Soil Saturation Extract Composition and Sulfate Solubility in a Tropical Semiarid Soil

Soil solution is the major source of plant nutrients, nutrient cycling in ecosystems, and pollutant transformation and transport in soil. I examined the composition of soil saturation extract of a cultivated and an uncultivated savanna Alfisol to determine the relations of Ca 2+ , Mg 2+ , and K + concentrations in soil solution with exchangeable Ca, Mg, and K; and to determine the solubility relations of SO 4 for which there is little information in savanna soils. The soil saturation extract had ionic strength ( I ) below 0.001 except for one field under intensive fertilization with NPK fertilizer and farmyard manure (FYM) whose ionic strength was between 0.001 and 0.002 at a depth below 40 cm due to leaching. The Ca 2+ , Mg 2+ , and K + in the soil saturation extract correlated weakly with exchangeable Ca, Mg, and K respectively, but K + intensity was fairly well predicted by percentage of K saturation of cation‐exchange capacity (CEC), whereas the intensities of Ca 2+ , Mg 2+ , and SO 4 2− in soil solution were best predicted by electrolytic conductivity of the saturation extract. The K activity ratios in solution defined as a K/( a Ca + a Mg) 1/2 were <0.01 in the cultivated soil, suggesting a preponderance of K adsorbed to edge rather than planar sites. The Ca 2+ activity ratios defined as a Ca/[∑ a Cations] were <0.15 for the cultivated soil, indicating possible Ca deficiency. The SO 2− 4 activities in solution were in apparent equilibrium with basaluminite in the cultivated fields, whereas in the uncultivated field, SO 2− 4 activities were in apparent equilibrium with alunite.