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.