Potassium→ammonium exchange of two benchmark soils from Botswana and its implication for nitrogen economy of the soils

Ion exchange preferences for NH4+ and K+ by soil exchanger surface can greatly affect the NO3- leaching into groundwater and nitrogen-use efficiency in agricultural production. Since NH4+ and K+ salts are usually applied together as fertilizers, the binary KNH4 exchange of two benchmark Botswana soils, Pellustert and a Haplustalf, was studied to determine the selectivity coefficients and the thermodynamic exchange constant with special reference to N economy. The Vanselow and the Gaines and Thomas coefficients indicated preference for NH4+ by the Pellustert and K by the Haplustalf across the exchanger phase composition. The equilibrium constant (K-ex) was 1.807 for the Pellustert and 0.174 for the Haplustalf. The exchange free energy (G(ex)(0)) was -1.467kJmol(-1) for the Pellustert and 4.334kJmol(-1) for the Haplustalf. Negative G(ex)(0) for the Pellustert is consistent with preference for NH4+ to K+ in contrast to positive G(ex)(0) for the Haplustalf. The greater stability of NH4X than KX complex in the Pellustert, and KX than NH4X in the Haplustalf, would mean increased residence time of NH4+ in the Pellustert than the Haplustalf. The implication of short residence time of NH4+ in soil is rapid nitrification, thereby leading to NO3--N leaching losses and possible groundwater contamination.