Phosphorus sorption by three cultivated savanna alfisols as influenced by pH

Abstract

An earlier study of phosphate sorption by some savanna soils from Nigeria suggested that increased P sorption when pH was raised might be due to precipitation of exchangeable Al as amorphous polymeric Al species with increased sorption sites. But these savanna soils have Ca as the dominant cation in their exchange sites, and low exchangeable Al. The objective of this study was to determine the role played by Ca in pH-induced P sorption of three savanna soils under continuous cultivation. 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-phosphates, Ca-induced P sorption or co-adsorption of Ca and H₂PO ⁻₄ or HPO ²⁻₄ as ion pairs or complexes. Available evidence suggests that all three mechanisms can operate together to enhance P retention as pH increases. The paper proposes that increased P sorption by savanna soils when pH is raised is likely to be related to the chemistry and retention of Ca rather than to hydrolytic reactions of Al.