Can new fertilizer technology facilitate a reduction in fertilizer-N rates and improved water quality without compromising sugar production?

Optimizing nitrogen (N) application rates to both sustain high levels of productivity and minimize any impacts on the surrounding ecosystem is challenging, especially under monsoonal wet- season conditions in northern Australia. The inability of existing application strategies and fertilizer-N products to achieve synchrony of mineral N supply with crop demand or prevent rapid formation of nitrate-N (that is vulnerable to loss via gaseous or aqueous loss pathways) increases risks of inefficient N use. A blend of enhanced efficiency fertilizers (EEFs) with different modes of action has the best chance of lowering the risk of N losses and increasing crop-N recovery, providing an opportunity to reduce fertilizer-N rates without increasing the risks of productivity loss. Seven field trials were established from Mackay to the wet tropics, with data from consecutive ratoon crops at sites in Tully, Silkwood and the Burdekin reported here. Yields and indices of N use efficiency were developed for crops receiving urea-N at rates equivalent to that derived from the local SIX EASY STEPS guidelines, or as urea or a blend of EEFs applied at N rates calculated using a block-specific yield target (PZYP) based on mill records. Results showed that yields at all sites responded to the application of urea-N fertilizer, and there were suggestions of a slight productivity drop if rates were lowered to those determined using PZYP at some sites. The apparent crop uptake of urea-N was generally poor (15–30% of applied N) and the agronomic efficiency of fertilizer-N use varied significantly among sites and seasons (2.5–11.4 kg N applied/t additional cane yield). The use of the EEF blend consistently delivered improved fertilizer-N recovery (30–47% of applied N), but the lack of any yield increases at sites other than Silkwood resulted in similar agronomic efficiencies (2.5–7.6 kg N applied/t additional cane yield). The impacts of the EEF blend on runoff and drainage losses at Silkwood were confounded by inadequate closure of the fertilizer trench during stool splitting. Key words Urea, enhanced efficiency fertilizers, nitrogen, productivity zone yield potential, dissolved inorganic nitrogen