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. Core s from below the plough layer were equilibrated at a range of matric p otentials 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, rel ative diffusivity, diffusion time delay and air permeability were meas ured at each potential, and pore continuity and pore organisation indi ces were calculated. Diffusion time delay is a measure of the time tak en 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 wettin g to -2 kPa from - 10 kPa than after drainage to -2 kPa from saturatio n. 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 ef fects were partly attributed to the sampling method but gave evidence of greater vertical than horizontal orientation of macropores in ploug hed soil and greater horizontal than vertical orientation of macropore s in direct drilled soil.