STUBBLE RETENTION AND NITROGEN-FERTILIZATION IN A FALLOW-WHEAT RAIN-FED CROPPING SYSTEM .1. SOIL-WATER AND NITROGEN CONSERVATION, CROP GROWTH AND YIELD

In semi-arid environments where water supply is the factor usually limiting grain yield, fallowing provides a way to increase water and nitrogen supply, An experiment was maintained from 1980 to 1990 at the Wimmera Research Station, Dooen, Australia. In each year, water use, growth and yield of wheat (Triticum aestivum L.) were measured following three tillage methods of year-long fallow. The three methods were; chemical (no tillage), blade plough (sub-surface tillage) and a fully tilled fallow following burning of stubble residues (conventional method). From 1984, the comparison was extended to include two levels of applied nitrogen (zero and from 20 to 60 kg N ha(-1)). Over the decade of the experiment, annual rainfall varied from 190 to 586 mm and yield from 1.03 to 5.72 Mg ha(-1), There were small but significant differences in water conservation and resultant wheat yield by the three fallow methods. The differences were not consistent from year to year but a pattern was evident, In years of low yield (less than 3.7 t ha(-1), 5 years in ten), chemical fallow had a yield advantage over both tillage methods, fully tilled fallow (up to 0.8 Mg ha(-1)) and blade plough (up to 0.6 Mg ha(-1)). In seasons with yield greater than 3.7 Mg ha(-1) (4 years in ten), yield following the fully tilled fallow exceeded that of the other methods (up to 0.75 Mg ha(-1)). In the driest year (1982), there was no difference between methods. During the 7 year sequence when nitrogen was applied, yield of the unfertilised treatments exceeded 3.5 Mg ha(-1) in 5 years. Under those conditions, the addition of nitrogen increased yield by a maximum of 0.6 Mg ha(-1). The advantage of chemical fallow in years of low rainfall and yield potential was shown to result from greater water conservation (average of 53 mm year(-1)), It is proposed that the advantage of the fully tilled fallow in years of greater yield potential can be attributed to better N supply.