Farmers have known for millennia that beans and other food legumes provided a benefit when grown with grain species or when grains or vegetables followed beans in rotation. It wasn’t until Rhizobia bacteria was isolated and identified in 1889, that we knew what caused that legume ‘advantage’.

Do Legumes Really Fix Nitrogen?

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 symbiotic (mutually beneficial) association with Rhizobia bacteria that ‘fix’ nitrogen from the air and share it with their host plant. There is a tradeoff, though. Legume plants expend energy supplying the bacteria with carbohydrates and mineral nutrients. For both the legumes plants and the farmers who grow them, the tradeoff is worth it!

Rhizobia bacteria are free-living soil bacteria that, during a portion of their life cycle, can infect the roots of legumes and form nodules on the plants’ roots. During this portion of the bacteria’s life cycle, their numbers increase.

Different rhizobia species co-evolved at their geographic centers of origin with their legume hosts and are fairly specific to legume plant species. For instance, for alfalfa plants to nodulate and fix N, the rhizobia species, Sinorhizobium meliloti must be present in the soil or introduced with the seed. This rhizobia also can colonize sweet clover as its host species.

Red clover, though, needs the bacteria Rhizobia leguminosarum (biovariant) trifolii to develop its N fixing capabilities. Soybeans require the bacteria species, Bradyrhizobium japonicum to nodulate and fix N. This bacteria has determinant growth and forms round nodules on soybean roots, compared with the nobby or irregularly-shaped nodules that form from indeterminate growth type rhizobia on most forage legume roots.

Most of our legume oilseed, forage, and cover crops aren’t native to the U.S. and neither are their specific companion Rhizobia species.

Because of this, 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 or may not need to inoculate each time that crop is planted. Factors that affect rhizobia survival in the soil in years where there host legume isn’t grown include: low pH (less that 5.5-6.0), extremely hot or extremely dry soil conditions.

When Should You Inoculate Legumes?

Albert Lea Seed recommends inoculating your legume species if:

1. The legume has never been grown before in your cropping system, for example: hairy vetch, dry beans or sunn hemp.
2. The legume was grown in the past, but you aren’t sure that the plant were well nodulated. Was leaf color and yield performance poor? Plants may not have been well-nodulated.
3. 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. It is impossible (in practice) to know the populations of Rhizobia remaining in soil following a diverse mix of species.
4. The legume crop has not been grown for several years. In this situation, Rhizobia levels in the soil will decline with time.

Farmers often ask, “How long after growing a legume crop do Rhizobia bacterial populations decline enough that I need to inoculate this same crop?” Much work has been done with soybeans to answer this question. In the Upper Midwest, research indicates that if soybeans have not been grow for three to five years or more, you should inoculate the next soy crop. Rhizobia species specific to forage legume species have been less studied for their longevity in soil.

How to Inoculate Legumes

One hundred years ago, farmers were advised to inoculate a ‘new’ field by transferring 300 to 500 pounds of 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 and are formulated to either apply directly to seeds or drop in the seed furrow at planting.

Peat-based inoculant 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 planter and drills may not be equipped for this type of application. Clay-based inoculants are applied to seeds and maintain viable Rhizobia fora year or more.

Albert Lea Seed provides pre-inoculation of all of our organic and conventional alfalfa, red clover, and white clover seed varieties, with either Apex^TMGreen Hydroloc or Prevail^TM. These clay-based inoculants are OMRI-approved.

For other legumes (soybeans, field peas, forage and cover crops) we carry OMRI-approved, peat-based inoculants specific to each legume species. For soybeans, peas, vetch and lentils, we also carry both a liquid formulation and a talc/graphite- based biological seed treatment that contains rhizobia in addition to a beneficial fungi, Trichoderma. Ask about these inoculants when you order seed.

At about $1 to $3.50 per acre, inoculation is an inexpensive ‘insurance’ for your soybean, forage and cover crop legumes.