Final answer:
Nitrogen-fixing bacteria are found in the soil and the nodules of legumes, where they form a symbiotic relationship with the plants. These bacteria convert atmospheric nitrogen into a usable form for the plants, helping to enrich the soil and promote plant growth.
Step-by-step explanation:
Nitrogen-fixing bacteria are found in the soil and in the nodules of legumes. These bacteria form a symbiotic relationship with legume plants, providing them with a source of fixed nitrogen. The nodules provide an oxygen-free area for nitrogen fixation to occur, allowing the bacteria to convert atmospheric nitrogen into a usable form for the plants.