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http://sonomamg.ucanr.edu/files/29954.pdf

Brochure

Worms you buy from a worm farm or supplier are generally either Red Wiggler Worms, Eisenia fetida, or European Night Crawlers, Eisenia hortensis. But many worms are called Composting Worms. The Red Wiggler, Eisenia fetida, is best as is lives in the top 18” of the soil and eats 1/2 its body weight each day in optimal conditions.

These worms live just under the surface, ingesting soil along with decaying organic matter and leaving tunnels that carry oxygen to plant roots and improve drainage. This activity breaks up heavy dirt clods, and the resulting castings keep the soil loose and provide nutrients to plants.