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How Nitrogen Fixation Happens In Your Soil

By on August 18, 2011

Tricia adds inoculant to our Premium Soil Builder Mix cover crop seeds.

Tricia adds inoculant to our PVFS Premium Soil Builder Mix.

In our new video on cover crops, and in other blog posts, we keep talking about how important it is that legume cover crops are able to fix nitrogen in the soil. You might be asking:

Why is nitrogen important?

Available nitrogen in the soil is the essential nutrient for green leaves, and new roots and shoots.

What happens with legumes to fix nitrogen in the soil?

Legumes work in harmony with a group of bacteria that live on their roots. These bacteria take nitrogen from the air and “fix” or concentrate it in pink root nodules, adding nitrogen to the soil in a form the plants can absorb.

To get scientific about it, Rhizobia spp. bacteria have a symbiotic relationship with legume plant roots. The bacteria live in nodules in the legume roots. Plants cannot absorb nitrogen (N2) from the air. The bacteria convert nitrogen (N2) from the air by adding hydrogen to create ammonium ions (NH4+) which the plants can absorb. Converting the nitrogen by adding hydrogen is what is called “nitrogen fixation” in legumes.

For other plants, other bacteria add oxygen to fix nitrogen.

Inoculate the legume seeds with rhizobacteria to make sure this process occurs.

Inoculate with nitrogen-fixing bacteria

We advise using inoculants with legume cover crop seeds to give an extra boost of rhizobacteria. Average soil has some rhizobacteria, but not enough to fix a large amount of nitrogen. Some of our legume cover crop seeds come pre-coated with inoculant (“rhizocoated” or “nitrocoated”) and others need to have it added just before planting. Colorado State University Extension has a good overview on using inoculants.

For more information on the wonders of soil, and how cover crops help build healthy soil, take a look at our favorite books on soil and cover crops.

Teaming with Microbes is an award-winning book that tells the scientific story of soil in easy to understand language—with action photos too. Wait till you see the photo of the nematode being lassoed by a fungal hypha. You’ll be glad you’re as tall as you are.

  Comments (4)


What happens with legumes to fix nitrogen in the soil?

Posted by moh.belaid on Dec. 29, 2012 at 2:29:22 AM

Moh.Belaid, The third paragraph of this article has a good explanation of the nitrogen fixing process.

Posted by on Dec. 29, 2012 at 2:35:17 PM


Garden plot is low in nitrogen what ammendments can i add to fix nitrogen into the soil when planting during the spring.

Posted by Carolyn on Sep. 27, 2014 at 11:34:34 PM

Cover crops are planted usually in the fall and will add nitrogen, as well as organic matter to your garden when tilled under in the spring. Cover crops can also be spring planted and usually quickly grow and used as a “green manure” for your soil. Watch our video on cover crops to obtain more information.

Posted by Suzanne at Peaceful Valley on Oct. 07, 2014 at 2:17:11 PM

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