More than 3,000 species now exist, and they live almost everywhere. One acre of cultivated land may be home to as many as 500,000 earthworms, each making your soil a better place for plants. Earthworms are among a gardener's best allies, so whenever you work the soil, consider how your actions affect the worms below its surface. The earthworm pushes through soft earth with the point of its head. If the soil is hard, the worm eats its way through, forming interconnected burrows, some several feet deep.
Burrows loosen the soil, admitting air and water and helping roots grow.
As an earthworm feeds, organic matter passes through its body and is excreted as granular dark castings. You may see these small casting piles in your garden. An earthworm produces its weight in casts daily. Wormcasts are rich in nutrients otherwise unavailable to plants. When you add nitrogen-rich compost to your soil, you help worms.