EFFECTS OF UNIAXIAL COMPACTION ON AERATION AND STRUCTURE OF PLOWED OR DIRECT DRILLED SOILS

Cores of intact soil and of soil aggregates in the field condition were compacted uniaxially in the laboratory in a series of progressive increments of stress. Before compaction and after each increment of stress, air-filled porosity, relative diffusivity, diffusion time delay and air permeability were measured. Pore continuity and pore organisation indices were calculated from these measurements. Diffusion time delay is a measure of the time taken for gas to diffuse through a core from the start of a diffusion measurement. During compaction relatively small changes in air-filled porosity were accompanied by large changes in relative diffusivity and air permeability. After the application of a given stress, these properties decreased less in direct drilled than in ploughed soil, particularly when wet. This was associated with the lower compactibility of the long-term direct drilled treatment as a result of improvement in soil structure associated with organic matter accumulation. Stresses of only 77 kPa were sufficient to reduce permeability and relative diffusivity values to zero in wet, intact ploughed cores. Pore continuity was lower under direct drilling than under ploughing, but was less affected by compaction. The direct drilled soil probably contained more dead-end pores than the ploughed soil. This dead-end porosity was less affected by applied stress than pore continuity.