Nitrogen prediction

Abstract

The Project had two elements; a review and an experiment.

I. Review of current advice on cereal crop requirements

The review was made through a sequence of 16 scientific papers, with additional press articles, giving information which forms the basis for advice on applying nitrogen to cereal crops. These are summarised as a series of questions with short answers.

II. Effect of previous cropping on responses of winter wheat to applied nitrogen

The experiment used winter wheat crops to test whether responses to applied nitrogen were different after peas, beans and cereals and, if so, whether variation could be related to characteristics of previous crops.

Conclusions

1. In deciding how to apply nitrogen to cereals, the tendency of growers is first to choose the fertiliser, then the amount and lastly the timing of their applications. Their choices are influenced as much by commercial considerations as by growing conditions.

2. If perfect recommendations were possible for all wheat crops, growers would benefit by about £20/ha. Current recommendation schemes are similarly poor because they fail to identify fields with aberrant responses to N.

3. Differences in effectiveness between urea and ammonium nitrate are normally too small to be proven by experiment.

4. Organic matter content is a useful yardstick of the nitrogen supplying power of Fenland soils, but applications of nitrogen should also account for subsoil acidity and residual effects of previous crops.

5. Measurements in spring show that sugar beet leaves behind no more N than cereals, but oilseed rape or peas leave about an extra 30 kg/ha of 'available' N. Despite the greater yields after oilseed rape and peas, optimum amounts of fertiliser nitrogen are about 50 kg/ha less than after cereals or sugar beet.

6. Measurements in autumn show about 25 kg/ha extra 'available' N after peas or about 10 kg/ha after beans but eventually their nitrogen contributions to a succeeding cereal crop are very similar.

7. Wheat appears to recover an average of 60% of the N that is applied, but this varies between about 40% and 90%. There is evidence that large crops recover large proportions of the fertiliser N that they receive.

8. The green surface area of a wheat crop appears to be directly related to the N it contains. If a canopy needs to expand by 1 ha, a crop needs to take up about 30 kg extra N.

9. Wheat crops appear to reach a limit for N uptake; further increases in fertiliser N give little further expansion of leaf area or increase in growth.

10. Where all quality characters of wheat crops grown for breadmaking are expected to be satisfactory other than protein, it is profitable to apply about 25 kg/ha extra N.

11. Only about 30% of wheat crops grown for breadmaking have grain protein concentrations where extra N would be expected to increase premium payments. Analysis of flag leaves might help identify which crops these are.

12. Apparent recovery of sprayed N can be improved by making small applications. It may be worth adding urea N to sprays applied for other purposes.

13. When the protein in wheat flour has been given a boost by extra applications of N this is not always translated into larger loaves.

14. It is best to estimate the optimum N amount for a cereal crop from a multi-level N experiment, fitting a curve to describe the yield response. Models help to justify how a particular fertiliser policy has been chosen, because models are likely to provide the best representation of what is understood of soil and crop processes.

15. Sub-optimal amounts of N have a small effect on the nitrate residues left in the soil by cereals, but effects can be large where N applications are super-optimal.

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