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Do Legumes Really Fix Nitrogen in the Soil?

By Margaret Smith, Ph.D., Forage Agronomist, Sponsored by Albert Lea Seed

Farmers have known for millennia that beans and other food legumes provide benefits when grown with grain species or when grains or vegetables follow beans in rotation. But do legumes really add nitrogen to the soil?

Most of the legumes important in agriculture are known as nitrogen fixers, but the plants themselves don’t really fix nitrogen from the atmosphere. These legumes can form a mutually beneficial association with rhizobia bacteria that “fix” nitrogen from the air and share it with their host plant.

Rhizobia bacteria are free-living soil bacteria that, during a portion of their life cycle, can infect the roots of legumes (in a good way) and form nodules on the plants’ roots. During this portion of the bacteria’s life cycle, their numbers increase. It wasn’t until 1889, when rhizobia bacteria were isolated and identified, that we knew what caused that legume “advantage.”

Nodules on legume roots help fix nitrogen
Root nodules occur on the roots of legumes that associate with symbiotic nitrogen-fixing bacteria (known as rhizobia)

Rhizobia bacterial species co-evolved with their legume hosts, and many of them are specific to individual legume species. In fact, most of our legume oilseed, forage, and cover crops aren’t native to the U.S. and neither are their specific companion rhizobia species. As an example, for alfalfa plants to nodulate and fix nitrogen, a specific rhizobia species (Sinorhizobium meliloti) must be present in the soil or introduced with the seed. This rhizobia can also colonize sweetclover as its host species, but not red clover, which needs a different bacteria (Rhizobia leguminosarum (biovariant) trifolii) to develop its nitrogen-fixing capabilities.

Soybeans require yet another bacteria species (Bradyrhizobium japonicum) to nodulate and fix nitrogen. This bacteria forms round nodules on soybean roots, compared with the knobby or irregularly shaped nodules that form from rhizobia on most other forage legume roots.

Because of this species-specific symbiosis, any legume new to a cropping system should be inoculated to provide the specific rhizobia species needed for nitrogen fixation. Where legume species are repeatedly grown in a crop rotation, you may not need to inoculate each time the legume is planted. Factors that affect rhizobia survival in the soil in years where their host legume isn’t grown include low pH (less that 5.5-6.0) and extremely hot or extremely dry soil conditions.

We recommend inoculating your legume species if:

  • The legume has never been grown before in your cropping system; for example, hairy vetch, dry beans or sunn hemp.
  • The legume was grown in the past, but you aren’t sure that the plants nodulated well. Was performance poor?
  • The legume was grown in the past, but only in a small proportion of the total crop mix, such as lentils in a cover crop mixture.
  • The legume crop has not been grown for several years. In this situation, rhizobia levels in the soil will decline with time. For example, if soybeans have not been grown for three or more years, you should inoculate the next soybean crop.

How to Inoculate Legumes with Beneficial Bacteria

A hundred years ago, farmers were advised to inoculate a “new” field by transferring soil from a field where their preferred legumes had already been grown. Fortunately, inoculation is far easier today.

An inoculant is a formulation of a carrier and the live rhizobia bacteria. Commercial inoculants may be powdered (peat-, clay-, or talc/graphite-based), granular or liquid. They are formulated for either application directly to seeds or to drop in the seed furrow at planting.

Peat-based inoculants contain the most bacteria per unit of carrier, but the bacteria in this formulation is very short lived. After opening a package and applying to seed, the seed should be planted within 24 hours. Granular applications are formulated for ease of application to apply directly in a seed furrow, rather than on the seed. Individual planters and drills may not be equipped for this type of application. Clay-based inoculants are applied to seeds and maintain viable rhizobia for a year or more.

Organic growers have access to many OMRI-approved inoculants specific to each legume species.

At a cost of about $1 to $3.50 per acre, inoculation is a relatively inexpensive “insurance” for your soybean, forage and cover crop legumes.

For more information, visit alseed.com or call 800-352-5247.

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