Application of Probability Analysis to Assess Nitrogen Supply to Grain Crops in Northern Australia

Grain yield and protein of cereal crops in northern Australia provide a useful indicator of the supply of available nitrogen (N) to the crop. Our intention was to utilize this principle on a site-specific basis through an associated probabilistic framework to identify the likelihood that grain yield was limited by N supply. Yield and protein data were taken at harvest from sorghum, wheat and barley crops near Dalby, southern Queensland, in 1999. Considerable variation was found in grain yield for the three crops, but less so for grain protein. Frequency–response relationships, derived from historical multiple N field experiments, were applied to identify areas where grain yield was limited by N supply. This approach indicated that there was a 60% or higher likelihood that plant-available N was yield-limiting for 17%, 23%, and 26% of the area sown to sorghum, wheat and barley, respectively. These areas were not necessarily those where crop yield was relatively low. Calculation of N removal and N supply, using N transfer efficiency relationships, verified that those areas with a high likelihood of response to N had considerably lower supplies of N compared to other areas. The application of probability analysis offers a unique strategy to identify within-field areas where N supply could be yield-limiting, and provides a rationale for predicting the spatial variation and likely range of N supplies for successive seasons.