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

Abstract

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 K→NH₄ 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 (ΔG_ex⁰) was −1.467 kJ mol⁻¹ for the Pellustert and 4.334 kJ mol⁻¹ for the Haplustalf. Negative ΔG_ex⁰ for the Pellustert is consistent with preference for NH₄⁺ to K⁺ in contrast to positive ΔG_ex⁰ 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.

Keywords:

• ion exchange
• cation selectivity
• exchange equilibrium constant
• free energy

Acknowledgements

We thank the Department of Crop Science and Production, Botswana College of Agriculture, for allowing us to use laboratory facilities for this study and Mr S.M. Machacha for providing the soil samples.