Impact of lupins, grazed or ungrazed subterranean clover,stubble retention, and lime on soil nitrogen supplyand wheat nitrogen uptake, grain yields, and grain protein

Abstract

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.