Sodium-calcium and sodium-magnesium exchange equilibria in soil for chloride- and sulfate-dominated systems
1983
Mehta, S. C. | POONIA, S. R. | PAL, RAJ
We investigated the effect of Cl and SO4 ions on Na-Ca and Na-Mg exchange equilibria in three soils that differed in OM, clay content, and CEC. Homoionic (Na-saturated) soils were equilibrated with Cl and SO4 salt solutions of Na + Ca and Na + Mg at 25 meq L total electrolyte concentration. The exchange equilibria data were analyzed, using the thermodynamic approach and Gaponʼs, Vanselowʼs, and Krishnamoorthy-Davis-Overstreetʼs (KG, KV, and KKDO) selectivity coefficients. At a given equivalent fraction of Na and SAR in the equilibrium solution, sodification of soils was more for SO4 than for Cl salts. The values of ΔGr were bigger for (1) Cl than for SO4 salts. (2) Na-Ca than for the Na-Mg system, and (3) soil high in organic matter and CEC than for soil low in OM and CEC. The values of KG, KV, and KKDO were bigger for SO4 than for Cl salts. However, when the calculations considered formation of ion pairs, viz., Ca SO4, Mg SO4, and Na SO4, the difference in the values of ΔGr, KG, and KKDO for the Cl and SO4 series was considerably narrowed. None of the exchange selectivity coefficients remained constant throughout the exchange isotherm.
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