Earthworms
Abundant in moist, heavy soils, earthworms are a natural component of healthy lawns. Their diet of dirt, organic matter, and plant litter, is excreted in the form of a rich digestive byproduct called castings. These hardened small piles can be found scattered across the ground. While initially they can be felt underfoot, castings will eventually break down, providing your lawn with a dose of natural fertilizer. Worm castings are sold commercially for this very purpose.

In addition to helping provide nutrients for plants, earthworms aid in thatch decomposition. They also improve soil aeration and increase water penetration through their extensive burrowing.