“The amount of nitrogen returned to the soil during or after a legume crop can be misleading. Almost all of the nitrogen fixed goes directly into the plant. Little leaks into the soil for a neighboring nonlegume plant. However, nitrogen eventually returns to the soil for a neighboring plant when vegetation (roots, leaves, fruits) of the legume dies and decomposes.”
Therefore, the only way to harness the nitrogen produced by the legume/rhizobial relationship is to use the legume as a cover crop and mulch it onto the soil (or where you need the nitrogen) at the end of the season. It will not provide nitrogen to other plants as it is growing. With the exception of perennial leguminous trees and shrubs (e.g. Tagasaste, wattles (Acacia) and Pigeon Peas), which can contribute nitrogen to the soil by periodically losing their leaves and branches.
- Stubble retention and mulching cover crops – these bacteria need to feed on carbon, so the more carbon available in stubble and mulch the better. Bare soil will cause them to starve.
- Don’t use nitrogen fertiliser – nitrogen fixation is only triggered if there is not already sufficient nitrogen in the soil, adding fertiliser will prevent these bacteria from fixing their own nitrogen
- High moisture levels and warm temperatures– nitrogen fixation occurs to a greater extent under these conditions, not that you can control them, except in an irrigated greenhouse maybe!
- Don’t use pesticides – synthetic pesticide chemicals kill bacteria, including free-living nitrogen fixing bacteria
- By all means, also plant legumes to be used as mulch, to maximise your access to free nitrogen!