FALLOWING PRACTICES, SOIL-WATER STORAGE, PLANT-AVAILABLE SOIL-NITROGEN ACCUMULATION AND WHEAT PERFORMANCE IN SOUTH WEST QUEENSLAND

The effects of tillage frequency (conventional, reduced and zero), primary tillage implement (disc, blade and chisel plough), stubble management (retention and removal), gypsum application and paraplowing were examined with respect to soil water storage, soil nitrate accumulation, seedling establishment, crop growth, yield and grain protein content for four successive years of wheat, grown on a sodic, duplex soil in South West Queensland, Australia. Stubble retention generally resulted in more soil water at sowing but less at harvest, compared with stubble removal. This led to higher grain yields in dry growing seasons and has implications for reducing runoff. Zero tillage with stubble removal had the lowest mean water storage and, in dry growing seasons, the lowest grain yield. This indicates that when stubble is absent or lacking, some tillage is needed, probably to break surface seals and increase surface roughness. Zero tillage with stubble retention accumulated the most soil water but the least soil nitrate. Consequently this treatment outyielded all others in the driest growing season but was outyielded by almost all others in the wettest. Increased frequency and aggressiveness of tillage, and stubble removal, increased the amount of soil NO3-N at 0-60 cm. There were no significant (P < 0.05) tillage X stubble interactions. In the wettest growing season, wheat grain yields reflected the different levels of NO3-N in the soil. In the other three years, grain protein contents reflected these levels. Both zero tillage and stubble retention reduced the efficiency of water use, probably because both also reduced nitrogen supply. So despite high yields from zero tillage with stubble retention in the dry growing seasons, the full yield potential of this treatment with respect to its water supply was not realized. Gypsum application at 5 t ha-1 had no commercially useful effect on establishment or grain yield and is not recommended. Similarly, paraplowing had no effect on yield. Reduced or zero tillage is recommended, provided that stubble is present and nitrogen supply is adequate.