Dr Peter Thorburn1, Mr Jody Biggs1, Dr Bree Martin1, Dr Jonathan Richetti2
1CSIRO Agriculture and Food, 2CSIRO Agriculture and Food
Biography:
Dr Peter Thorburn is a Chief Research Scientist and Research Group Leader with CSIRO. He is an internationally recognised cropping systems modeller, interested in developing tools that can help farmers better manage their crops.
Abstract:
A common concept in developing recommendations for nitrogen (N) fertilizer applications is the “mass balance paradigm” – that is, bigger crops need more N, and smaller less. However, it has been suggested that the optimum N rate (Nopt) is poorly correlated to the yield at Nopt (YNopt) – i.e., bigger crops do not necessarily need more N. We calculated Nopt and YNopt in previously published experiments on maize, sugarcane, sorghum, barley and wheat. Correlations between Nopt and YNopt ranged from 0.03 to 0.51, showing YNopt is a poor predictor of Nopt. Enough information was available to simulate the maize sugarcane experiments. Simulated N lost to the environment, expressed to relative yields (i.e., kg N/t) varied +/-124 and +/-164 % of the mean for the maize and sugarcane experiments. N exported in harvested produce (kg N/t) was also variable, being +/-11 and +/-48 % of the mean in the two crops. Given the experiments were uniformly managed across time and crops were grown consecutively, not in rotation with other crop, the variations result from crop-to-crop climatic differences explaining the cause of low “predictive power” of YNopt. Even if yield of the coming crop could be accurately predicted it would be of little use in determining the amount of N fertilizer farmers need to apply because of the variability in environmental N losses and/or crop N uptake. These results reinforce the need for new methods to guide N fertilizer management and/or approaches to explicitly mitigate the risk inherent in N decisions.