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 r ainfall 6 weeks before sowing made soil conditions unfavourable for ea rly growth of oil-seed rape. This resulted in differences in the size, vigour and population of young plants both between treatments and bet ween replicates. The crop was better under conventional mouldboard plo ughing and drilling than under either long-term (21 years) or medium-t erm (6 years) direct drilling. Under direct drilling the crop was poor est in wet and compact replicates. We attempted to identify the soil p hysical and structural properties which were impairing crop growth. Th e experimental site was located on a Cambisol and a Gleysol in south-e ast Scotland. Crop size and vigour were related to visual estimates of field soil structure and to soil transport properties and macroporosi ties. Infiltration rates were less than 1 mm min(-1) and air permeabil ities of the top 50 mm of soil were less than 10 mu m(2) (at -6 kPa ma tric potential) in direct drilled areas of low crop vigour. Structural indices were derived from air permeabilities and gas diffusion measur ements made in soil cores. These were pore organisation (from air perm eability), and pore continuity and the likely dead-end porosity (from gas diffusion). All of these structural indices related well to crop r esponses. Soil structures favourable to the crop (ploughed and some lo ng-term direct drilled plots) contained well-interconnected macroporos ity greater than 0.1 m(3) m(-3). Less favourable structure, confined m ainly to the medium-term direct drilled plots, contained macroporosity less than 0.1 m(3) m(-3), with few channels and a significant proport ion of dead-end pores.