Wheat yields are highly variable in northern Australia because rainfall is
variable. Economic benefits from applying nitrogen fertilisers are uncertai
n because yield responses depend on a good supply of soil moisture during t
he growing of the crop to provide a high potential yield. While an experime
nt in 1996 indicated that up to 90 kg N/ha of N fertiliser is profitable, i
t was not known whether this response is reliable or typical. A crop model
and historical climate records (1960-1993) were used in this study to produ
ce a long-term record of yield and grain protein responses to N fertilisers
, Responses in 1996 are shown to be atypical due to the favourable conditio
ns in that year. Under typical growing conditions, it is not economic to us
e N fertiliser. The simulations quantified the relationship between respons
es to fertiliser and the amount of soil moisture available at sowing. Appli
cations of N fertilisers will be most profitable if used when measurements
indicate that the plant-available soil moisture content before sowing is ab
ove average. (C) 1999 Elsevier Science Ltd.