The earthworm may seem like a simple animal, but Charles Darwin spent decades studying the slimy crawlers and once said they played a vital part in our world's history. Darwin isn't the only person to recognize their value. Most people who are into composting already know the benefit of earthworms. They are nature's farmers, plowing the soil as they tunnel through it. The United States Department of Agriculture (USDA) estimates that it takes earthworms 10 to 20 years on average to turn over the top 6 inches (15 centimeters) of soil. The tunnels they leave behind also serve a purpose by circulating air and water into the soil, keeping it fresh and nutrient-rich. Earthworm droppings, called castings, are also essential, as they are rich in nitrogen, calcium and other nutrients that are indispensable for a healthy ecosystem. According to the USDA, one square yard of soil can contain up to 300 earthworms, and while not all soil contains earthworms, their presence is usually a good sign of healthy dirt.