Changes in soil physical properties and crop root growth in dense sodic subsoil following incorporation of organic amendments

Measurements of soil physical properties, root growth and the water content in the subsurface layers of a clay Sodosol soil were carried out to determine why the incorporation of organic amendment (20 t/ha) resulted in marked increases in wheat yield in an earlier paper. The incorporation of lucerne or dynamic lifter® pellets at a depth of 30–40 cm resulted in an almost doubling of the macroporosity from <10% to >18%, together with reductions in bulk density and the volumetric water content (θvθv) at −1500 kPa, and a 50-fold increase on saturated hydraulic conductivity in this subsurface layer. These changes in physical properties in the 30–40 cm deep layer were highly correlated (r values 0.69–0.93, P < 0.01) with increased root growth in this layer, and increases in crop yield. The practice of incorporating an organic amendment in the top clay layer of the B horizon in soils where the high density restricts root growth, which is termed ‘subsoil manuring’, shows promise for increasing crop productivity on these soils in the high rainfall zone of southern Australia.