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Potassium-Calcium Exchange Equilibria in Volcanic-Ash Soils¹

ABSTRACT

Potassium-Ca exchange equilibria were determined at potassium adsorption ratios (PAR) of 0.2, 2.0, and 20 for pH values of 4.8, 6.0, and 7.5 for four volcanic-ash soils of southern Chile. Increases in pH produced large increases in CEC which were largely balanced by increases in the amount of Ca adsorbed. The increases in exchangeable K with increase in CEC were relatively small. The relative affinities for K versus Ca at a PAR of 0.2 indicated that for each soil a small, but nearly constant, amount of negative charge had a high specificity for K. Beyond this small amount of CEC with a high affinity for K, the affinities for K were relatively low.

¹ Contribution from the Facultad de Ciencias Quimicas of the Universidad de Chile, Santiago, and from the Dept. of Soil Science and Agricultural Engineering, Univ. of California, Riverside 92502. The support of the Univ. of Chile-Univ. of Calif. Program financed by the Ford Foundation is gratefully acknowledged.

² Professor of Analytical Chemistry, Univ. of Chile; Professor of Soil Science, Univ. of California; and former student, Univ. of Chile, respectively.

Received for publication May 21, 1975. Accepted for publication July 17, 1975.