Calcium-Ammonium Selectivity of Two Benchmark Soils from Botswana as Assessed by Competing Semi-empirical Ion Exchange Equations

Abstract

The effectiveness of lime-ammonium-nitrate (LAN) as a nitrogen (N) fertilizer in weathered soils depends on the respective selectivity for ammonium (NH₄) and calcium (Ca) by the soils. The study assessed Ca²⁺/NH₄⁺ exchange selectivity of two benchmark soils from Botswana and examined the soil fertility management implications. Surface horizons (0–20 cm) of Pellustert and Haplustalf were equilibrated with 50 ml stock solution containing variable concentrations of Ca²⁺ and NH₄⁺. The Ca²⁺/NH₄⁺ exchange data were fitted into the Vanselow (K_V), Gaines and Thomas (K_GT), Davies (K_D), and the regular solution (K_RS) equations. The selectivity coefficients for the Ca²⁺/NH₄⁺ exchange reactions varied widely with the soil exchanger composition except for the relatively stable K_RS. The selectivity coefficients indicated strong preference for NH₄⁺ to Ca²⁺. The thermodynamic exchange constant, K_ex, was 5.75 ± 1.24 in the Pellustert, indicating preferential adsorption of NH₄⁺, but not in the Haplustalf with K_ex = 0.92 ± 0.27. The free energy for Ca²⁺/NH₄⁺ exchange (ΔG°_ex) was negative (−4.26 ± 0.59 kJ mol⁻¹) in the Pellustert but slightly positive in the Haplustalf (0.34 ± 0.87 kJ mol⁻¹). In conclusion, the soil-NH₄ complex was more stable than soil-Ca complex in the Pellustert, indicating LAN as a N fertilizer would have greater potential effectiveness in the Pellustert than in the Haplustalf.

Keywords:

• Change in free energy of exchange (ΔG°_ex)
• equilibrium exchange constant (K_ex)
• ion exchange
• regular solution model coefficient (K_RS)
• selectivity coefficients

Acknowledgments

We thank Botswana College of Agriculture for permission to use laboratory facilities and S. M. Machacha for providing the soil samples for this study.