The relationship between aggregation and other soil properties in cracking clay soils

Abstract

This paper explores the processes responsible for clay dispersion, and the formation of large dry aggregates, in cracking clay soils. It also isolates the soil factors causing variations in dry aggregate size using regression analysis. Twelve cracking clay soil samples were selected on visual differences in dry aggregate size distribution following seedbed preparation, and a range of soil structural and chemical properties were measured. The per cent dry aggregates > 5 mm was found to increase with resistance to mechanical abrasion, stability to wet sieving after capillary wetting, and dispersion ratio, indicating that large dry aggregates are formed as a result of binding by dispersed clay. Both raindrop impact and puddling by cultivation may be involved in dispersion. There was a strong relationship between dry aggregate size in the 0-10 cm layer and salt content in the subsurface (60-90 cm) layer. Coarse surface aggregation is explained in terms of limited profile hydraulic conductivity. For the soils studied, the properties of the surface layer appear to be responsible, at least in part, for the limitation in profile hydraulic conductivity. Dry aggregate size in the 0-10 cm layer was not simply correlated with any of the chemical properties of that layer. However, equations containing two (ESP and CEC per gram of clay) or three (ESP, per cent clay and CEC) independent variables were derived to explain variations in dry aggregate size, both for the 12 soils studied and for a wider range of Queensland cracking clay soils.