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.
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.