Relationship between yield of grain sorghum (Sorghum bicolor) and soil salinity under field conditions

Three field experiments using grain sorghum (Sorghum bicolor), an important dryland summer crop on the Liverpool Plains in northern New South Wales, were conducted: (i) to determine the effect of dryland salinity on the yield of commercial crops at 2 sites; (ii) to see if ridging the soil would ameliorate the problem; and (iii) to compare 16 commercial varieties for tolerance to dryland salinity. Grain sorghum was shown to be more severely affected by dryland salinity than most literature would suggest. Over 3 seasons and 2 sites, sorghum yield was reduced by 50% at soil electrical conductivity (saturation extract, ECe) levels as low as 2.8 dS/m whereas advisory literature indicated a salinity threshold (no yield reduction) for sorghum of 6.8 dS/m, and 50% yield reduction at 9.9 dS/m. Current advisory literature is based on research where salinity was artificially imposed after plants were established in non-saline soil. The measurements described in this paper were on sorghum sown into saline soil. Soil and crop management strategies (ridging the soil or choosing a tolerant variety) showed limited potential for improving yields of grain sorghum on saline soil. At one site, the ECe varied widely across the paddock but little down the soil profile at any sampling point. Hence, analysing the surface soil would indicate the salinity hazard. However, at a second site, where ECe levels in the surface soil were low (< 2 dS/m) everywhere, ECe at soil depths of 1 m varied widely (from 2 to 15 dS/m) across the paddock. Soil sampling to assess salinity hazard before crop planting should therefore include the entire root zone.