Potassium–calcium and potassium–magnesium exchange equilibria in an acid savanna soil from northern Nigeria

Cation exchange reactions affect ionic composition of soil solution from which plants draw their nutrients. With routine potassium (K+) fertilization of savanna soils, there is little or no information on K exchange equilibria of the soils. We studied Ca–K and Mg–K exchange reactions of a surface (0–15 cm) and sub-surface layer (15–30 cm) of an acid savanna soil with a view to providing baseline data for the efficient management of Ca2+ and Mg2+ fertility without inducing their deficiencies through unfavourable cationic ratios. Calculated selectivity coefficients consisting of the Vanselow (KV), Gapon (KG) and Gaines and Thomas (KGT) selectivity coefficients were greater than one across a wide range of exchanger phase composition, indicating a preference for K+ to Ca2+ and Mg2+. The plots of the charge fraction of adsorbed K+ (EK(exch)) versus the charge fraction of K+ in aqueous solution (EK(aq)) showed that the experimental points were above the non-preference isotherm line, indicating a marked selectivity of K+ over Ca2+ or Mg2+. The thermodynamic exchange constant (Kex) for the 0–15 cm layer was 4.62 for Mg–K, and 4.31 for Ca–K, while in the 15–30 cm layer Kex was = 2.75 for Mg–K and 2.67 for Ca–K exchange. The free energy of exchange (ΔGex ), for Mg–K system was between − 3.79 and − 2.51 kJ mol− 1, while in the Ca–K system, it varied from − 3.62 kJ mol− 1 in the 0–15 cm to − 2.43 kJ mol− 1 for the 15–30 cm layer. The negative ΔGex values were consistent with preferential K+ retention and affinity by this savanna soil.