SODIUM-CALCIUM AND SODIUM-MAGNESIUM EXCHANGE EQUILIBRIA IN SOIL FOR CHLORIDE- AND SULFATE-DOMINATED SYSTEMS

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Abstract
The effect of Cl- and SO42- ions on Na+-Ca2+ and Na+-Mg2+ exchange equilibria in 3 soils that differed in OM [organic matter] clay content and CEC [cation exchange capacity] was investigated. Homoionic (NA+-saturated) soils were equilibrated with Cl- and SO42- salt solutions of Na+ + Ca2+ and Na+ + Mg2+ at 25 meq l-1 total electrolyte concentration. The exchange equilibria data were analyzed, using the thermodynamic approach and Gapon''s Vanselow''s and Krishnamoorthy-Davis-Overstreet''s (KG, KV and KKDO) selectivity coefficients. At a given equivalent fraction of Na+ and SAR [sodium adsorption ratio] in the equilibrium solution, sodification of soils was more for SO42- than for Cl- salts. The values of .DELTA.Gr0 were bigger for Cl- than for SO42- salts, Na+-Ca2+ than for the Na+-Mg2+ system and soil high in OM and CEC than for soil low in OM and CEC. The values of KG, KV and KKDO were bigger for SO42- than for Cl- salts. However, when the calculations considered formation of ion pairs, viz., Ca SO40, Mg SO40, and Na SO4-, the difference in the values of .DELTA.Gr0, KG, and KKDO for the Cl- and SO42- series was considerably narrowed. None of the exchange selectivity coefficients remained constant throughout the exchange isotherm.

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