Available Phosphorous


Available Phosphorous

The forms of soil P removed by ion in soil test extract. The ions covered will be H+, OH-, F-, HCO3, acetate and sulphate.

a. Hydrogen Ions:

It greatly increased solubility of all Ca-P including basic Ca-P such as hydroxyl appetite. This is the principle used in super phosphate and H3PO4 from rock phosphate.

b. Hydroxides:

Hydroxides have been found to have little effect on Ca-P but will dissolve Fe-P and Al-P in that order. The use of OH ions is not practical in soils with high organic matter content.

c. Fluoride Icons:
Fluoride ions specifically precipitate soluble Ca as CaF2 and thus will extract the more soluble Ca-P such as CaHPO4 from the soil.

d. Bicarbonate Ions:

Bicarbonate ions have the property of precipitating calcium as CaCO3, thus making the more soluble Ca-P extractable by lowering calcium activity.

e. Acetate Ions:

Acetate ions have the ability to form complexes with polyvalent metal cations in soil but without accompanying H ions, their ability to remove P in is small. The main utility of acetate ions seems to be preventing readsorption of phosphate removed by other ions.

f. Sulphate Ions:

Sulphate appears to prevent readsorption of phosphate replaced by the H ions.

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