Topic 9



Potassium (K) Potash (derived from the Dutch word for wood ashes) Kalium

Potassium Fertilizer Conversion Factors % K x 1.2 = % K2O

% K2O x 0.83 = % K

A. Soil K

1. Origin of soil K is primary minerals from which soil is formed.

a. K-feldspars

b. mica

c. clay- illite, vermiculite and chlorite

2. Plants take up K in the ionic form (K+)

3. Availability of K in the soil

a. Relatively unavailable K (90-98%)                                

       - part of the crystal structure of minerals

b. Slowly available (1-10%) K that is bonded in the interlayer position of clays.

c. readily available - 1-2%  - K on cation exchange sites and K in soil solution.

d. weathering usually moves K towards the available forms.  However, applying large amounts of fertilizer K can reverse this.

4. K fixation -- Trapped in the interlayer of illite and vermiculite (2:1 clays)

5. Factors affecting availability of K

a. Soil parent material

     - feldspars and micas are high in K. If these minerals are present the soil will be high in K.

b. Soil texture - Fine textured soils have more K than coarse textured soils.

      - Norfolk Sandy Loam - 1,600 lbs. total K2O / acre

      - Cecil Sandy Clay Loam -11,800 lbs. total K2O / acre

c. Intensity of weathering - High temperature and rainfall cause faster breakdown of minerals but also more leaching loss.

       - Although soils contain large total amounts of K it is usually necessary to add K with fertilizers.

6. Effect of pH on retention of applied K

    - Leaching loss of K from a sandy soil was greatly reduced when the soil was limed.

Soil pH
Percent Added K Lost by Leaching


     - at pH 5.1 a large portion of the exchange complex is occupied by Al3+ which is held more tightly than K. Therefore,  K is blocked from the exchange site.

    - When limed Ca2+ occupies the exchange sites and K can displace the Ca2+ and be held in the soil.

7. Soil factors to consider in K fertilization

a. Ability of the soil to retain and supply K ]

    - Sandy Soils have low CEC and low K reserves

    - Fine textured soils have higher CEC and K reserves

b. Fixation of K is generally not a problem in NC

c. Leaching - Occurs on sandy coastal plain soils

  1. Cannot detect buildup in topsoil over the years with soil testing
  2. Will accumulate in the B horizon if present
  3. Annual applications are necessary because of leaching and high crop requirements

d. Placement - Broadcast application is generally as effective as banding

B. K fertilizer Mining, manufacture, and properties

a. Found as water-soluble salts in large deposits.  New Mexico and Canada. Composed mainly of KCl (sylvite), KCl + NaCl (Sylvinite),or K2SO4 + MgSO4 (Langbeinite). These salts are mined and refined to produce K fertilizers. Only 15% of potash used in the U.S. is produced domestically. Most of the rest is imported from Canada.

Potassium Fertilizer Production and Technology (IPI)

Shaft, underground mining

Solution Mining

b. K Fertilizer Materials

1. KCl - muriate of potash 60%

    - 90% of U.S. consumption

    - completely water soluble

    - contains Cl

c. K2SO4 - Potassium sulfate

    - 50% K2O  18%  Sulfur

    - provides SO4 as well as K and must contain less than 2.5% Cl   

    -  used only on Cl sensitive crops because it is more expensive than KCl

d. K2SO4 * MgSO4 - sulfate of potash-magnesia, Sul-PO-Mag, K-Mag

     - 22% K2O,  11% Magnesium,  22% Sulfur

     - max Chloride 2.5 %

e. KNO3 potassium nitrate

manufactured by reacting with KCl with sodium nitrate

    - 44% K2O  13% N

    - no Cl

C. K fertilization of Crops

a. Benefits to plants

    - improves stalk strength and size (reduces lodging )

    - improves drought resistance

    - winter hardiness improved - reduces winter kill of perennial grasses like coastal bermuda.

    - increases resistance to some diseases and nematodes

    - crop quality is improved - tobacco, peach storage qualities, corn grain, soybeans

b. Choice of K fertilizer materials

  1. I f  no special conditions are a factor, one K source is as good as another. Choice based on price per pound KCl is cheapest and used in most fertilizers.
  2. Consider the accompanying elements Mg or SO4 may be needed Cl may be detrimental.
  3. Consider crop sensitivity to chlorine

          - Tobacco is sensitive to chlorine; an excess affects burning quality

          - Irish potatoes are sensitive to chlorine.   K2SO4 produces better keeping and chipping qualities.

c. Peanuts

    High K levels can inhibit Ca uptake by the peg. K may be applied to the crop preceding or several months before to avoid this.

d. Soils low in Magnesium - High K rates can supress Mg uptake causing Mg deficiency.   Important in pastures; Mg deficiency in forage can cause grass tetany a disease of grazing animals.

e. Luxury consumption of K, Plants will take up more K than they need.

lbs. K2O
% K in Clover
Yield Per/A


f. Crop removal of K

g. Deficiency symptoms

h. Fertilizer list


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