PHOSPHORUS SORPTION BY 3 CULTIVATED SAVANNA ALFISOLS AS INFLUENCED BYPH

An earlier study of phosphate sorption by some savanna soils from Nige ria suggested that increased P sorption when pH was raised might be du e to precipitation of exchangeable Al as amorphous polymeric Al specie s with increased sorption sites. But these savanna soils have Ca as th e dominant cation in their exchange sites, and low exchangeable Al. Th e objective of this study was to determine the role played by Ca in pi -I-induced P sorption of three savanna soils under continuous cultivat ion. Phosphorus sorption increased when pH was raised from 4.5 to 7.0. Similarly, Ca retention increased with increasing pH. Regression of P sorption on Ca retention indicated a significant linear relationship in the three soils. Three possible mechanisms were proposed to explain the increasing P sorption with increasing pH: precipitation of Ca-pho sphates, Ca-induced P sorption or co-adsorption of Ca and H2PO4- or HP O42- as ion pairs or complexes. Available evidence suggests that all t hree mechanisms can operate together to enhance P retention as pH incr eases. The paper proposes that increased P sorption by savanna soils w hen pH is raised is likely to be related to the chemistry and retentio n of Ca rather than to hydrolytic reactions of Al.