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.
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.
“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.”
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.
 |
| Free-living bacteria? |
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!
Oh well well I grew pigeon peas I kept pruning them back and leaving the prunings lying on the ground - it sounds as though I did the right thing. I love your picture of the free living bacteria. :)
ReplyDeleteThanks for writing on this topic. It comes in the nick of time as the winter broad beans just seem to keep on keeping on. The lady who is growing them will probably pull them up soon, meaning lots of mulchy goodness for the garden, and a much needed nitrogen fix!
ReplyDeleteGreat info Liz. Love the pic
ReplyDeleteYay, sounds like I'm on the right track with my pea and lucerne straw mulch. I do so little to fertilize my garden, it's good to know that one of the things I actually do works. Thanks!
ReplyDeleteYep, feed the microbes in your soil on lots of nitrogen rich legume leaves and they will feed your plants....
ReplyDelete