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Sodium-Calcium Exchange Behavior in Organic Soils¹

ABSTRACT

The exchange complex of two organic soils from the Sacramento-San Joaquin delta of California was easily saturated with calcium. However, the Ca²⁺ of the calcium-saturated soil was difficult to replace with Na⁺ even when the calcium-saturated soils were leached with 1N NaCl. Exchange of the Ca²⁺ by Na⁺ was found to be reasonably stoichiometric and complete after extensive leaching of the calcium-saturated soils with the 1N NaCl solution. A large fraction of the adsorbed Ca²⁺ was extremely difficult to replace with Na⁺ when the soils were leached with large volumes of 0.01N NaCl.

The Ca-Na ion-exchange isotherms of the soils were determined through the use of Vanselow's and Gapon's ion-exchange equations. Vanselow's ion-exchange "constant" indicated an increased selectivity for Ca²⁺ over Na⁺ as the amount of exchangeable Na⁺ increased while Gapon's equation yielded a constant selectivity throughout the isotherms. Both equations suggested that the soils exhibit a very large selectivity for Ca²⁺ over Na⁺, especially as the amount of exchangeable sodium becomes appreciable.

¹ Contribution from the Department of Soils & Plant Nutrition, University of California, Berkeley. Part of a thesis submitted by the senior author in partial fulfillment of the requirements of the Ph.D. degree.

² Former Research Assistant, University of California, now Assistant Professor of Soils, Department of Agr. Biochem. & Soils, University of Idaho, Moscow; and Professor of Soil Chemistry, deceased, University of California, respectively.

Received for publication March 6, 1969. Accepted for publication March 23, 1969.