We all know that earthworms in our soil are a good thing,and they improve drainage and soil structure, but perhaps the biggest benefit is that earthworms leave behind a richer soil than was there before. Worms feed on plant debris (dead roots, leaves, grasses, manure and soil) and their digestive systems concentrate the organic and mineral contents of the food they eat. What they cast off is a rich compost that is readily available for plants to thrive. In other words, castings or vermicompost.