Field Experiments to Help Optimise Nitrogen Fixation by Legumes on Organic Farms

Field Experiments to Help Optimise Nitrogen Fixation by Legumes on Organic Farms

During an organic rotation, the aim is to increase the nitrogen (N) content of the soil (and hence build up soil fertility) by recycling crop residues, applying manures/composts and from N fixed by legumes. IGER, with ADAS, Duchy College Cornwall and Abacus Organic Associates are developing improved guidance for organic farmers in the use of fertility-building crops. The main questions are: how to maximise N fixation and how to make the most efficient use of the fixed N? Available soil mineral N, which is generally thought to reduce N fixation, will be increased by manure applications, cutting/mulching and grazing. We describe an experiment to establish the extent to which animal and green manures can adversely affect N fixation. The results from the first year (2003) were reported recently (Hatch et al., 2004). Here we summarise the findings from 2 years’ results (2003-4) to show the changes that occurred after the legume crop became fully established.