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Heterovalent Cation Exchange Equilibria in Soils with Variable and Heterogeneous Charge¹

ABSTRACT

Divalent cations were incrementally displaced, from limed and unlimed samples of three soils, by suspending the soils in increasing volumes of 2mM KCl. Mass action coefficients for exchange between Ca and Mg were constant. But for the exchange of divalent cations by K, the exchange coefficients of Kerr, Vanselow, Gapon, Krishnamoorthy and Overstreet, and Gaines and Thomas increased markedly as potassium saturation or pH increased.

For each cation species, separate fractions which were relatively tightly or relatively loosely adsorbed could be estimated by a simple regression analysis. The loosely adsorbed Ca, which may control Ca availability to plants, could be independently estimated by a leaching procedure. Mass action exchange coefficients for the loosely adsorbed cations were approximately constant under all conditions tested.

The variable charge mobilized by liming differed from the fixed charge in having a slightly higher proportion of sites with high divalent affinity, in an Oxisol, and a much lower proportion in a Vertisol and a Mollisol.

¹ Contribution from the Dep. of Soils and Plant Nutrition, Univ. of California, Davis CA 95616. Supported in part by subcontract under a grant from the Agency for Int. Develop. to the Univ. of Hawaii (AID/TA-C-1207).

² Associate Professor of Soil Science.

Received for publication May 20, 1976. Accepted for publication July 19, 1976.