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The potassium fixation is the result of re-entrapment

of K+ ions between the layers of 2:1 minerals

especially illite. The 1: 1 minerals such kaolinite do

not fix K. K+ is sufficiently small to enter the

interlayer regions, where it is firmly held by

electrostatic forces.

NH4+ has nearly the same radius with K+ , so it can

be fixed in a manner similar to K , its presence will

alter both the fixation of added K and release of

fixed K.

K fixation is generally more important in fine textured soils. Although fixation reactions are not considered a serious factor in limiting crop response to either applied NH4 or K, increasing K concentration in soils with a high fixation capacity will obviously encourage greater fixation.

Air drying some soils high in exchangeable K can result in fixation and decline exchangeable K. In contrast drying of the soils low in exchangeable ,particularly subsoils, will frequently increase exchangeable K. The release of K upon drying is thought be caused by cracking of the clay edges and exposure of inter layer K, which can the be released to the exchange sites.

1. Clay minerals

The soils containing 2:1 type of clay minerals like illite,

vermiculite and montmorillonite can fix considerable

amounts of potassium. A laterite soil containing kaolinite

type or clay mineral fixed very little amount of potassium.

The potassium and ammonium ions are attracted

between the crystal units by the same negative charges

responsible for the internal absorption of these and other

cations. The tendency for fixation is greater in minerals

where the major source of negative charge is in the silica

sheet.

2. Potassium concentratioAn increase in K concentration is likely to increase K

fixation because more K goes into the exchange complex.

3, Wetting and drying

The K fixation is strongly influenced by wetting and

drying of soils. Fixation occurs when initial level of exchangeable and soluble K is high and release occurs when the level of such K is low. Thus, the process of drying favours attainment of equilibrium in distribution of K in soils.

4. Temperature

Higher temperature favours dehydration and contraction of the cryst- al lattice resulting higher K

fixation

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9. Anions The k fixation from KH2PO4 was greater than from

KCl and K2SO4, but there was no difference in K fixation

from K2SO4 and KCl.

10. Organic matter The addition of organic matter decreases

the k fixation by inorganic colloids.

Thank you.