Soil structure and the saturated hydraulic conductivity of subsoils

The saturated hydraulic conductivity, Ksat, was measured on soil samples collected from the plough layer and the subsoil. A range of naturally occurring soil bulk densities was obtained by sampling in different years, with different crops and within and without wheel-tracks, etc. It was found that, for the plough layer, quite good linear relationships exist between the logarithm of Ksat and the bulk density. However, for the subsoils, the value of Ksat usually lies above the regression line for found for the corresponding plough layer. This “excess” hydraulic conductivity of subsoils is attributed to the presence of biopores, especially root channels. The lower hydraulic conductivity of the plough layer, relative to the subsoil, is attributed to the destruction of these biopores by tillage. A simple model for the separate contributions of soil texture and root channels to the overall value of Ksat is presented. It is concluded that subsoil tillage could cause significant reductions in Ksat with potentially serious environmental consequences unless it is repeated periodically.