THERMODYNAMIC CHARACTERIZATION OF CA-NA EXCHANGE IN VERTISOLS AND ASSOCIATED ZONAL SOILS

Thermodynamic parameters of Ca-Na exchange were derived for chernozems , cinnamonic soils, and associated Vertisols on the basis of ion-excha nge reaction simulation. An increase in the soil selectivity for Ca wa s found, when the share of smectites in clay decreased and the content of organic matter increased. Vertisols exhibited a lower affinity for Ca because of the lower content of clay-bonded organic matter. A shar p increase in affinity for Na was found for all sails at low fractions of exchangeable Na.