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Sodium-Calcium-Magnesium Exchange Reactions on a Montmorillonitic Soil: III. Calcium-Magnesium Exchange Selectivity¹

ABSTRACT

The Vanselow selectivity coefficient was calculated for Mg Ca exchange on a montmorillonitic soil separate containing organic matter. Seven sets of cation exchange data were analyzed numerically, encompassing pH values ranging from 5 to 7 in a 50 mol m^–3 perchlorate medium and exchangeable sodium percentages ranging from 0 to 25%. The selectivity coefficient for Mg Ca exchange was observed to decrease from a value near 1.5 at 0% Ca saturation to a value near 1.0 at 100% Ca saturation with no effect of pH on this trend in the range examined. At low Ca saturation of the soil, the selectivity coefficient increased by about 15% as the ESP increased from 0 to 25%. This behavior was shown through a model calculation to be consistent with a mechanism in which Na adsorbs preferentially on the montmorillonitic component of the exchange complex in the soil, leaving the nonmontmorillonitic portion of the exchange complex to adsorb Ca selectively over Mg.

¹ Joint contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, CA 92521 and the Soil Science Laboratory, Parks Road, Univ. of Oxford, Oxford 0X1 3PF, Great Britain.

² Professor of Soil Science and Postdoctoral Research Associate, respectively.

Received for publication December 17, 1984. Accepted for publication April 8, 1985.

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