Crop response to fertiliser nitrogen on a sodic soil in the Central Region

The SIX EASY STEPS™ nutrient-management program encourages a process of ongoing refinement, and any change to a nutrient-management program should be based on evidence. This may include leaf testing, farm records, on-farm evaluation or the combination of these and other factors. Crop response to applied fertiliser should be one of the main factors assessed when refining a nutrient-management program. Crop response to applied nitrogen (N) may be reduced on sodic soils, associated with a reduced crop-N requirement where growth is constrained. However, the sodic conditions may also limit the crop’s ability to acquire N. A trial was established in the Central Region to investigate crop response to fertiliser N on a sodic soil. N rate treatments (0, 50, 100, 150 and 200 kg N/ha as urea) were established in the first-ratoon crop and repeated for the remainder of the crop cycle. The trial contained two zones, one with low yield potential and high electrical conductivity (LYHEC) and a second zone with higher yield potential and low electrical conductivity (HYLEC). Both zones were highly sodic (ESP>15%) at depth. An N deficiency developed over the crop cycle, with the 0N and 50N treatments showing lower leaf %N, and reduced cane and sugar yield. Optimum N fertiliser rates increased from the first ratoon (18 kg N/ha) to the third ratoon (109 kg N/ha) but declined in the fourth ratoon (85 kg N/ha). Nitrogen-use efficiency declined with N application rate and the highest N uptake efficiency of fertiliser N was 27% at the 50N rate. Overall, the trial showed that a small reduction (~20 kg N/ha or 14%) from the recommended rate of 140 kg N/ha would have been appropriate at this site. The results provide support for the recently developed SIX EASY STEPS Toolbox guidance for refining nutrient inputs for specific on-farm circumstance, including sodic soils. This guidance encourages amelioration of the sodic soil condition as the grower’s priority before amendment of nutrient inputs is considered.