Earthworms are constantly plowing through the soil, crisscrossing it over and over as they dig their burrows. Their burrows aerate compacted soil and provide a path for water and oxygen to penetrate to a plant's roots. The burrows increase water penetration to the roots and increase the amount of water that the soil can hold. They also reduce erosion by trapping water below ground level instead of allowing it to run off of the surface.
Earthworm's tunnels extend as deep as 8 feet into the soil. They often return to the surface to pull plant debris into their tunnel. In the tunnel the plant material is shredded, partially digested, mixed with intestinal fluid and incorporated into the soil in the tunnel walls. Moving plant debris from the surface to below ground level and mixing it with the soil is a natural form of composting and results in the release of nutrients into the soil as the organic matter decomposes.
As earthworms crawl through the ground they eat small soil particles, decaying roots and leaves, fungi, bacteria and smaller worms. Enzymes in their digestive tract transform their food into plant fertilizer. Their excrement, or casts, can be up to five times higher in nitrogen, contain seven times the potash and have 11 times the phosphates compared to soil with no earthworms. Each worm can eat half their body weight each day and can produce 10 lbs. of high quality fertilizer each year if there is enough organic material available for food.
In addition to providing nutrients to soil, earthworms provide nutrition and are an important food source to a wide variety of animals. Their high numbers, lack of formidable defense, and ease in catching make them easy prey for toads, beetles, hedgehogs, moles, slugs, snails, spiders, ducks and birds. They are especially important as food for snakes and are the primary food source of the common garter snake and the only food source of the worm snake.