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Sulfate Retention by Kaolinitic Alfisols from Nigerian Savanna

Department of Soil Science, Faculty of Agriculture/Institute for Agricultural Research, Ahmadu Bello University, Zaria, Nigeria

ABSTRACT

The low amount of available SO₄ in savanna Alfisols necessitates investigation of mechanism of SO₄ retention and availability in these soils. The study examines the mechanism and SO₄ retention capacity of kaolinitic Alfisols from Nigerian savanna. Soil samples from 0- to 20-cm depth from three cultivated plots and a native site were equilibrated with 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 (PZSE) and point of zero net charge (PZNC) of the soils determined by potentiometric titration and ion adsorption method, respectively. The results of the study indicated that the soils had high SO₄ retention capacity ranging from 15 to 17 mmol kg^–1 soil, and the sorption isotherms were described by a modified Freundlich equation. Sulfate sorption was, however, not significantly affected by ionic composition and strength. If the forces of SO₄ adsorption were simply electrostatic, increasing ionic strength would have significantly decreased SO₄ adsorption above the PZSE or PZNC of the soils. The study suggested that SO₄ retention by kaolinitic Alfisols might involve inner-sphere complexation despite the difficulty of making mechanistic, molecular level interpretation from gross solution data of whole soils.

Received for publication March 22, 1995.