Slaking characteristics of some Australian and British soils

Aggregates (9.5–12.7 mm) from ten soils were equilibrated at a range of matric suctions (Ψ_a) between 1 kPa and 100 MPa before immersion in water or wetting on a porous plate at zero suction. The soils were from cultivated and grassland sites and included hardsetting and non-hardsetting Australian and British soils as well as a Vertisol. The initial rate of wetting of each aggregate, and the composition and size distribution of the slaked fragments were measured. There was a significant inverse linear relation between the amount of slaking produced by plate wetting air-dry soil (Ψ_a=100 MPa) and its organic carbon content (r= 0.82***). The three cultivated hardsetting soils shared several common features. Their slaking was the most pronounced after plate wetting and occurred at the smallest Ψ_a(10 kPa). Their slaking also increased linearly with rate of wetting and the particle-size distribution of their slaked fragments varied significantly and considerably with Ψ_a. This last observation demonstrates that it is not always helpful to call the fragments produced by slaking, microaggregates. Possible explanations for our results and their agricultural implications are discussed.