Potassium-Calcium and Sodium-Calcium Exchange on Kaolinite and Kaolinitic Soils
- G. J. Levy ,
- H. v. H. van der Watt,
- I. Shainberg and
- H. M. du Plessis
SIRI, Private Bag X79, Pretoria 0001, Republic of South Africa (RSA)
Dep. of Soil Sci. and Plant Nutrition, Univ. of Pretoria, Pretoria 0002, RSA
Inst. of Soil and Water, Agricultural Research Organization, The Volcani Ctr., Bet-Dagan, Israel
Water Research Commission, P.O. Box 824, Pretoria 0001, RSA
Potassium-Ca and Na-Ca exchange isotherms were obtained for two kaolinite samples and two kaolinitic soils (Hutton and Avalon forms). The Gapon, Vanselow, and Gaines-Thomas corrected selectivity coefficients were usually greater for K-Ca exchange than for Na-Ca exchange, as was expected. The exchange isotherms and selectivity coefficients suggested a preference for K relative to Ca on all four exchangers. The contribution of micaceous impurities to the observed high affinity for K was taken into consideration, by reasoning that exchange sites with high affinity for K resulting from these impurities are first saturated with K [i.e., at low exchangeable potassium percentage (EPP) values]. It is suggested that the high charge density of kaolinite enhances K dehydration, resulting in a high affinity of kaolinite per se for K.
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