The value of nitrogen applied to wheat during early development

Forty eight field trials were conducted at sites representing the major arable areas of the UK to test the effectiveness of nitrogen applied during early growth. A family of linear + exponential curves of constant shape fitted to data from all the trials accounted for 94% of the variation in grain yield. This approach identified soil nitrogen supply as the largest source of variation in the optimum requirement for fertiliser nitrogen. Changes in optimum amounts of fertiliser N indicated that N applied at sowing was about half as effective as N applied in spring. Where some of the spring N was applied in February, the mean response to N applied at sowing was -0.02t/ha of grain. Although same sites responded economically to N applied at sowing, varietal type and delayed spring N application were the only factors identified as possible predictors in practice. Grain yields with optimum N varied between 6.1 to 11.5 t/ha but this wide range failed to account for variation in optimum amount of spring applied N for individual sites. However, about one third of the variation in optimum N was accounted for by a knowledge of soil type, drilling date, winter rain and amount of seedbed N.