CROP PERFORMANCE AND SOIL-CONDITIONS ON IMPERFECTLY DRAINED LOAMS AFTER 20-25 YEARS OF CONVENTIONAL TILLAGE OR DIRECT DRILLING

Crop yield and soil conditions under four different tillage regimes were monitored over five growing seasons from 1986/1987 to 1990/1991 as part of a long-running investigation into reduced tillage effects. Two long-term treatments (direct drilling and conventional mouldboard ploughing, dating from 1968) were compared with two begun in 1983: a shortterm direct drilled treatment and a treatment consisting of three seasons of broadcast sowing plus rotovation and one season of mouldboard ploughing and straw incorporation. Autumn and spring nitrogen treatments were also compared. The crop was winter barley except in 1988/1989 when it was oil-seed rape. Straw was removed, often incompletely, by a mixture of baling, burning and raking, except in the straw incorporation treatment where it was chopped by the harvester. The experimental site is located on a Cambisol (15% clay in the topsoil) and a Gleysol (17% clay in the topsoil) in southeast Scotland. For winter barley, grain yields under long- and short-term direct drilling were comparable but were lower than those under conventional ploughing and drilling mainly because of problems associated with straw residues, grass weeds and seedbed compaction. Yield responses to nitrogen were not consistently related to tillage, although uptake of nitrogen in the grain was frequently least in the direct drilled treatments. soil aeration, strength and structure were more favourable under ploughing than under direct drilling. Bulk density and soil strength did not show any long-term progressive changes in the long-term direct drilled treatment. Weather and drainage status varied markedly between seasons and determined the number of available workdays during the harvesting and tillage period. Available workdays influenced crop responses to reduced tillage more than soil type or physical condition. In some seasons workdays were insufficient to permit the high standard of management necessary for successful reduced tillage, especially direct drilling. Due to this reduction in management opportunities some direct drilled plots became so infested with soft brome (Bromus mollis L.) that oil-seed rape was grown in 1988/1989 as a break crop.