Earthworms help maintain fertile soil. An earthworm literally eats its way through soil and decaying vegetation. As it moves about, the soil is turned, aerated, and enriched by nitrogenous wastes. Charles Darwin (1809–1882) calculated that a single earthworm could eat its own weight in soil every day. Much of what is eaten is then excreted on Earth’s surface in the form of “casts.” The worms then re-bury these casts with their burrowing process. In addition, Darwin claimed that 2.5 acres (1 hectare) of soil might contain 155,000 earthworms, which in one year would bring 18 tons of soil to the surface and in 20 years might build a new layer 3 inches (11 centimeters) thick.