Clover field at Daylesford Organic, Gloucestershire. Martin Morrell

Nitrogen fixation

Department for Environment, Food & Rural Affairs (Defra)
Duchy College
All rights reserved.
Academic research

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Resource explained: 

This information is part of the SWARM Hub website’s ‘Fertility building’ section. It condenses information taken from ‘A review of leguminous fertility-building crops, with particular reference to nitrogen fixation and utilisation’ and ‘Advisory Leaflet Soil Fertility Building Crops in Organic Farming.’ It explains that nitrogen (N) is key in helping you to achieve acceptable yields and crop quality. Legumes can provide you with ‘free’ N through their capacity to take up mineral N in the soil and fix N2 from the atmosphere. The page is divided in to sections describing: The N fixation process; factors affecting N fixation; ways you can minimise N losses; ways you can maximise N fixation; ways you can improve the efficiency of N availability; and other considerations (managing cover crops, crop sequences, bi-cropping, costs, cultivation practices, pests and diseases, nutrient deficiencies, and organic manures). 

Findings & recommendations: 
  • By growing legumes in rotation you can build up nitrogen (N) below and above ground; N is broken down by microbial activity following incorporation and released for uptake by the following crop.
  • More N will be fixed in most organic rotations where legumes follow crops that have previously depleted soil N levels. 
  • You can increase the soil mineral N pool on your farm by applying manure, by cutting and mulching, and by grazing livestock.
  • Research has shown that the proportion of N that is fixed from legumes is greater when the crop is cut and removed than when it is cut and directly mulched.
  • You can maximise N fixation in various ways such as growing a legume in a mixture with a non-N fixing species and growing permanent beds of legumes alongside a cash crop.
  • Greater N fixation has been demonstrated in minimum tillage systems.
  • For most legume species, yield increases as the quantity of fixed N increases.
  • One way to improve efficiency is to combine several legume and grass species in a mixture, incorporating a number of slower growing species. The Legume LINK project is referred to in the section on improving the efficiency of N availability.