CATION-TRANSPORT DURING UNSATURATED FLOW-THROUGH 2 INTACT SOILS

In the quest for better understanding of cation movement through undis turbed soils, leaching experiments on 300-mm long undisturbed soil col umns of two contrasting soils were carried out. One soil was a weakly- structured alluvial fine sandy loam, the other a well-structured aeoli an 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 collect ing effluent under suction at the base. In the sandy loam anion transp ort 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 eq uations. Good simulations of the Mg2+ and Ca2+ concentrations in the e ffluent and on the exchange sites were obtained if 80% of the exchange able cations, as measured using the 1 M ammonium acetate method, were assumed to be active. Local physical or chemical disequilibrium did no t need to be explicitly taken into account. About 400 kg ha(-1) of nat ive potassium was leached from the alluvial soil, but only about 10 kg ha(-1) was leached from the aeolian soil. The convection-dispersion e quation coupled with exchange theory was found to describe cation tran sport under unsaturated how through undisturbed soil satisfactorily.