We are commonly told to plant legumes in pasture or in the garden to increase the nitrogen in the soil. It is true that given the right minerals and microbes in the soil, legumes will develop a symbiotic relationship with rhizobial bacteria, which can “fix” gaseous nitrogen from the air and make it available to the plant (more here). The important point is that this nitrogen is used by the growing legume plant and only minimal amounts are transferred to the soil orother plants.
“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.
Don’t despair though, there is still free nitrogen to be had! Fortunately, as well as the rhizobial bacteria, there also exist bacteria known as free-living nitrogen fixing bacteria (is anyone else picturing hippy bacteria? Free living, man!). They are also called “non-symbiotic” nitrogen fixing bacteria, but that doesn’t sound as funny.
Anyway, these bacteria live in the soil and convert gaseous nitrogen in the air into ammonia in the soil, which can be accessed by plant roots. There are a number of farming practices that can encourage the presence of these bacteria and effectively give us access to free nitrogen, for example:
- 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!