Soil water and nitrogen interaction in wheat in a dry season under a fallow-wheat cropping system

Measurements of water use, growth and yield of wheat were made in a dry year following 3 methods of long fallow: chemical (CH) (no tillage), blade plough (CB) (subsurface tillage), and a fully tilled fallow (CT) following burning of stubble residues (the popular method). Crops in each fallow method were fertilised with and without nitrogen. All fallow systems produced relatively high yields in the range 2.7-3.8 t/ha, despite less than average rainfall over the 18-month fallow period (539 mm) and a significant dry period extending from before anthesis until maturity. CH fallow, however, accumulated 83 mm more water than CT fallow by sowing time and produced 0.74 t/ha additional yield. This can be attributed to increased water use through greater leaf area index duration during grain filling. The application of nitrogen fertiliser had no effect on the yield of the stubble-retained crops (CH and CB). However following nitrogen application, 1 t/ha less yield was produced under CT fallow than under CH fallow. There were marginal advantages in growth and biomass partitioning from applied nitrogen with increased leaf area index during the early stages of growth. At sowing there was a significant accumulation of mineral nitrogen below 120 cm as a result of a 6-year history of fertiliser application under the chemical fallow. This, however, disappeared by anthesis with a greater nitrogen use. In dry years, that are not catastrophic, fallow with stubble retention and zero tillage (CH) offers some insurance against low yield. This was achieved through improved water availability (320 v. 240 mm in CT) which also may reduce the risk of yield loss through haying off by overdosing with nitrogenous fertilisers when levels of soil mineral nitrogen at sowing are high (200 kg nitrogen/ha).