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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
.
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.