For thousands of years, the Maya, in what is now Central America, have planted their fields with alternating rows of corn and beans. Together corn and beans provide all the protein needed for a human diet. But that's not the only reason this combination has been so successful.
If you pull up a bean plant you can see that the roots are covered with little, reddish nodules. Bacteria on the roots of the plant trigger the plant to produce these nodules, which provide a home for the bacteria. These "nitrogen-fixing bacteria," as they're called, live only on the roots of legumes, such as beans and alfalfa, and provide virtually all the nitrogen used by plants and animals.
The atmosphere is about seventy-eight percent nitrogen an essential element for most forms of life. But atmospheric nitrogen consists of pairs of nitrogen atoms bonded so tightly to each other that they can't be used by either plants or animals. And that's where nitrogen-fixing bacteria come in.
These bacteria split the nitrogen atoms apart and combine them with other molecules, making the nitrogen available to the plants that the bacteria are living on. In exchange, sugars produced by the plant's photosynthesis fuel the process of nitrogen-fixation and enable the bacteria to grow.
These bacteria not only help the plants they live on, but release useable nitrogen into the soil which is taken up by other plants, and in turn passed on to animals that eat those plants.
The Maya knew that corn and beans grow well together, just as in this country some farmers rotate their crops, planting alfalfa or beans every few years to replenish the soil's nitrogen.