Cation transport during unsaturated flow through two intact soils

Abstract

SummaryIn the quest for better understanding of cation movement through undisturbed soils, leaching experiments on 300‐mm long undisturbed soil columns of two contrasting soils were carried out. One soil was a weakly‐structured alluvial fine sandy loam, the other a well‐structured aeolian silt loam. About 2000 mm of solutions of MgCl2 and Ca(NO3)2 of 0·025 M were applied at unsaturated water flow rates of between 3 and 13 mm h−1. Solute movement was monitored over several weeks by collecting effluent under suction at the base. In the sandy loam anion transport was influenced by exclusion from the double layer, whereas in the Ramiha soil anion adsorption occurred. Cation transport was described by coupling the convection‐dispersion equation with cation exchange equations. Good simulations of the Mg2+ and Ca2+ concentrations in the effluent and on the exchange sites were obtained if 80% of the exchangeable cations, as measured using the 1 M ammonium acetate method, were assumed to be active. Local physical or chemical disequilibrium did not need to be explicitly taken into account. About 400 kg ha−1 of native potassium was leached from the alluvial soil, but only about 10 kg ha−1 was leached from the aeolian soil. The convection‐dispersion equation coupled with exchange theory was found to describe cation transport under unsaturated flow through undisturbed soil satisfactorily.