Nitrogen Fixing Bacteria

If you grow any plants that are a member of legume family there is a very important bacteria needed in the ground you may not be aware of.

Leguminous nitrogen-fixing plants as they are called include all green and dry beans, lima beans, garbanzo beans, soy beans, fava beans, peas, field peas, clover, alfalfa, vetch, peanuts, lentils and probably others.

These plants are unique in their ability to absorb nitrogen from the air and fix it into the ground. This means these plants are normally able to provide most of their own fertilizer, and so can often be grown in very poor soil with little or no added nutrients. In fact these plants will usually leave the soil richer than before they were planted. In between growing other crops, farmers and gardeners often plant ‘cover crops’ that are intended to fix nitrogen into the ground and otherwise improve it.

These plants depend on the presence of a rhizobia bacteria in the ground. This bacteria does not normally occur naturally in the ground, and many gardeners will have to establish it themselves. Gardeners are often unaware they need to do this. Once it is established, it normally persists for a very long time and so is no longer an issue.

There are four different strains of rhizobia bacteria, and you must have the proper strain for the particular crop you are trying to grow. There is one strain for soy beans, one strain for garbanzo beans, one strain for clover and alfalfa and one last strain for most other plants. If you establish one strain of bacteria then grow something that requires a different strain, you will need to establish that strain separately.

If this bacteria is not present in the ground you can expect very poor crop performance.

Sometimes packages of seeds are labelled with a warning about this bacteria, but few warnings go so far as to tell you you will probably have a partial or total crop failure if you lack this bacteria. It is very important to establish this bacteria in your ground if you are growing these crops!

There are several ways to establish this bacteria. If you have been growing these crops already for a number of years, you probably already have it. If you or a neighbor have some land where this bacteria is already established, a few shovelfuls of dirt are often all that is required to establish it in a new spot. The bacteria will also usually establish itself, so you may be able to just wait through a year or more of poor harvests until it is finally in the soil. Finally you can establish this bacteria by using an inoculant (see below).

A common technique to firmly establish the bacteria in a particular spot is to make two successive plantings of the same sort of plant, even if you have to temporarily set aside good crop rotation practices.

Finding a supplier for inoculant can be difficult. In the UK there is a single manufacturer, Legume Technology Ltd. People living in the US can often simply find it at a garden center, but it is usually only the strain of the bacteria used for garden peas and beans. If you want other strains, or you want to order it by mail, Bountiful Gardens is a good source. Bountiful Gardens will also ship overseas, so they are a good source for people living outside of the US as well.

14 thoughts on “Nitrogen Fixing Bacteria”

  1. Many fertilizers nowadays often have rhizobia in them, usually a few strains. Not sure if they have this in Amsterdam, though!


  2. This was quite an interesting article for me, as a beginner gardener. My friend was just telling me about his failed attempts to grow soybeans, and how the plants always produced fewer beans than he used to plant them.

    thanks for the tips :)

  3. I have planted red clover in November to add Nitrogen to my soil. Since I live in Illinois and we have freezing temps up until April, when should I till the clover and turn it under? Thanks

  4. Sooooo do only legumes have the nitrogen fixing ability thing? or can like garlic “fix” the nirtogen?

  5. Hello Howdy, no garlic can’t fix nitrogen. Mostly it’s only peas, beans, clover and the other plants I mention near the top of this post.

    You can tell if a plant fixes nitrogen after you dig it up by looking at the roots. If it has been fixing nitrogen it will have nodules on the roots. These are small round white balls, that look something like what’s in a bean bag chair only the root nodules are usually smaller.

  6. Hi, I planted peas this spring and have harvested most of the crop by now. I hope they fixed some nitrogen for me. Once the plant dies completely, can I uproot them or do I have to cut them down, leaving the root in the ground, in order to fully benefit from the nitrogen-fixing that I hope has gone on along the roots?

  7. Hi Perplexed,

    I think most of the nitrogen fixing has already happened, but indeed there is still some in the roots, and it’s no problem to leave them where they are.

    It all depends on what you want really. You can also pull them up and put them in the compost, where they will add a little extra nitrogen there. You could also put them someplace else.

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