Improving Soybean and Nitrogen Management in Sugar Cane Systems in Subtropical NSW

Some sugar cane growers are not fully realising the economic benefit of soybean crops to their cane production systems or the environmental benefit through minimising the use of N fertilisers. This paper reports on the measurement of nitrogen (N) inputs from soybean crops at a range of sites in order to fine-tune N fertiliser recommendations for sugar cane following soybean in subtropical New South Wales (NSW). Replicated field trials were established in March 2009 in each of the three sugar mill areas of NSW. Residual N cycling following soybean is being assessed by measuring the contribution of the soybean crops through N₂ fixation (nodule assessment, biomass, grain yield, grain protein and δ15N analysis) and soil tests to determine plant-available and total soil N. Responses to different rates of fertiliser N in the plant cane following soybean will be assessed in the next phase of this project (sugar cane leaf analysis, cane tonnage and sugar yield). This paper reports on data from soybean crops in the first phase of the project. Results indicate effective soybean nodulation at all sites with soybean shoot biomass ranging from 5.7 to 8.3 t dry matter/ha, soybean grain yields of 2.7 to 4.0 t/ha and grain protein levels of 37.5 to 44.4% (dry matter basis). The soybean Ndfa values ranged from 72 to 79% with, on average, 90 kg N fixed for each tonne grain produced. Residual soil mineral-N levels (0-90 cm depth) ranged from 50 to 260 kg/ha with soil organic carbons (0-25 cm depth) ranging from 1.9 to 2.9%. A key finding was that in developing final fertiliser recommendations for sugar cane, the conditions experienced after soybean harvest must be taken into account, particularly extended periods of flooding.