Growing Legumes to Increase Soil Nitrogen - December 1, 2004
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Approximately 79% of the air surrounding the earth is nitrogen gas. However, it is not in a form that plants can use. Legumes are plants such as alfalfa, peas, beans, clover, vetch, and their relatives (including mesquite and palo verde trees). Legumes help convert nitrogen gas into plant-available nitrogen. In reality, the legumes cannot convert nitrogen from the air without help. They need the assistance of Rhizobium bacteria which live in small tumor like structures on the legume roots called nodules.

Rhizobium bacteria can convert nitrogen gas from the air in the soil and transform it into ammonium (NH4), which can be used by the plant. This ammonium is the same form as in ammonium nitrate (34-0-0) and ammonium sulfate (21-0-0) fertilizer. This process is called nitrogen fixation because the Rhizobium bacteria “fix” the nitrogen by converting it from the unavailable form found in the atmosphere to a form which plants can use.

The relationship between legumes and Rhizobium bacteria is symbiotic (both organisms benefit). The legume receives nitrogen that will stimulate growth and the Rhizobium receives carbohydrates produced during photosynthesis. The relationship is actually much more complex than the aforementioned explanation and has resulted because the two organisms have coevolved for hundreds of thousands of years.

Some Rhizobium bacteria exist in soils where native legumes are found. However, the quantities present in soil will usually not be adequate to inoculate an entire crop of legumes. Furthermore, there are multiple strains of Rhizobium bacteria and each is specific to a certain type of legume. For example, the Rhizobium bacteria strain found in association with vetch will also work with peas, but not alfalfa. When legumes are inoculated with the proper strain of Rhizobium bacteria, they produce large, pink nodules on the roots of the host plant. The pinkish color comes from the presence of a hemoglobin-like molecule that is necessary for nitrogen fixation to occur.

Several Rhizobium bacteria strains are often mixed together to create commercial inoculants. The typical inoculant is a mixture of finely ground peat moss and live Rhizobium bacteria packaged in a small plastic bag. The inoculant has a certain shelf life and must be stored under cool, dark conditions. Reputable dealers should have high quality inoculants of the correct strain for the legume seeds that they sell.

To inoculate, moisten seeds with just enough water to coat the seeds in a shallow bowl, pour on the inoculant and mix well to coat seeds. Some growers use a 10% mixture of sugar or molasses to moisten the seeds before adding the inoculant. This makes the solution stickier and can increase the amount of inoculant surrounding each seed. Plant seeds immediately after inoculation. A seed drill is best to get the inoculated seeds underground where the inoculant will be protected from light and heat. Smaller garden areas can be broadcast seeded and raked into the soil.

I seeded half of my vegetable garden this fall with a mixture of hairy vetch and cereal rye. I used an inoculant purchased from the supplier that was recommended for hairy vetch. After digging up a vetch seedling this morning, I found that it had nodulated. My plans are to allow this cover crop to grow until 50% of the flowers are in bloom before I mow it and incorporate it into the soil as green manure. The usage of cover crops and green manure is an important sustainable agriculture practice that adds nitrogen and organic matter to the soil and reduces the need for chemical fertilizers.

The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: December 1, 2004
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