SULFATE RETENTION BY KAOLINITIC ALFISOLS FROM NIGERIAN SAVANNA

The low amount of available SO4 in savanna Alfisols necessitates inves tigation of mechanism of SO4 retention and availability in these soils . The study examines the mechanism and SO4 retention capacity of kaoli nitic Alfisols from Nigerian savanna. Soil samples from 0- to 20-cm de pth from three cultivated plots and a native site were equilibrated wi th varying amounts of SO4 (0-0.003 M) in three supporting electrolytes , 0.01 M CaCl2, KCl, and NaCl, and three ionic strengths, 0.0 1, 0.1, and 0.2 M KCl at pH 5.6+/-0.2, above the point of zero salt effect (PZ SE) and point of zero net charge (PZNC) of the soils determined by pot entiometric titration and ion adsorption method, respectively. The res ults of the study indicated that the soils had high SO4 retention capa city ranging from 15 to 17 mmol kg(-1) soil, and the sorption isotherm s were described by a modified Freundlich equation. Sulfate sorption w as, however, not significantly affected by ionic composition and stren gth. If the forces of SO4 adsorption were simply electrostatic, increa sing ionic strength would have significantly decreased SO4 adsorption above the PZSE or PZNC of the soils. The study suggested that SO4 rete ntion by kaolinitic Alfisols might involve inner-sphere complexation d espite the difficulty of making mechanistic, molecular level interpret ation from gross solution data of whole soils.