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.
Subject
General Agricultural and Biological Sciences
Cited by
182 articles.
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