Heterovalent cation exchange in two savanna soils of different lithogenic origin as described by the Rothmund–Kornfeld model

Prediction of cation selectivity is imperative for soil fertility management and disposal of metal-polluted wastes through the land. Predictive thermodynamic approaches based on the Vanselow–Argesinger and Gaines and Thomas conventions require complex expressions. We examined the two-parameter Rothmund–Kornfeld model for accessing thermodynamic equilibrium constant (Kex) for K → Ca and K → Mg exchange in two soils formed on Basement Complex (BC) rocks and sandstones (SST) in the Nigerian savanna. The Kex values based on Vanselow–Argesinger convention for K → Mg exchange (0.112–0.182 mol kg− 1) and K → Ca exchange (0.069–0.103 mol kg− 1) were in close agreement with Gaines and Thomas convention (0.115–0.183 mol kg− 1 for K → Mg and 0.072–0.103 mol kg− 1 for K → Ca exchange). Applying the Rothmund–Kornfeld approach to either Vanselow–Argesinger or Gaines and Thomas convention gave Kex values close to those based on classical Vanselow–Argesinger approach (0.130–0.156 mol kg− 1 for K → Mg and 0.075–0.106 mol kg− 1 for K → Ca) and the Gaines and Thomas convention (0.134–0.146 mol kg− 1 for K → Mg and 0.082–0.144 mol kg− 1 for K → Ca exchange). Regardless of lithology, the Rothmund–Kornfeld model sufficiently represented the K → Mg and K → Ca exchange isotherms of the soils, and its two parameters adequately predicted equilibrium exchange constant. Evidence from Kex and the positive change in free energy of exchange (ΔGexo) indicated non-preference for Ca2+ and Mg2+ to K+ in both soils, suggesting prediction of cationic selectivity and fertility can be extrapolated from one soil to the other in the savanna.