Potential impacts of subsoil constraints on canola productivity in southern NSW

Research investigating declining canola yields in southern NSW identified subsoil constraints as a potential contributing factor. We investigated the impact of subsurface compaction (hardpans), acidity, and/or subsoil sodicity or salinity on canola yields in replicated on-farm experiments undertaken between 2007 and 2009. Treatments included combinations of surface and/or deep applied lime or gypsum, with or without deep-ripping to 25-30cm. After 3 years of experimentation at 7 locations between Greenethorpe north of Young and Corowa near the Victorian border that differed in underlying subsoil constraints, we concluded that canola was surprisingly tolerant to most subsoil constraints. Specifically: (a) Canola would not be expected to respond to deep-ripping to remove subsurface compaction where compaction was determined by measures of penetrometer resistance to be < 3 MPa at field capacity. Above 3 MPa crop responses may be possible; however, the economic viability of deep-ripping will depend upon seasonal conditions and whether a residual value persists over several years; (b) Canola appeared to be relatively tolerant of subsurface acidity where the surface soil was limed, except where exchangeable aluminium exceeded 20%, concentrations of manganese were toxic, or where the acid 'throttle' was thicker than 20cm deep; (c) Canola would not be expected to respond to deep placement of gypsum to ameliorate subsoil sodicity unless subsoil exchangeable sodium levels were >15% and the growing season rainfall exceeded 400mm; (d) Canola rooting depth and yield were sensitive to subsoil salinity, but effects were masked in a favourable season. Electro-magnetic (EM) surveys combined with diagnostic soil sampling can identify saline zones where canola should not be grown.