ADSORBED ION ACTIVITY-COEFFICIENTS IN K-CA EXCHANGE ON SOIL FRACTIONSDERIVED FROM VOLCANIC MATERIALS

yThe effects of temperature and iron oxide coatings on the adsorbed io n activity coefficients obtained by the Gaines-Thomas treatment for th e K-Ca exchange equilibria on clay fractions of 3 Chilean Andisols wer e studied. The comparison with the values calculated through the Vanse low approach, using conversion equations, was also considered. Adsorbe d Ca activity coefficient curves present a similar shape for the Gaine s-Thomas and Vanselow approaches; for K, maxima were observed only whe n the Gaines-Thomas treatment was used. Iron oxide coated samples show ed higher adsorbed ion activity coefficients than allophanic samples. The adsorbed K activity coefficients increased when temperature increa sed from 4 degrees C to 26.5 degrees C, but only a slight effect on Ca ions was observed. In both types of samples, the presence of 3 possib le selective sites was detected: K-selective sites, Ca-selective sites , and sites where K-Ca competition occurs.