Physicochemical Fate of Sulfate in Soils

Abstract

Sulfur occurs in soils in organic and inorganic forms, with the inorganic S accounting for < 5 percent of the total S in most soils from humid and semihumid regions. Sulfate in soils may occur as water-soluble salts, adsorbed by soil colloids, or as insoluble forms. Considerable seasonal fluctuations in the concentration of soluble SO₄ ²⁻ in surface soils occur as a result of interaction of seasonal conditions on the mineralization of organic S, leaching, and uptake by plants. Soils vary widely in their capacity to adsorb SO₄ ²⁻ . Sulfate adsorption is readily reversible and the amount of SO₄ ²⁻ adsorbed is concentration and temperature dependent. The capacity of soil to adsorb SO₄ ²⁻ depends upon clay content, nature of clay minerals, pH, associated cation, and the amount of hydrous Al and Fe oxides. The mechanisms of SO₄ ²⁻ retention by soils involve coordination with hydrous oxides, exchange on edges of silicate clays, incorporation in mineral structures, and molecular adsorption. For acid soils, SO₄ ²⁻ adsorption essentially involves the chemistry of Fe and Al. Sulfate adsorption increases as the pH decreases below zero point of charge. Several forms of insoluble SO₄ ²⁻ are likely to occur in soils, including BaSO₄, SrSO₄, SO₄ ²⁻ associated with CaCO₃, and basic Fe and Al sulfates.