Subplasticity in Australian soils. V. Factors involved and techniques of dispersion

Abstract

The subplastic soils studied were two from the Riverina area of New South Wales, two montmorillonite rich clays formed on basalt, from South Australia and Queensland, and a krasnozem from New South Wales. To assess the effectiveness of physical and chemical methods of dispersion, theoretical clay contents were calculated from the ratio of the CEC of the soil to that of separated clay. The composition of the clay from the soils showed little or no change with degree of dispersion. To disperse the soils without chemical pretreatment, a method of disaggregation was devised that involved vigorous shaking of a soil paste. Following this technique the Riverina soils and the krasnozem yielded almost all their clay. Lithium saturation was the only chemical treatment that aided dispersion of the montmorillonite clays, and this, together with the high tetrahedral lattice charge, suggests that aggregate stability is mainly due to a large electrostatic interaction between clay sheets. Any pretreatments involving the use of sodium hydroxide improved clay yields for the Riverina soils. The data indicated that the loss of subplasticity was accompanied by the solution of clay, suggesting that aggregate stability was due to non-ionic bonds between clay particles, possibly as the result of intergrowth of clay mineral crystals. Organic matter and/or free iron oxide was responsible for cementation of the krasnozem.