IMPACT OF LUPINS, GRAZED OR UNGRAZED SUBTERRANEAN CLOVER, STUBBLE RETENTION, AND LIME ON SOIL-NITROGEN SUPPLY AND WHEAT NITROGEN UPTAKE, GRAIN YIELDS, AND GRAIN PROTEIN

Soil nitrogen (N), N uptake, and wheat production in relation to rotat ion with wheat, lupin, or subterranean clover, mulched or grazed,were examined on a red earth at Wagga Wagga, New South Wales. Data over 4 y ears (1992-95) are presented from a long-term trial commenced in 1979. The effects of the various rotations on wheat productivity changed wi th seasonal rainfall during the wheat and the previous legume growing year. Generally, low rainfall (1991 and 1994) during the legume growin g season resulted in lower N uptake, grain protein, and grain yield by wheat grown in a 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 ferti lised with N were usually lower than those after a lupin growing seaso n, although total soil N levels were similar. Subterranean clover prod uced higher total soil N and grain protein than lupin but yields were normally less. Lodging and take-all diseases were higher after a growi ng season with subterranean clover than after lupins and most likely r educed grain yields. Grazing, as opposed to mowing and; mulching subte rranean clover, increased soil total N, grain protein, and usually soi l mineral N, but not grain yield. The addition of lime at 1.5 t/ha rai sed the soil pH(CaCl2) (0-10 cm) of the most acidified treatment, cont inuously cropped wheat fertilised with N, from 4.04 to a mean of 4.7, and increased yields and N uptake in 1993 and 1995.