On the nature of soil aggregate coalescence in an irrigated swelling clay

Aggregate coalescence in irrigated cracking clays constrains crop yields, yet little is known about it or how it can be managed. A measure of coalescence is introduced to separate the effects of natural aggregate-bed densification from those of age-hardening; this measure,, comprises a ratio of the net change in (tensile or penetrometer) strength, Y, that occurs in relation to the corresponding net change in dry bulk density, rho (b), as follows:chi = Y/Delta rho (b). A laboratory study was conducted to illustrate the variation in for a virgin and cultivated cracking clay exposed to 16 weekly cycles of wetting and draining. Penetrometer resistance and tensile strength at -100 kPa, plus bulk density and other physical and chemical properties, were measured throughout the experiment. The cultivated soil rapidly became denser and stronger, it developed larger aggregates, and its water-uptake rate in the air-dry state was significantly greater than that for the virgin soil. The values suggested that age-hardening processes constituted a greater component of coalescence in the cultivated soil than it did in the virgin one, and this was thought to be mediated by the large differences in the content and composition of organic matter in the two soils.