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 lim iting grain yield, fallowing provides a way to increase water and nitr ogen supply, An experiment was maintained from 1980 to 1990 at the Wim mera Research Station, Dooen, Australia. In each year, water use, grow th and yield of wheat (Triticum aestivum L.) were measured following t hree tillage methods of year-long fallow. The three methods were; chem ical (no tillage), blade plough (sub-surface tillage) and a fully till ed fallow following burning of stubble residues (conventional method). From 1984, the comparison was extended to include two levels of appli ed nitrogen (zero and from 20 to 60 kg N ha(-1)). Over the decade of t he experiment, annual rainfall varied from 190 to 586 mm and yield fro m 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 m ethods. The differences were not consistent from year to year but a pa ttern was evident, In years of low yield (less than 3.7 t ha(-1), 5 ye ars in ten), chemical fallow had a yield advantage over both tillage m ethods, 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 excee ded 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 sh own to result from greater water conservation (average of 53 mm year(- 1)), It is proposed that the advantage of the fully tilled fallow in y ears of greater yield potential can be attributed to better N supply.