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 (KV), Gaines and Thomas (KGT), Davies (KD), and the regular solution (KRS) equations. The selectivity coefficients for the Ca2+/NH4+ exchange reactions varied widely with the soil exchanger composition except for the relatively stable KRS. The selectivity coefficients indicated strong preference for NH4+ to Ca2+. The thermodynamic exchange constant, Kex, was 5.75 ± 1.24 in the Pellustert, indicating preferential adsorption of NH4+, but not in the Haplustalf with Kex = 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.