Rate processes of calcium, magnesium and potassium desorption from variable-charge soils by mixed ion-exchange resins

There is growing incidence of calcium (Ca), magnesium (Mg) and potassium (K) deficiencies in tropical soils dominated by variable-charge colloids. Management strategy to improve cationic balance and fertility of soils requires kinetic data to predict their release, and analytical methodology to assess phytoavailability. This study examines the rate processes and energy relations governing the simultaneous release of Ca, K and Mg from variable charge soils to ion-exchange resins. The desorption of Ca, K and Mg was adequately described by an apparent first-order rate, a film diffusion equation and an Elovich equation. Agitation of soil samples increased desorption rate two to threefolds, suggesting that film diffusion rather than chemical exchange reaction was rate-limiting. The magnitude of activation energy (E-a) of desorption further supported a film diffusion mechanism. Compared to reports from temperate soils, the free energy Delta G* and enthalpy Delta H* of activation for desorbing Ca, Mg and K were lour. The low E-a and Delta G* suggested a more rapid release of Ca, Mg and K to ion-exchange resins from tropical soils than temperate soils probably because of the pre-ponderance of low-activity clays which have no interlayer binding sites for cations. The trend in Delta H* and Delta S* for desorption of Ca, Mg and K from the soils by the mixed resins seemed to indicate stronger binding energy for K than Ca and Mg. (C) 1999 Elsevier Science B.V. All rights reserved.