A comparison of soil porosity and solute leaching after six years of direct drilling or conventional cultivation

Abstract

The soil porosity, pore size distribution, micro-morphology and the leaching of surface-applied solutes after six years of continuous winter wheat planted by direct drilling (DD) or conventional cultivation (PL) were compared. The total porosity of the PL soil was greater than that of the DD soil throughout most of the cultivated depth. This was mainly due to the greater volume of pores >300 µm diameter in the PL than DD soil. Pores in this size range were, however, more stable in the DD than PL soil, with the differences between tillage treatments being more apparent at the winter than summer sampling occasion. The most common macropores in PL soil were compound packing voids with only a few variably oriented biotic channels (>5 mm) being present. In contrast, the DD soil consisted mainly of a system of vertically or subvertically oriented biotic channels which were frequently linked with planar voids (< 1 mm). The probable differences in tortuosity and continuity of these contrasting pore systems were related to the leaching characteristics observed under the two tillage treatments. The breakthrough curves showed extensive preferential flow under both ponded infiltration and simulated-rainfall input conditions. More rapid leaching through DD than PL soil was observed for both 'equilibrated' and 'non-equilibrated' surface-applied solutes under ponded infiltration conditions. This implied that large pores were more continuous in uncultivated than cultivated soil. In contrast, it is suggested that there was no difference in the continuity of the smaller pores involved in water and solute flow under simulated rainfall since both tillage treatments had similar breakthrough curves under these conditions. More solute was recovered in 1.5 pore volumes of leachate under simulated rainfall than ponded infiltration conditions.