Hint: The nitrogen-fixing bacteria are prokaryotic microorganisms that transform nitrogen gas from the atmosphere into fixed nitrogen compounds, for instance, ammonia, nitrite and nitrate which are used by plants.
Complete step by step answer: A lot of heterotrophic bacteria are found in the soil. These bacteria fix a substantial amount of nitrogen from the atmosphere without any direct interaction with other organisms. Nitrogen-fixing bacteria are mainly of two types; symbiotic or mutualistic. These species of bacteria are found in root nodules of certain plants like legumes. The plants of the pea family are called legumes. They are some of the major hosts for the nitrogen-fixing bacteria. Some other plants also provide shelter to the nitrogen-fixing bacteria. Few examples of symbiotic nitrogen-fixing bacteria are Rhizobium (it is generally associated with plants in the pea family also known as legumes) and several Azospirillum species (these are associated with cereal grasses). The free-living nitrogen-fixing bacteria include the cyanobacteria (or blue-green algae) for example, Anabaena, Nostoc, and other genera, for instance, Azotobacter, Beijerinckia and Clostridium.
Additional information: Nitrogen fixation is of two types, namely, physical nitrogen fixation and biological nitrogen fixation. Nitrogen is the most prevalent and essential macro-elements in living organisms apart from carbon, hydrogen and oxygen. Nitrogen comprises 75% of the air in the atmosphere.
Note: Nitrogen fixation is the method which includes the conversion of nitrogen gas from the atmosphere into other compounds like ammonia, nitrate and nitrites. These compounds can be used directly by plants and animals. The three major ways of nitrogen fixation are by lightning, by industrial methods and by bacteria living in the soil.