As every gardener -- and grape grower -- knows, it takes good dirt to grow tasty crops.

While a well-tended compost pile remains the traditional way to build nutrient-rich soil for plants, it requires full sun and a strong back. For high-quality compost without all that shoveling and turning, many home and commercial growers have been enlisting worms to do the work instead.

One Napa vermiculturist -- or worm farmer -- has even turned professional, selling worm compost, concentrated "compost tea," and red, wriggling worms to gardeners and teachers eager to start their own worm bins.

Professional growers with larger orders call Lois and Jack Chambers at Sonoma Valley Worm Farms, where the minimum order for worms is two pounds.

Among the Chambers' Napa Valley customers are wineries and Copia, where garden director Colby Eierman maintains a half-ton worm bin but sometimes needs even more vermicompost for the extensive Copia beds.

Eat. Excrete. Repeat.

Worm castings -- what worms leave behind after they chow down -- make a dark, rich, crumbly matter that smells like wet earth. Called vermicompost, it's rich in nutrients from organic matter, making it an ideal soil builder.

The most popular worms for the job are red wigglers -- Latin name Eisenia Foetida, better known as "the Cadillac of worms" in a famous jingle (for bait) on the '70s-'80s sitcom "WKRP in Cincinnati."

Add a batch of red wigglers to a properly prepared bin, provide fresh vegetable and fruit scraps, and the worms immediately get to work doing what they appear to have been put on earth for: Eating, excreting and making more worms.

"All they do is eat, poop and reproduce," said Debby Stevens of Napa's Worm Endings Unlimited.

Stevens began worm composting as a hobby when she moved from a house to a condo five years ago.

"I felt so guilty," because she could no longer compost her food scraps, she said, so she attended a city-sponsored workshop on worm composting and started a bin in her pantry.

"Now I'm one of those speakers" at gardening workshops, Stevens said. She's also Napa's only known worm entrepreneur, charging $4 a pint for vermicompost, $12.50 a pint for compost tea -- touted for foliar and root applications -- and $25 a pound for worms, though she'll sell smaller quantities.

Stevens does a bustling business at the Napa and Sonoma farmers markets, and also maintains a Web site (www.wormendingsunlimited.com). On local roadways, she's easy to spot in her purple "Worm Mobile," with the vanity plate WORMT4U.

Stevens said initially she didn't plan to sell worms, just compost and tea. "I thought, 'I don't want to sell my workers!'" she recalled.

But now, she says "worms, not compost, are the big product."

Worms go to school

Napa teacher Karla Bunter recently purchased worms from Stevens to start a bin for the after-care program at Stone Bridge School in Napa. The kids will feed the worms scraps from their afterschool snacks, and use the compost in school gardening projects.

Worm bins are a popular teaching tool because they're easy to set up, maintain and observe, providing kids with a hands-on lesson in how the worms process food, create compost and multiply in numbers.

Light-textured fruit and vegetable scraps -- not heavy stalks and roots -- are easiest for the worms to process. At Copia, the red wigglers in Eierman's half-ton bin get all the trimmings from nearby beds of tender greens.

Meats, dairy and other fatty foods are tough for worms to digest, and attract flies and other pests. Coffee grounds, on the other hand, are fine, as long as they don't make the bin too wet.

Worms need moisture to thrive, but too much water and they'll drown. They prefer their bedding -- whether thinly shredded newspaper or barnyard straw -- to be about as damp as a wrung-out sponge. Worm bins must be well-ventilated and have good drainage.

Along with the right amount of moisture, worm bins need a shady spot -- unlike traditional composters, which have to "cook" in full sunlight. Worms tolerate cold temperatures far better than hot ones; Stevens lost eight pounds of worms in last summer's triple-digit heat wave.

Once up and squirming, worm bins need little maintenance beyond ensuring that they have the right amount of moisture, bedding, air and food.

The Internet is full of Web sites with simple instructions for setting up a home worm bin, and Stevens is always glad to dispense advice along with worms; she can be reached at 255-1148, 287-0891 or via www.wormendingsunlimited.com.