Deep placement of organic amendments in dense sodic subsoil increases summer fallow efficiency and the use of deep soil water by crops

Physical subsoil constraints limit crop production in many areas of southern Australia. There has been limited success in ameliorating these constraints. A field study commenced in 2005 at two adjacent field sites with and without a 4-year history of grazing lucerne, to determine whether the incorporation of organic (lucerne pellets and dynamic lifter at rates of 10-20 t ha(-1)) and inorganic (gypsum, coarse sand and MAP) amendments into a depth of 30-40 cm improves crop performance on a Sodosol with dense sodic subsoil. We reported previously that the organic amendments increased wheat yield in the first year by 70% above the untreated control. This paper reports the change in soil water dynamics and the performances of a wheat crop in 2006 and a canola crop in 2007. A drought occurred in 2006 with only 55% of the average annual rainfall. The growing season in 2007 was difficult for canola, due to an extended dry period in the spring. However, there was generally more water captured and stored in deeper soil layers during the summer fallow period in both years, in plots treated with organic amendments, compared to control plots, particularly at the non-lucerne site. The application of organic amendments also increased the crop shoot biomass prior to anthesis at both sites, and increased wheat yield by up to 54% at the non-lucerne site in 2006, and the canola yield at both sites in 2007. The residual effect from the incorporation in 2005 of organic amendments in the subsoil, a practice known as subsoil manuring, significantly increased wheat yield at the non-lucerne site in 2006, and canola yield at both sites in 2007. The increases in crop yield were mainly attributed to the use of extra soil water (stored during the summer fallow) at critical growth stages.