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

Summary

Direct drilling has often been reported to increase density and strength and to affect pore continuity and tortuosity of the upper soil layers. In this study these aspects were studied for three texturally differing soils 4–6 years after initiation of continuous trials with direct drilling and mouldboard ploughing. The soils studied were a coarse sandy soil (Korntved, 5% clay), a sandy loam (Ballum, 8% clay) and a silty loam (Højer, 19% clay). The crop rotation at Korntved was spring barley and winter rye while at Ballum and Højer it was spring barley and winter wheat. Both crops were grown every year. All fields had been mouldboard ploughed for decades prior to the trial period. The ploughed treatment (PL) was imposed in the autumn and the seedbed preparation and drilling were performed with an S-tined seedbed harrow and a traditional drill. The direct drilled (DD) treatment received no tillage other than the drilling which was performed by a triple-disc drill. Straw and stubble were burned. In the 4th, 5th and 6th years of the trial 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 20 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 shear strength was measured in the field at 2-week intervals in the spring with a vane shear tester in the two upper layers mentioned. All samplings and measurements took place in the field grown with spring barley. In the laboratory air diffusivity and air permeability were measured at field-sampled water content and again when the soil cores were 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 Højer soils were used for estimating soil cohesion and soil internal friction by a shear annulus method at field-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 shear strength for the DD compared to the PL treatment. The shear annulus measurements in the laboratory revealed no differences between tillage treatments for the Korntved soil, while a tendency of higher cohesion and internal soil friction was found for the DD treatment on the Højer soil. The cone penetration measurements indicated a stronger top-soil and fewer high-strength soil elements in 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 m) 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 was the 14–18 cm depth of the Ballum soil, where increased air diffusivity and air permeability was measured at field-sampled water content. Continuity 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 continuous and more tortuous macropores than the ploughed soil.