Keywords: crop, fertiliser, nitrogen, spreading, uniformity, yield
The yields of cereals and oilseed rape are closely related to nitrogen supply. The relationships are measured in field experiments and can be described by various non-linear equations. In the field experiments, care is taken to spread fertiliser uniformly within every plot. The quantified relationships, and fertiliser recommendations based on them, apply to situations where fertiliser is spread uniformly. In practice, fertiliser is not spread perfectly uniformly at a field scale and the non-uniformity can affect both crop yield and the calculated economic optimum rate of nitrogen application. In the UK, a linear plus exponential (LEXP) equation often is used to relate crop yield to the rate of fertiliser application and was used in formulating the nitrogen recommendations for cereals and oilseed rape published by the government. A method is described here for modifying this equation to include a measure of the non-uniformity of fertiliser spreading. The equation then can be used to predict crop yield and the economic optimum nitrogen rate for different degrees of non-uniformity. Using typical nitrogen response curves for winter wheat and winter oilseed rape, the calculated effects on yield and economic optimum nitrogen rates were unimportant where the coefficient of variation of spreading (V) was up to 0.1. The calculations supported the objective of V ~ 0.1 for fertiliser spreaders in field operation. Fertiliser recommendations derived using LEXP and data from field experiments where fertiliser is spread evenly should not be affected where V ~ 0.1.
I R Richards and R D Hobson, Ecopt, Dewells Farm House, Ufford Road, Bredfield, Suffolk, IP13 6AR, UK.
12 pages, 2 figures, 1 table, 17 references.