POTASSIUM‐ALUMINIUM EXCHANGE IN ACID SOILS I. KINETICS

Abstract

Summary A kinetic method is described and investigated for determining exchangeable Al in acid soils using M NH 4 Cl solutions the pH and leaching rate of which was critical for obtaining accurate and reproducible values. Molar concentrations of ammonium acetate displaced at least part of the Al chelated to functional groups in the soil organic matter. The adsorption kinetics of K and Al from 10 −3 M chloride solutions on the NH 4 ‐saturated forms at the original soil pH of these, under leaching conditions similar to the ‘extraction’ method, showed that, when the atom ratio K/A1 > 1 in the equilibrating solution, initially more K was adsorbed than at equilibrium, although Al did not ‘over‐equilibrate’ when K/A1 < 1. At least 24 h were required for attaining equilibrium in K :A1 equilibria work with soils and clays. The kinetics of desorption (extraction method) and adsorption of K and Al obeyed the first‐order and parabolic rate respectively. This is attributed to the large difference in anion concentrations in the two methods.