Getting legumes to work for you

The Haber process is an energy intensive process operating at high temperature and pressures that takes nitrogen from the air and turns it into nitrogen (N) fertilisers. The roots of legumes and specialist soil bacteria can do the same job more effectively in field soil. Including legumes in a sward or crop rotation can be an effective way of reducing bills for N fertiliser.

Fixation of atmospheric nitrogen (N) by both free-living soil micro-organisms and micro-organisms within nodules associated with the roots of leguminous plants were both demonstrated over 100 years ago.  Subsequently it has been shown that a wide range of free-living soil bacteria can fix atmospheric N but the high energy requirements of the process means that both the number of these organisms and their N fixation are very low. On average less than 1 kg N ha-1 year-1 is fixed by this route.

An estimated 40 million tonnes of nitrogen is fixed due to the symbiotic relationship between bacteria (largely Rhizobium species) with leguminous field crops and pasture species. The ranges of N fixation that can be expected in the UK have been estimated for most common forage species. However, it still remains almost impossible to give detailed site and season specific estimates of N fixation.

The formation of the nodules, which act as N fixing factories, is the result of complex interactions between plant and microbial processes.  It is important to note that both nodule formation and the effectiveness of N fixation is reduced when N is readily available in the soil. Legumes would rather take up easily available N than have to expend a lot of energy to set up N fixing factories by working with the bacteria. If a particular legume is being cultivated for the first time (especially if it is not a native species) then it is often necessary to inoculate the seed with the associated rhizobial species.

Current advice to farmers that effective nodulation and N fixation in soils is increased where crop rotations include regular legume phases, soil pH is maintained in the neutral to slightly alkaline range and soil organic matter levels maintained or increased.