Increased profile wettability in texture-contrast soils from clay delving: case studies in South Australia

Clay delving is becoming a popular practice to increase productivity of texture-contrast soils in southern Australia. The practice brings subsoil clay to the surface to be mixed with the sandy topsoil, and unlike clay spreading, it combines the addition of hydrophilic material with a ripping effect that disrupts the sharp boundary between the sandy topsoil and clayey subsoil. Our objective was to evaluate the magnitude of effects caused by delving on the spatial distribution of water through the profile for three Sodosols (Stagnic Solonetz soils) in the south-east of South Australia. We also wished to evaluate the extent to which clay delving might reduce water ponding at the A-B horizon interface. We wetted both delved and undelved texture-contrast soils with a Brilliant Blue dye solution under initially dry and wet conditions (to evaluate the effect of antecedent water content), and then took digital images of the stained profiles for quantitative comparison of the wetted areas. The stained soil profiles indicated that clay delving reduced preferential water flow (finger flow) and resulted in deeper and more uniform wetting of the A horizon, particularly under initially dry conditions. Under wet conditions (where water repellence was largely overcome), finger flow was significantly reduced regardless of delving but it still occurred to varying degrees depending on site characteristics. Delving significantly reduced ponding of water at the A-B horizon boundary and allowed greater penetration into the B horizon. At all sites, greater effects occurred directly on the delving lines and diminished with distance, implying that closer spacing of delving tines would increase uniformity of wetting throughout the profile. The effectiveness of delving on profile wetting was highly variable across the three sites, indicating that the outcome depends inter alia on the intrinsic soil characteristics and the delving equipment used in the field.