Sulfate adsorption and its relationships with properties of representative soils of the Sao Paulo State, Brazil

Relationships between sulfate adsorption and physical, electrochemical and mineralogical properties of representative soils of the Sao Paulo State, Brazil, were evaluated. Ten subsurface soil samples were characterized for particle size distribution, electrochemical and mineralogical properties and for their capacities of sulfate adsorption. The experimental results were submitted to analysis of variance and to correlation and regression analyses. The composition of the clay fraction was more important than the absolute clay content for sulfate adsorption. Although correlation analyses showed that dithionite- and ammonium-oxalate-extractable iron and aluminum were similarly related to sulfate adsorption, regression analyses indicated that the retention of sulfate was dependent mainly on Fe-o and Al-o. When the adsorption was evaluated in the clay fraction, the kaolinite content was associated with low capacities of sulfate adsorption; however, no relationship was observed between the kaolinite soil content and the sulfate adsorption by the whole soil. No significant effects on sulfate adsorption were observed for individual hematite and goethite soil contents; on the other hand, the sum of hematite and goethite contents was related to sulfate adsorption. Soil pH in 1 M NaF (pH(NaF)) was strongly related to sulfate adsorption and was influenced by soil properties very similarly to sulfate adsorption. Thus, it can be used as an estimator of the sulfate adsorption capacity of weathered subsoils. Finally, the current rules for gypsum amendments of agricultural soils of the Sao Paulo State should be revised. In this context, more research involving the use of pH(NaF) as an ancillary criterion for defining gypsum requirement should be encouraged. (C) 2003 Elsevier Science B.V. All rights reserved.