Legumes are flowering plants, and most produce seed pods. Legumes are one of the most prolific flowering plants in the world, and most varieties are edible. One important characteristic of legumes is that they contain a high level of protein compared to other plants; soybeans contain the only naturally-occurring complete vegetable protein, and so are valuable as food for both humans and animals. Legume seed pods can grow above and below ground. One major difference between legumes and other plants is that legumes contain root nodules that can fix nitrogen from the atmosphere instead of taking it from the soil. By seeding legumes alternately with other crops, farmers can replenish soil without costly nitrogen fertilizer. The bacteria in the root nodules is called Rhizobia, and the nitrogen collected in these nodules helps the legume to grow; when the plant dies, the nodules break down, allowing the nitrogen to spread throughout the soil.