Sodium‐Calcium Exchange Behavior in Organic Soils

Abstract

Abstract The exchange complex of two organic soils from the Sacramento‐San Joaquin delta of California was easily saturated with calcium. However, the Ca 2+ of the calcium‐saturated soil was difficult to replace with Na + even when the calcium‐saturated soils were leached with 1 N NaCl. Exchange of the Ca 2+ by Na + was found to be reasonably stoichiometric and complete after extensive leaching of the calcium‐saturated soils with the 1 N NaCl solution. A large fraction of the adsorbed Ca 2+ was extremely difficult to replace with Na + when the soils were leached with large volumes of 0.01 N 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 Ca 2+ 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 Ca 2+ over Na + , especially as the amount of exchangeable sodium becomes appreciable.