The first part is material such as clay, sand and gravel, which is really small rocks. This material doesn't support much plant life. Humus helps hold water in the soil and keeps the soil from getting too hard. Trees, shrubs and most other plants require soil that contains humus. In forests and woodlots, the leaves that fall off the trees each year add large amounts of organic material to the soil.
Earthworms have no teeth, so they coat their food with saliva to aid digestion.
The leaves and soil enter the earthworm through its mouth and end up in an area known as a gizzard. Within the gizzard, the food is ground up with the help of tiny stones. The food then travels through the long intestine and exits as waste material. As the earthworm travels through the soil digging, eating and digesting, it actually improves the quality of the soil. The nutrients contained in the castings are one of nature's most important fertilizers.