The comparative effects of exchangeable calcium, magnesium, and sodium on some physical properties of red-brown earth subsoils. II. The spontaneous dispersion of aggregates in water

Abstract

Aggregates from the subsoils of four red-brown earths were brought into equilibrium with chloride solutions of known SAR, 1 mM in calcium or magnesium. The solutions were replaced by water and the amount of clay which subsequently dispersed from the aggregates measured. For the three soils from the same area, the ESP required to induce dispersion was about 3 for magnesium-sodium aggregates compared with 5 for calcium-sodium aggregates. For the other soil the respective ESP values had to be increased by a factor of 2. The increased resistance to dispersion of the latter was maintained even after sodium saturation. The results are used to show that differences in the dispersion of the natural aggregates in water are due primarily to the composition of the cations on the exchange sites, including aluminium. Shepparton aggregates of known ESP were wetted with water at 12 cm suction. Compared with calcium or magnesium aggregates, there was a marked further water uptake by magnesium-sodium aggregates for ESP > 3 and for calcium-sodium aggregates for ESP >9. The subsequent dispersion in water of such aggregates was much less than that of aggregates treated in the standard way. It is suggested that the spontaneous dispersion of clay from a wet aggregate of low ESP immersed in water depends on the gradient of the soluble salt concentration at the boundary of the intact portion of the aggregate, as well as the actual concentration.