THE ROLE OF CROP AND PASTURE LEGUMES IN ROTATIONS ON DUPLEX SOILS

Duplex soils are prominent in southern Australia and are generally low in fertility. Their agricultural performance is, therefore, suboptimal in most circumstances without an exogenous source of nitrogen. This is often supplied by legumes which are grown in rotation with non-leguminous crops. Both crop and pasture legumes are now widely used in southern Australia and the contribution that they make to the non-legume phase of rotations is through nitrogen fixation and through other mechanisms such as cereal disease breaks. We use a mathematical programming model, MIDAS (Model of an Integrated Farming Dryland Agricultural System), to investigate the role of legumes in the low rainfall wheatbelt of Western Australia. The impact of legumes on farm profitability is assessed with a special focus on the contribution of legumes grown on a duplex soil. By using the model, the sensitivity of rotation choice on this duplex soil to changes in biological and economic parameters is explored. We conclude that crop legumes, in particular, have a firmly established role on sandy-surfaced duplex soils in low rainfall regions and that substantial increases in both the productivity and legume content of pasture would be required to outperform rotations which include crop legumes.