SULFATE SORPTION BY TWO OXISOLS AND AN ALFISOL OF THE TROPICS1

Abstract

Sulfate availability in tropical soils is highly variable, and factors influencing its sorption in the soil profile are incompletely understood. The influence of pH and SO4 - concentration on the amount of SO4= sorbed and on negative charges of the soil was investigated through equilibration procedures using soil samples representing two Oxisols and one Alfisol of Säo Paulo, Brazil. These experiments revealed that the amount of SO4= sorbed was significant in subsoil (B2) samples. At a pH of about 4.0, the amount sorbed reached more than 2 me/100 g of soil in one sample, an amount that is high in relation to the low cation retention capacity of these highly weathered tropical soils. Sulfate sorption was clearly dependent on the pH of the equilibrium solution; the amount sorbed decreased as the pH increased with each soil. In the surface horizons with relatively high organic matter content, the SO4= sorption capacity was smaller, and the pH effect was less evident. When SO4= sorption occurred, the pH of the equilibrium solution increased, probably in part at least a result of SO4= substitution for OH− on the surface of oxides. Sorbed SO4= was desorbed only partially in some soil samples, while in others SO4= was desorbed in larger amounts than were actually sorbed, indicating a release of native SO4=. Potassium desorbed, considered as a measure of cation exchange capacity, suggested an increase in cation retention sites as a result of increasing levels of sorbed SO4=. Sulfate availability in tropical soils is highly variable, and factors influencing its sorption in the soil profile are incompletely understood. The influence of pH and SO4 - concentration on the amount of SO4= sorbed and on negative charges of the soil was investigated through equilibration procedures using soil samples representing two Oxisols and one Alfisol of Säo Paulo, Brazil. These experiments revealed that the amount of SO4= sorbed was significant in subsoil (B2) samples. At a pH of about 4.0, the amount sorbed reached more than 2 me/100 g of soil in one sample, an amount that is high in relation to the low cation retention capacity of these highly weathered tropical soils. Sulfate sorption was clearly dependent on the pH of the equilibrium solution; the amount sorbed decreased as the pH increased with each soil. In the surface horizons with relatively high organic matter content, the SO4= sorption capacity was smaller, and the pH effect was less evident. When SO4= sorption occurred, the pH of the equilibrium solution increased, probably in part at least a result of SO4= substitution for OH− on the surface of oxides. Sorbed SO4= was desorbed only partially in some soil samples, while in others SO4= was desorbed in larger amounts than were actually sorbed, indicating a release of native SO4=. Potassium desorbed, considered as a measure of cation exchange capacity, suggested an increase in cation retention sites as a result of increasing levels of sorbed SO4=. © Williams & Wilkins 1979. All Rights Reserved.