Thermodynamics of Potassium–Magnesium Exchange in Two Alfisols of Northern Greece

Abstract

The exchange behavior of potassium (K) in soils has been extensively studied relative to a dominant cation, which in most cases was considered to be calcium (Ca). Magnesium (Mg) was tacitly assumed that existed in minor quantities or that it had the same exchange behavior with Ca. However, under certain conditions and practices (Mg-rich soils, greenhouse crops) Mg can become an important component of the soil exchange complex participating drastically in the exchange reactions. Consequently, the exchange behavior of the pair K–Mg has merit and in this study the thermodynamics of exchange for this pair were evaluated in two Alfisols with different mineralogical composition of the clay fraction. The exchange reaction was studied in both directions, namely, Mgsoil→Ksoil and Ksoil→Mgsoil. Comparison of the experimental exchange isotherms to the non-preference isotherms revealed that both soils exhibited preference for K relative to Mg. Standard free energies of exchange (ΔG0) for the reaction Mgsoil→Ksoil were negative (−5.7 to −6.7 kJ mol−1) indicating the spontaneity of the reaction and that the formation of a K-soil is favored. Also, ΔG0 values for the reaction Ksoil→Mgsoil were positive (7.3 to 7.5 kJ mol−1) which indicated the reluctance of K-soil to be converted to Mg-soil, thus corroborating the previous findings. Standard enthalpies of exchange (ΔH0) were negative (−8.7 to −13.5 kJ mol−1) for the reaction Mgsoil→Ksoil and positive (13.8 to 15.1 kJ mol−1) for the backward reaction suggesting a strong binding of K with some sites of the soils. Vanselow selectivity coefficients (KV) showed a stronger preference for K in the soil where mica-type minerals dominated the clay fraction. Selectivity for K, as judged by the KV values, was also strongly K-saturation dependent; the lower the degree of K saturation the higher the selectivity for K. Under the conditions of the experiment the exchange reaction of the pair K–Mg was not reversible but exhibited hysteresis, the magnitude of which was more pronounced in the soil where mica-type minerals were prevalent in the clay fraction.