Growers reduce uncertainty around adjusting nitrogen rates following legume cover crops in the Murray district

The SIX EASY STEPS? Toolbox provides guidelines for estimating the amount of nitrogen (N) contained in legume cover crops and refining plant-cane N rates after a legume break crop. However, grower uncertainty remains regarding the appropriate adjustment to make to plant-cane fertiliser N rates. Six growers from the Murray subdistrict of the Tully mill area measured changes in soil mineral N across seven sites over the 2022/2023 growing season to better understand how plant-cane N rates could be adjusted following a legume cover crop without adversely affecting productivity. Sites varied by location, soil type, legume cover crop species, planting date and the amount of N fertiliser applied at planting. Most sites had a plant-cane N requirement of 120 kg N/ha (for a bare fallow) according to SIX EASY STEPS? guidelines for the Wet Tropics region. The amount of N applied at planting ranged from 24 to 61 kg N/ha. The mean values of total soil mineral N ranged from 10 to 365 kg N/ha in the row shoulder over the season. Sharing the mineral N results with the participating growers gave them confidence to not to apply additional N fertiliser at top-dressing. Crop N uptake was not restricted during the growing season because of this decision. Third-leaf samples collected from all sites confirmed N concentrations (% dry matter) were above the critical value and indicated the crops were not N deficient.