Nitrogen is one of the four most abundant elements in living things. Nitrogen is a major component of DNA and RNA, proteins, and cellular energy molecules. Needless to say, it is very important to all living things. Most nitrogen on Earth is in a gas form that living things can't access to process—the air we breathe is about 78 percent nitrogen gas (N2).
Humans and other animals get nitrogen from the foods they eat. Some autotrophs, like plants, get their nitrogen from products that bacteria make. The bacteria take the nitrogen from the air and "fix" it in a form that plants can use. This process is called nitrogen fixation. Bacteria use enzymes to convert N2 into ammonia compounds that plants can absorb through their roots.
Some bacteria live in the roots of plants like soybeans, alfalfa, peas, and peanuts. Farmers plant these kinds of plants on sections of their farms to enrich the soil with nitrogen. The next season they plant other crops that use the nitrogen left in the soil in the roots of the nitrogen-fixing plants the year before. This is called crop rotation.