Estimation of the nitrogen requirement of winter wheat in the UK: a multiple regression approach

In this study, multi‐site crop response data were examined so as to improve guidelines for crop nitrogen requirement and efficient fertiliser use on winter wheat over a range of soil types. The parameters which had most individual effect on nitrogen requirement (the economic optimum amount of fertiliser N (Nopt)) were soil type and previous cropping. Interactions between soil type and soil N supply as measured by soil mineral nitrogen analysis were also important. Yield had no effect. Descriptive models were developed for Nopt. The simplest, based on N index, rainfall and soil type, accounted for 40% of the variation in Nopt. This was improved to 51% by adding spring analysis of soil mineral nitrogen. The model could be improved by inclusion of measured yield or a seasonal term, but the precision of predicted Nopt was not improved significantly, 41% of cases being within ±25 kg ha−1 of the optimum without yield and 42% with yield. The study showed that some of the inputs currently used to determine N requirement need to be revised and additional terms added. However, even the best estimates have poor predictive precision.