PHOSPHATE SORPTION BY SOIL IN RELATION TO EXCHANGEABLE CATION COMPOSITION AND PH

The influence of exchangeable cation composition on the sorption of phosphate (P) was studied using samples of an acid, clay loam subsoil which had been saturated with Ca2+ or Na+. The Na-soil always sorbed less P than the Ca-soil but the difference between the two systems was strongly pH-dependent. Sorption by the Na-soil declined rapidly as pH increased, to the extent that at pH 7.6 it was only 22% of that sorbed at pH 4.1. Although sorption by the Casoil also decreased with an initial pH increase, it increased again once pH exceeded approximately 5. The difference in sorption between the Ca, and Na-soils was only about 20% at low pH but it increased considerably above pH 5. At the highest comparable pH value (6.7), the Ca-soil sorbed almost 4 times as much P as the Na-soil. The observed cation-induced differences in P sorption by this soil cannot be explained simply by the formation of insoluble Ca-P compounds or surface Ca-P complexes in the Ca-saturated samples. A more plausible interpretation appears to derive from the effect of cation saturation on electrostatic potential. A model of P sorption which incorporates electrostatic potential effects indicated that the pH-dependence of the cation effect is likely to be due to a more rapid decrease in potential in the Na- than in the Casoil as pH is increased. However, in the absence of any measured values for potential, it is not possible to apply this model in a general way.