Anion and Mineralogical Effects on K+, Ca2+, and Mg2+ Leaching in Oxisols

Gypsum (CaSO4 center dot 2H(2)O) application favors root growth in acidic subsoils. Although previous research has shown that both sorption and cation leaching can be increased after gypsum application, little is known on the mineralogical effects on those opposite phenomena. In this article, the simultaneous leaching of K+, Ca2+, and Mg2+ in the presence of either nitrate or sulfate was compared in column experiments carried out with two Oxisols with identical contents of clay-size particles composed of different amounts of the same minerals. In addition, the possible use of remaining phosphorus as predictor of cation fate in gypsum-treated soils was evaluated. The results showed a weathering-progressive retarding effect of sulfate on K+, Ca2+, and Mg2+ leaching. Competitive batch sorption experiments showed that for both soils, the presence of sulfate increased the cation sorption, which was more intense in the soil richer in gibbsite and goethite, where the total cation sorption in the presence of sulfate exceeded its measured cation exchange capacity. Noncompetitive batch sorption experiments carried out with single minerals (i.e., kaolinite, gibbsite, hematite, and goethite) suggested that the cosorption of sulfate and K+, Ca2+, and Mg2+ seems to be the main cause of the sulfate retardant effect on exchangeable base leaching in Oxisols. The results also suggest that gypsum application on weathered soils rich in gibbsite and goethite can increase their capacity for retaining exchangeable bases in excess of their measured cation exchange capacity values. The sulfate retardant effect on cation leaching is greater in soils with low remaining phosphorus values.