EFFECTS OF SOIL-WATER HYSTERESIS AND THE DIRECTION OF SAMPLING ON AERATION AND PORE FUNCTION IN RELATION TO SOIL COMPACTION AND TILLAGE

Cores of intact soil were taken in vertical and horizontal directions in two tillage experiments from horizons subjected to compaction. Cores from below the plough layer were equilibrated at a range of matric potentials by drainage. Cores from near the soil surface were subjected to hysteresis by drainage to -2 kPa from saturation, further drainage to -10 kPa and finally re-wetting to -2 kPa. Air-filled porosity, relative diffusivity, diffusion time delay and air permeability were measured at each potential, and pore continuity and pore organisation indices were calculated. Diffusion time delay is a measure of the time taken for gas to diffuse accross a core at the beginning of a measurement of diffusion. Hysteresis influenced both air-filled porosity and the relationship between air-filled. porosity and gas diffusivity and air permeability. Soil aeration was generally more favourable after wetting to -2 kPa from - 10 kPa than after drainage to -2 kPa from saturation. Hysteresis effects were greater in ploughed than in direct drilled soils. Direction of sampling influenced diffusion and flow properties, mainly in compact direct drilled soil from near the surface. These effects were partly attributed to the sampling method but gave evidence of greater vertical than horizontal orientation of macropores in ploughed soil and greater horizontal than vertical orientation of macropores in direct drilled soil.