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Inoculation of Legumes

Written by Anowar Islam

As spring progresses, good weather shows up and soil temperature warms up, farmers begin to think about spring planting. Recently, I started receiving many phone calls asking questions about forage crops, especially legumes – for example, alfalfa and sainfoin. The questions inquired about are varieties of legumes, seeding rates and planting time. One important aspect of forage production that farmers haven’t asked about, though, is inoculation of legumes.

For high-level forage production, nitrogen is required in large amounts. Deficiencies of nitrogen can cause a significant loss of forage and animal production. Nitrogen is available everywhere in the atmosphere, with the atmosphere consisting of about 80 percent nitrogen. You may be surprised to learn that one acre of land can have an estimated 37,000 tons of nitrogen in the air above it. Although lots of nitrogen is present in the atmosphere, unfortunately, it is not readily available to plants because these molecules are in the form of inert gas.

What are the sources of nitrogen? There are several sources of nitrogen that plants can use for their growth. Free-living soil bacteria and blue-green algae can supply small quantities of nitrogen. Precipitation can also add small amount of nitrogen to the soils. Organic manures and compost can add some nitrogen to the soils, as well. However, nitrogen content in organic sources is limited.

Commercial fertilizers are the number one and major source of nitrogen. Nevertheless, they are expensive.

Legumes can produce substantial amount of nitrogen, and they are low-cost compared to commercial sources.

Legume plants are unique in that they have the ability to fix atmospheric nitrogen. Most legumes have a symbiotic association – or mutual benefits – with bacteria, called Rhizobium. These bacteria infect the roots of the legumes from which they get their own food to survive. On the other hand, the bacteria obtain nitrogen from soil air and fix it in the root in a useable form. The nitrogen accumulated in the root forms small outgrowth structures called nodules.

The accumulated nitrogen in the root nodules can be used by legumes, by grasses mixed with legumes or by other crops planted in rotation with legumes. In addition, this fixed nitrogen can be transported long distance from one field to another by grazing livestock through dung and urine.

Many factors affect nitrogen fixation. These include Rhizobium populations in the soil, temperature, soil moisture, soil pH, soil fertility, amount of shading and, most importantly, the species of legumes. Nitrogen fixation can be adversely affected by reduced growth of plants and some nutrient deficiencies, such as calcium and potassium. Adding more than 25 pounds nitrogen per acre can reduce nitrogen fixation and enhance weed growth.

For good nitrogen fixation, it is important to have large numbers of live bacteria in the soils and legume roots. It is generally recommended to inoculate legume seeds with the right bacteria before planting. Please remember that legume seeds must be inoculated with species-specific bacteria. That means alfalfa seeds need to be inoculated with alfalfa-specific bacteria, sainfoin with sainfoin-specific bacteria and so on. Remember, alfalfa bacteria will not work for sainfoin and vice versa.

Many legume seeds, such as alfalfa, white clover, red clover and birdsfoot trefoil,  are pre-inoculated and commercially available. They do not require inoculation. However, it is highly advisable to check whether seeds are pre-inoculated or not before purchase. If not, species-specific bacteria need to be purchased, and the legume seeds should be inoculated before planting.

Remember, the correct, adequate bacteria will result in a significant amount of nitrogen fixation in the soils. As an example, in the right situation, alfalfa can fix 200 pounds or more nitrogen per acre per year.

Legume inoculation is important and deserves careful attention.

Anowar Islam is an associate professor and the University of Wyoming Extension forage specialist in the Department of Plant Sciences in the College of Agriculture and Natural Resources. He can be reached at 307-766-4151 or This email address is being protected from spambots. You need JavaScript enabled to view it..