The process of converting atmospheric nitrogen into ammonia is known as nitrogen fixation. One of the most critical aspects of the nitrogen cycle is nitrogen fixation.
There are two types of nitrogen-fixing bacteria. The cyanobacteria (or blue-green algae) Anabaena and Nostoc, as well as genera like Azotobacter, Beijerinckia, and Clostridium, belong to the free-living (nonsymbiotic) bacteria. Mutualistic (symbiotic) bacteria are the second type; examples include Rhizobium, which is associated with leguminous plants (e.g., various pea family members); Frankia, which is associated with certain dicotyledonous species (actinorhizal plants); and certain Azospirillum species, which are associated with cereal grasses.
The symbiotic nitrogen-fixing bacteria infiltrate the root hairs of host plants, multiplying and stimulating the creation of root nodules, plant cell enlargements, and bacteria in close proximity. The bacteria in the nodules convert free nitrogen to ammonia, which the host plant uses for growth. Seeds of legumes (e.g., alfalfa, beans, clovers, peas, soybeans) are frequently inoculated with commercial cultures of appropriate Rhizobium species to ensure sufficient nodule formation and optimum growth, especially in soils weak in the bacterium.
