SOIL STRUCTURAL AND TRANSPORT-PROPERTIES ASSOCIATED WITH POOR GROWTH OF OILSEED RAPE IN SOIL DIRECT DRILLED WHEN WET

In a long-term tillage experiment on soil of impeded drainage, heavy rainfall 6 weeks before sowing made soil conditions unfavourable for early growth of oil-seed rape. This resulted in differences in the size, vigour and population of young plants both between treatments and between replicates. The crop was better under conventional mouldboard ploughing and drilling than under either long-term (21 years) or medium-term (6 years) direct drilling. Under direct drilling the crop was poorest in wet and compact replicates. We attempted to identify the soil physical and structural properties which were impairing crop growth. The experimental site was located on a Cambisol and a Gleysol in south-east Scotland. Crop size and vigour were related to visual estimates of field soil structure and to soil transport properties and macroporosities. Infiltration rates were less than 1 mm min(-1) and air permeabilities of the top 50 mm of soil were less than 10 mu m(2) (at -6 kPa matric potential) in direct drilled areas of low crop vigour. Structural indices were derived from air permeabilities and gas diffusion measurements made in soil cores. These were pore organisation (from air permeability), and pore continuity and the likely dead-end porosity (from gas diffusion). All of these structural indices related well to crop responses. Soil structures favourable to the crop (ploughed and some long-term direct drilled plots) contained well-interconnected macroporosity greater than 0.1 m(3) m(-3). Less favourable structure, confined mainly to the medium-term direct drilled plots, contained macroporosity less than 0.1 m(3) m(-3), with few channels and a significant proportion of dead-end pores.