Soil water behaviour in response to changes in soil structure

Surface horizons of two Australian alfisols which had been cropped for 3 years to wheat by zero, minimum and ploughed tillage were compared for differences in structure. Total porosities and pore size distributions differed between treatments, but values for water and air permeability, sorptivity, diffusivity and evaporation rate were not necessarily ranked in the same order. The stability of soil structure was usefully described by the ratio of water to air permeability (k_w/k_a), which indicated the relatively fragile nature of the ploughed structures, despite their initially greater proportion of coarse porosity. Time of sampling after seeding also influenced hydraulic properties which were found to vary significantly over a 10-week period. Variations in vertical distribution of organic matter between tillage treatments is postulated as influencing the differences in structural stability.