Nitrogen supply and demand in Australian agriculture

Abstract

The supply of and demand for nitrogen by whole industries and individual crops is discussed in relation to changes in farming systems, particularly the relative importance of fertiliser and biologically fixed nitrogen. The use of fertiliser nitrogen (N) in Australia has grown at an annual rate of 14% since the early 1990s, after growing at half that rate since the 1950s. The accelerated growth occurred during a period when world demand has been almost constant. Most of the additional demand has been for the dryland cereal and canola industries of southern Australia, where crops previously obtained almost all their N from mineralisation of soil organic matter and the residues of legume pastures. The most likely reasons for the belated increase in use of fertiliser N in Australia are to replace the supply from pasture residues as the area of pasture decreased and to satisfy the increased demand of cereals following break crops and of the break crops themselves, particularly canola.

For a dryland cereal, there is a problem of matching soil N supply with an unpredictable N demand. For winter cereals in Australia, crop N demand is poorly synchronised with soil N supply. The time of greatest demand is normally during the stem-elongation phase when the crop is growing fastest. For crops targeted for high-protein grain, there is an even greater demand around the flowering phase. The peak N demand for well-managed crops growing with no water limitations exceeds the capacity of the soil to supply N from mineralisation at the time, so additional N is required to meet the shortfall, either from fertiliser or mineral N retained in the soil from earlier mineralisation. Predicting the optimum supply of fertiliser N at sowing is difficult in cases where N demand is influenced by variable rainfall. Topdressing and banding fertiliser offer prospects for more closely matching N supply and demand for dryland crops. The future role of legumes in supplying residual N is discussed in relation to the trend towards continuous cropping.