Skip to main content

The truth about legumes and nitrogen fixation?

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. 
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!
More information here and here, now go get yourself some free nitrogen!


Comments

  1. 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. :)

    ReplyDelete
  2. Thanks 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!

    ReplyDelete
  3. Yay, 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!

    ReplyDelete
  4. Yep, feed the microbes in your soil on lots of nitrogen rich legume leaves and they will feed your plants....

    ReplyDelete

Post a Comment

Thanks, I appreciate all your comments, suggestions and questions, but I don't always get time to reply right away. If you need me to reply personally to a question, please leave your email address in the comment or in your profile, or email me directly on eight.acres.liz at gmail.com

Popular posts from this blog

Chicken tractor guest post

Sign up for my weekly email updates here , you will find out more about chickens, soap and our farmlife, straight to your inbox, never miss a post!  New soap website and shop opening soon.... Tanya from Lovely Greens invited me to write a guest post on chicken tractors for her blog.  I can't believe how many page views I get for chicken tractors, they seem to be a real area of interest and I hope that the information on my blog has helped people.  I find that when I use something everyday, I forget the details that other people may not be aware of, so in this post for Tanya, I tried to just write everything I could think of that I haven't covered in previous posts.  I tried to explain everything we do and why, so that people in other locations and situations can figure out how best to use chicken tractors with their own chickens. The dogs like to hang out behind the chicken tractors and eat chicken poo.  Dogs are gross! If you want to read more about chicken tractor

Getting started with beekeeping: how to harvest honey

While honey is not the only product from a beehive, its the one that most beekeepers are interested in and it usually takes a year or so to let the bees build up numbers and store enough honey before there is enough to harvest.  There are a few different ways to extract honey from frames.  We have a manual turn 2-frame certifugal extractor.  A lot of people with only a few hives will just crush and strain the comb.  This post is about how we've been extracting honey so far (four times now), and there are links at the end to other bloggers who use different methods so you can compare. Choose your frames Effectively the honey is emergency food stores for the bees, so you have to be very careful not to take too much from the hive.  You need to be aware of what is flowering and going to flower next and the climate.  Particularly in areas with cold winters, where the bees cannot forage for some time.  We are lucky to have something flowering most of the year and can take honey

Homekill beef - is it worth it?

We got another steer killed a few weeks ago now, and I weighed all the cuts of meat so that I could work out the approximate value of the meat and compare the cost of raising a steer to the cost of buying all the meat from the butcher.   My article has been published on the Farm Style website , which is a FREE online community for small and hobby farmers to learn everything about farming and country living . If you want to know more, head over the Farm Style to  read the the article  and then come back here for comments and questions.  Do you raise steers?  Is it worth it?  Do you have any questions? More about our home butchering here .