INFLUENCE OF AGGREGATE DIAMETER, SURFACE AREA AND ANTECEDENT WATER CONTENT ON THE DISPERSIBILITY OF CLAY

Abstract

The percentages of spontaneously dispersed clay, Ms, and mechanically dispersed clay, Mm, in a suspension of a given ionic strength were hypothesized to be controlled by the specific area of exposed aggregate surfaces and the dispersibility of clay per unit specific aggregate surface area. This hypothesis was evaluated using different sized aggregates collected in 1988 from wheat-fallow and continuous pasture rotations established in 1925 on a red-brown earth in southern Australia. Aggregates which were initially air-dry were wetted to matric water potentials ranging from −10 to −0.3 kPa, then placed in distilled water and Ms and Mm measured. Ms increased, as hypothesized, with increasing surface area of aggregates. However, the spontaneously dispersed clay per unit surface area of aggregates increased with increasing size of aggregates and increasing antecedent soil water content. The effect of water content was greatest in the larger aggregates of the less stable wheat-fallow soil. Mm was approximately 12 times larger than Ms and increased with increasing initial aggregate size and increasing antecedent water content. The sensitivity of Mm to water content was greatest on the least stable soil. Calculations showed that the higher values of Mm on unstable soils were due to both larger exposed aggregate surface areas and higher dispersibility of the clay on these surfaces. Keywords: Soil structure, aggregate stability, dispersion