Sub sections:
Improving soybean and nitrogen management in subtropical NSW cane systems
Project leader: Dr Natalie Moore, I&I NSW Grafton
This project, in collaboration with Professor David Herridge, and funded by the Sugar Research and Development Corporation (SRDC) through the Grower Group Innovation Program aims to (i) provide information on residual N cycling following soybean rotations on different cane soil types in subtropical NSW, (ii) use this information to produce soil-specific fertiliser recommendations following soybean to achieve more efficient use of nitrogen fertiliser in NSW cane crops, and (iii) compare different methods for dealing with soybean crop residue in terms of cost in a subtropical environment.
The benefits of using legumes such as soybean and peanut as break crops in sugar cane monoculture on the wet tropical coast of Queensland are well documented with yield increases of the following cane crop in the order of 20–30%. In addition to reducing the levels of cane pests and diseases, soybean rotations also provide economic benefits from the harvested grain and economic and environmental benefits from the N-rich residues reducing subsequent fertiliser N inputs. However, it has been observed that some cane growers in subtropical NSW may not realise the full benefits of the soybean rotation due to over fertilisation of the following cane crop whilst, in other situations, N stress has been observed in cane following a well-grown soybean crop.
During 2009, replicated field trials were established in each of the three sugar mill areas of NSW. Residual N cycling following soybean was assessed by measuring the contribution of the soybean crops through N2 fixation (nodule assessment, biomass, grain yield, grain protein and 15N analysis) and soil tests to determine plant-available and total soil N. 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 (nitrogen derived from the atmosphere) 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%.
Various fertiliser N rates were applied to all Phase 1 cane crops after sowing in October-December 2009. The fertiliser rates ranged from zero to 196 kg of N/ha. Cane leaf analysis and sugar cane yield (tonnage and sugar yield) will be assessed during the next two years. The N response curves should allow current fertiliser N recommendations, developed for cane following fallowed land, to be revised to account for the N benefits of soybean.