variability among 24 1-acre cells in a field, 2001
0
50
100
150
200
250
300
350
123456789101112131415161718192021222324
Cell
Soil Test K
Area STK Point STK

11. 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)

12. 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.

13. 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 averageof 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.

14. 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.

15. 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”