Soil nitrogen (N), N uptake, and wheat production in relation to rotation with
wheat, lupin,or subterranean clover, mulched or grazed, were examined on a red
earth at Wagga Wagga, New South Wales. Data over 4 years (1992{95) are
presented from a long-term trial commenced in 1979.
The effects of the various rotations on wheat productivity changed with
seasonal rainfall duringthe wheat and the previous legume growing year.
Generally, low rainfall (1991 and 1994) during thelegume growing season
resulted in lower N uptake, grain protein, and grain yield by wheat grown ina
following season. The addition of N fertiliser (100 kg N/ha) to continuous
wheat increased soil N supply, N uptake, grain yield, and grain protein.
Yields from continuously cropped wheat fertilisedwith N were usually lower
than those after a lupin growing season, although total soil N levels
weresimilar. Subterranean clover produced higher total soil N and grain
protein than lupin but yields werenormally less. Lodging and take-all diseases
were higher after a growing season with subterraneanclover than after lupins
and most likely reduced grain yields. Grazing, as opposed to mowing
andmulching subterranean clover, increased soil total N, grain protein, and
usually soil mineral N, butnot grain yield. The addition of lime at 1·5
t/ha raised the soil pH(CaCl2) (0-10 cm) of the
mostacidified treatment, continuously cropped wheat fertilised with N, from
4·04 to a mean of 4·7, andincreased yields and N uptake in 1993
and 1995.