Effect of gypsum and deep ripping on lucerne (Medicago sativa L.) yields on a red-brown earth under flood and spray irrigation

Abstract

Lucerne yield and soil responses to gypsum and deep ripping treatments were studied under flood and spray irrigation on 2 field experiments on a heavy red-brown earth. One application of gypsum at 12 t ha-1, both alone (G) and in combination with ripping to 0.6 m depth (GR) before sowing, resulted in sustained high yields of lucerne during a 4- year experiment. Under flood irrigation. there was no apparent yield advantage of GR treatments over G alone; yields averaged over 3 full production years were 143 and 175 t ha-1 for the control and the mean of the G and GR treatments, respectively. However, under spray irrigation, the mean annual hay yields for the control, G and GR treatments of 12.6, 17.3 and 19.4 t ha-1, respectively, were significantly different (P = 0.05). Yield was increased in proportion to the additional water made available to the crop resulting from deeper wetting of the ameliorated soils at irrigation. Increased water intake was attributed to increased conductance of the B horizon for water due to gypsum and/or deep ripping. Under flood irrigation. macroporosities measured at 0.30 m depth 4 years after treatment were higher in the ameliorated soils than the control but were no different between the G and GR treatments. In the spray experiment, the GR soils had consistently higher porosities and lower bulk densities than the G soils; the latter were no different to the control in these properties. The increased conductance is attributed to reduced clay swelling and the increased macroporosity of the B horizon due to ripping. Where measured on the flood experiment, the GR treatment promoted root growth in the R horizon. The longer-term stability of the ameliorated soils is discussed in relation to physico-chemical and biological mechanisms.