ARCHIVE: Fertiliser Manual (RB209)

Nitrogen for field crops

(Section 8 contains additional information for grassland)

Most agricultural soils contain too little, naturally occurring plant-available nitrogen to meet the needs of a crop throughout the growing season. Consequently, supplementary nitrogen applications have to be made each year. Applying the correct amount of nitrogen at the correct time is an essential feature of good crop management.

Crop nitrogen requirement

’Crop nitrogen requirement’ is the amount of nitrogen that should be applied to give the on-farm economic optimum yield. Nitrogen recommendations for all crops in this Manual, except grass, are crop nitrogen requirements defined in this way. Crop nitrogen requirement should not be confused with total nitrogen uptake by the crop or with the total supply of nitrogen (including that from the soil) that is needed by the crop. For grassland the new systems based approach described above provides recommendations based on the economic need to produce the amount of home grown forage necessary to maintain a target intensity of production, rather than the on-farm economic optimum of just the grass crop.

Basis of the Recommendations

Provided there are adequate supplies of water and other nutrients, nitrogen usually has a large effect on crop growth, yield and quality. The diagram below shows a typical nitrogen response curve. Applying nitrogen gives a large increase in yield but applying too much can reduce yield by aggravating problems such as lodging of cereals, foliar diseases and poor silage fermentation. When too much nitrogen is applied, a larger proportion is unused by the crop. This is a financial cost and can also increase the risk of nitrate leaching to water and contribute to other environmental problems such as climate change: Appendix 11 explains in more detail these other costs of fertiliser use.

A Typical Nitrogen Response Curve

With reference to diagram above:

  • Without applied nitrogen, yield typically is low (A). 
  • As nitrogen use increases from very small amounts, there is a large increase in yield up to the ‘on-farm economic optimum’ nitrogen rate (B). This rate depends on the cost of the applied nitrogen and on the value of the crop (‘breakeven ratio’) as well as on the shape of the response curve. All recommendations in this Manual are calculated using a typical breakeven ratio (see page 32) to provide the best on-farm economic rate of nitrogen to apply. Substantial changes in the value of the crop produce or in the cost of nitrogen are needed to alter the recommendations. Where appropriate, different recommendations are given to achieve crop quality specifications required for different markets. Appendix 11 describes on-farm economic optimum (or profit maximisation) in more detail. 
  • Application of nitrogen above the on-farm economic optimum will increase yield slightly but this yield increase will be worth less than the cost of the extra nitrogen. 
  • Maximum yield (C) is reached at a nitrogen rate greater than the on-farm economic optimum and this is never a target if farm profits are to be maximised. Application of nitrogen above point C does not increase yield, and with further applications yield falls and the need for agro-chemicals such as fungicides and growth regulators may increase. 
  • The nitrogen can be supplied from fertiliser and/or organic manure. 
  • At nitrogen rates up to the on-farm economic optimum, there is a roughly constant amount of nitrogen left in the soil at harvest. At nitrogen rates above the on-farm economic optimum, there will be a larger surplus of residual nitrogen, usually as nitrate, in soil after harvest. This nitrate is at risk of loss in ways that can cause environmental problems like leaching to ground or surface water and denitrification to nitrous oxide (a greenhouse gas). For this reason, the amount of nitrate leached begins to increase by larger amounts above the on-farm economic optimum. There are social costs associated with these environmental problems, and this is explained in more detail in Appendix 11.
ADLib logo Content provided by the Agricultural Document Library
© University of Hertfordshire, 2011