Sustaining productivity of a Vertisol at Warra, Queensland, with fertilisers, no-tillage or legumes. 6. Production and nitrogen benefits from annual medic in rotation with wheat

Abstract

Continuous cereal production in the summer-dominant rainfall region of north-eastern Australia has depleted native soil nitrogen fertility to a level where corrective strategies are required to sustain wheat grain yields and high protein levels for domestic and export markets. Annual medic pastures, along with other strategies, were evaluated from 1986 to 1998 on a Vertisol at Warra, southern Queensland, for their benefit to subsequent grain yield and protein content of wheat.
Annual medic production and nitrogen yields were closely correlated with the growing season rainfall (March–September). Each 100 mm of growing season rainfall resulted in 1.39 t/ha of dry matter and 40 kg/ha of N yield.

There were significant increases in soil mineralisable nitrogen following annual medic leys compared with continuous wheat in most years, but increases in total soil nitrogen were only observed in 1990, 1991 and 1992. However, pre-plant nitrate-nitrogen following an annual medic ley was always higher than that of continuous wheat without nitrogen fertiliser. This resulted in significant increases in wheat yield (6 of 9 seasons) and grain protein (in all seasons) compared with continuous wheat. The overall responses in yield and protein were similar to those of 50 kg/ha of fertiliser nitrogen applied to continuous wheat crops. A nitrogen harvest budget for the annual medic–wheat rotation over 11 years showed that it contributed 131 kg N/ha more than continuous wheat. Thus, using the seasonal conditions experienced from 1987 to 1998 as a reasonable representation of the rainfall range in the region, sustainable productivity can be maintained where annual medics are grown in short rotations with wheat.