The air we breathe contains more than 78 percent nitrogen in the form of nitrogen gas (N2). Legumes have the unique ability to form a symbiotic relationship with rhizobia (Rhizobium and Bradyrhizobium) bacteria to convert atmospheric nitrogen gas to ammonia nitrogen, a form usable by the plant. This relationship occurs in specialized root tissue called nodules. Some legumes, such as alfalfa, can produce enough ammonia to supply all their nitrogen needs (Table 1), hence nitrogen fertilization usually is not needed.

The relationship between the legume and rhizobia is symbiotic, or mutually beneficial. The bacteria invade plant root hairs and multiply in the outer root tissue. The plant forms tissue that acts as a protective enclosure around the bacteria. The plant also supplies energy to the bacteria from photosynthesis. For their part, the bacteria convert nitrogen gas to ammonia in the nodules.

Rhizobia species are identified by their ability to form nodules on specific legume species. Each legume requires a specific species and strain of rhizobia. Commercial inocula are labeled according to the plant species for which the rhizobia are highly effective.Successful nodulation and effective nitrogen fixation requires the correct inoculant.

Inoculation

Legume inoculation is the process of introducing commercially prepared sources of rhizobia to promote nitrogen fixation. This usually is done by applying inoculum directly to the seed prior to planting, or by metering the inoculum into the seed furrow during planting.

If the legume crop was grown in the field previously, there is a good chance that the soil already contains the correct rhizobial species for nodulation. However, native rhizobial populations found in soil often are less effective in nitrogen fixation potential. A nodule that is actively fixing nitrogen will be pink to reddish when cut open, rather than tan (ineffective) or green (dying).

Commercial inoculants are composed of rhizobial strains selected for maximum fixation potential. However, even when more efficient strains are introduced into the soil, there is no guarantee these strains will compete well with native strains for entry into plant roots.

Many studies have been conducted on the application of commercial inoculants into soils that already contain the correct rhizobial bacteria. In some studies, a significant yield increase has been observed. In other studies, no response occurred. Acid soils often have poor survival of the rhizobia. One way to evaluate the response of inoculants is to test several inoculants and an untreated control in fields using replicated strip tests. When in doubt about the rhizobial population in a field, it is a good practice to apply inoculum, especially if the legume has never or not recently been grown in that field.

Maintain proper soil fertility to ensure nodulation and nitrogen fixation. Some legumes normally get most of their nitrogen from the atmosphere through symbiotic nitrogen fixation. Attempts to supplement the legume nitrogen supply by fertilization usually are counterproductive, because plants tend to stop nitrogen fixationwhen soil nitrogen is high. Phosphorous and potassium can affect nodulation and nitrogen fixation. Research shows that additions of phosphorous and/or potassium increase the number of nodules formed, fresh weight of nodules, and amount of nitrogen fixed per nodule.

An important micronutrient for nitrogen fixation is molybdenum. Soils with a pH below 6.0 usually have low molybdenum availability. Other soils that could be low in molybdenum include those that are strongly weathered or leached, sandy soils, or soils high in manganese and iron. If molybdenum is a limiting factor, apply it as a seed treatment with the inoculum. Some inoculants have molybdenum already incorporated. Read the package label.

Quick Facts...

  • Legumes convert atmospheric nitrogen to usable ammonia nitrogen for the plant.
  • Inoculation is the process of introducing commercially prepared rhizobia bacteria into the soil.
  • Each legume species requires a specific species of rhizobia to form nodules and fix nitrogen.
  • Store inoculum and preinoculated seed in a cool environment without exposure to sunlight.
  • Inoculum packages usually are labeled with an expiration date.