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DIVISION S-2—SOIL CHEMISTRY

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

Dep. of Soil Science, Faculty of Agriculture, Ahmadu Bello Univ., Zaria, Nigeria

^* Corresponding author (joagbenin{at}yahoo.com)

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 Ca²⁺, Mg²⁺, and K⁺ concentrations in soil solution with exchangeable Ca, Mg, and K; and to determine the solubility relations of SO₄ 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 Ca²⁺, Mg²⁺, 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 Ca²⁺, Mg²⁺, and SO₄^2- in soil solution were best predicted by electrolytic conductivity of the saturation extract. The K activity ratios in solution defined as ^aK/(^aCa + ^aMg)^1/2 were <0.01 in the cultivated soil, suggesting a preponderance of K adsorbed to edge rather than planar sites. The Ca²⁺ activity ratios defined as ^aCa/[^aCations] 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.

Abbreviations: AR,_K, activity ratio of K • CEC, cation-exchange capacity • FYM, farmyard manure • I, ionic strength • IAP, ion activity product, OM, organic matter • pIAP, negative logarithm of ion activity product, pK_sp negative logarithm of solubility product constant