Salt sieving within clay soil aggregates

Abstract

The rates at which salt diffuses from the stable non-swelling aggregates of a heavy clay soil are studied in relation to both exchangeable cation type and free electrolyte content. Although the material is completely stable, it exposes a large clay surface of normal 'activity'. The results support a hypothesis that salt is constrained in some pores by salt-sieving within the aggregate; regions of anion exclusion, associated with surfaces in the narrower pores, restrict the cross section for diffusion from larger interior pores. It is shown that, when circumstances favour negative adsorption, that is with sodium cations and salt of relatively low concentration, salt is trapped within the microfabric; with calcium ions and salt of high concentration anion exclusion is suppressed and salt moves out of the aggregates much more easily. When the aggregates are in a saline sodic condition salt diffuses from them into saturated gypsum solution much more rapidly than it does into distilled water. This is in accord with the suggested model and implies that the ameliorative use of gypsum may involve a mechanism additional to those associated with swelling and flocculation. Application of the concept to soil in the field is briefly discussed.