Legumes and Nitrogen Fixation - February 15, 2012
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Approximately 80% of the air surrounding the earth is nitrogen gas. However, this nitrogen 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 in the atmosphere 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 nodules which are part of the legume root system.

Rhizobium bacteria transform nitrogen gas into ammonium, which can be used by the plant. This ammonium is the same form of plant available nitrogen as that found in ammonium nitrate (34-0-0) and ammonium sulfate (21-0-0) fertilizers. This process is called nitrogen fixation because the Rhizobium bacteria “fix” the nitrogen by converting it from the unavailable form 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 by the plant during photosynthesis. The relationship is actually much more complex than the aforementioned explanation.

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. Gardeners often use this symbiotic relationship to their advantage by planting legumes as a crop.

Rhizobium bacteria strains are available commercially for inoculation of legume seed prior to planting. 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 by broadcasting the seed and raking it lightly to cover it with soil.

I seeded my vegetable garden this fall with a mixture of hairy vetch (a legume) and cereal rye (a non-legume). I used an inoculant purchased from the supplier that was recommended for hairy vetch. 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 added fertilizers and organic matter.

Many of our native plants are also legumes. These include mesquite, catclaw, palo verde, New Mexico locust, and several other shrubs and forbs. Nitrogen fixation in the roots of these plants produce small amounts of nitrogen which meet the needs of natural ecosystems. When nitrogen is in limited supply, subsequent growth remains in balance with the precipitation and availability of other plant nutrients.

So, learn to identify and appreciate legumes for the service they provide – nitrogen fixation. If you have a vegetable or flower garden, consider planting a nitrogen fixing cover crop during the winter season. Finally, do not apply nitrogen fertilizers to natural and xeriscape landscapes – these plants are adapted to limited amounts of nitrogen and many are legumes that fix their own nitrogen.

If you have other gardening questions, call the Master Gardener help line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: February 7, 2012
Content Questions/Comments: jschalau@ag.arizona.edu
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