Soil strength and soil pore characteristics for direct drilled and ploughed soils

Direct drilling has often been reported to increase density and strength an d to affect pore continuity and tortuosity of the upper soil layers. In thi s study these aspects were studied fur three texturally differing soils 4-6 years after initiation of continuous trials with direct drilling and mould board ploughing. The soils studied were a coarse sandy soil (Korntved, 5% c lay), a sandy Loam (Ballum, 8% clay) and a silty loam (Hojer, 19% clay). Th e crop rotation at Korntved was spring barley and winter rye while at Ballu m and Hojer it was spring barley and winter wheat. Both crops were grown ev ery year. All fields had been mouldboard ploughed. For decades prior to the trial period. The ploughed treatment (PL) was imposed in the autumn and th e seedbed preparation and drilling were performed with an S-tined seedbed h arrow and a traditional drill. The direct drilled (DD) treatment received n o tillage other than the drilling which was performed by a triple-disc dril l. Straw and stubble were burned. In the 4th, 5th and 6th years of the tria l period, minimally disturbed soil cores were taken from the 4 to 8, 14 to 18 and 24 to 28 cm depths, i.e. two horizons above the ploughing depth of 2 0 cm, and one horizon below this depth. Longer cores were sampled in the 18 -27 cm depth in order to include this transition layer. Furthermore, in the 4th year of the trial period sheer strength was measured in the field at 2 -week intervals in the spring with a vane shear tester in the two upper lay ers mentioned. All samplings and measurements took place in the held grown with spring barley. In the laboratory air diffusivity and air permeability were measured at held-sampled water content and again when the soil cores w ere drained to a matric potential of -100 hPa. Cone penetration resistance was measured with a 2 mm diameter penetrometer. Separate core samples from the 14 to 18 cm depth of the Korntved and Hojer soils were used for estimat ing soil cohesion and soil internal friction by a shear annulus method at h eld-sampled water content. For all soils, DD increased soil bulk density in the two upper soil layers. The shear vane tester also generally estimated higher sheer strength For t he DD compared to the PL treatment. The sheer annulus measurements in the l aboratory revealed no differences between tillage treatments for the Korntv ed soil, while a tendency of higher cohesion and internal soil friction was found for the DD treatment on the Hojer soil. The cone penetration measure ments indicated a stronger top-soil and fewer high-strength soil elements i n the 24-28 cm horizon for the DD than for the PL treatment. Generally the DD treatment had lower volume of macropores (i.e. pores > 30 mum) in the 4- 8 and 14-18 cm depths than the PL treatment. This was reflected in reduced air diffusivities and air permeabilities for these horizons. An exception w as the 14-15 cm depth of the Ballum soil. where increased air diffusivity a nd, air permeability was measured at field-sampled water content. Continuit y indices calculated from air diffusivity and air permeability measurements showed that the DD soil from the 4 to 8 and 14 to 18 cm depths had less co ntinuous and more tortuous macropores than the ploughed soil. (C) 2000 Else vier Science B.-V All rights reserved.