INFLUENCE OF PHOSPHATE ON CADMIUM ADSORPTION BY SOILS

In batch equilibrium experiments, it was found that orthophosphate inc reased Cd adsorption and decreased its desorption by three representat ive soils. Ion activity products in the equilibrium solution indicated that cadmium activity was under saturated with respect to Cd-3(PO4) ( a) on the Red earth(Typic Hapludult) and the Yellow-brown earth(Typic Hapludalf). With the calcareous Yellow fluvo-aquic soil(Typic Haplaque nt), phosphate enhanced Cd adsorption at lower Cd concentration but it decreased the adsorption at high Cd level. Ion activity products and solubility diagrams suggested that at high Cd level Cd-3(PO4)(2) had f ormed and the activity of Cd in solution was controlled by this precip itate. This implies that the formation of Cd-3(PO4)(2) (s) is unable t o decrease Cd activity so long as soil adsorption sites are unsaturate d by Cd. It is suggested from these results that one could not expect decreasing soil Cd availability by phosphate fertilization unless a si multaneous rise in soil pH also occurs.