SAN JUAN BAUTISTA, Calif. — Earthbound Farm's fields of organic baby spinach and romaine lettuce are a living symbol of the organic food movement's explosive growth in recent years.

What started two decades ago as a three-acre roadside farm in this valley 90 miles south of San Francisco has grown into the country's largest grower of organic produce, with more than 100 types of fruits and vegetables on 28,000 acres in the U.S. and abroad.

Earthbound's extraordinary growth is only the most visible example of how organic farming is changing. Small family farms created as an alternative to conventional agriculture are increasingly giving way to large-scale operations that harvest thousands of acres and market their produce nationwide.

And with Wal-Mart, Safeway, Albertson's and other supermarket chains expanding organic offerings, the transformation may only be in its early stages.

"I don't think (consumers) have any idea just how industrialized it's becoming," said Michael Pollan, a journalism professor at the University of California, Berkeley, and author of "The Omnivore's Dilemma." "There are some real downsides to organic farming scaling up to this extent."

Pollan and others worry that the expansion of "Big Organic" will lower food quality, weaken standards and hurt small family farms. As organic goes mainstream, critics say, the movement loses touch with its roots as an eco-friendly system that offers a direct connection between consumers and the land where their food is grown.

Byron Albano, who handles marketing for Cuyama Orchards, his parents' 210-acre organic apple orchard in Santa Barbara County, worries the entry of Wal-Mart and other supermarket chains will "lead to organic produce becoming a commodity with prices being dictated by those buyers."

Other experts say the trend simply gives more consumers access to high-quality food and keeps prices down. It's also good for the environment because fewer pesticides and fertilizers will pollute the air and water.

Despite its size, Earthbound Farm follows the same practices as smaller organic farms. It rotates crops to enrich the soil and avoid disease, doesn't use chemical fertilizers or herbicides, and brings in syrphid flies and other beneficial insects to control pests.

Earthbound's bagged salads and other organic products are now sold in more than 80 percent of U.S. supermarkets.

"Earthbound Farm's mission is to bring the benefits of organic to as many people as possible," said Myra Goodman, who founded the company with her husband Drew.

Organic food only makes up 2.5 percent of U.S. food sales, but it's the fastest growing segment of the market. Sales reached nearly $14 billion last year, up from $6 billion five years earlier, according to the Organic Trade Association in Greenfield, Mass.

"Consumers see organic products as fitting in with a healthful life," trade association spokeswoman Holly Givens said.

To meet demand from health-conscious consumers and supermarket chains, farmers and ranchers are scaling up production and converting land to meet organic standards set by the U.S. Department of Agriculture.

According to the USDA's rules, organic produce must be grown without synthetic fertilizers or bioengineering and animals must be raised without antibiotics or growth hormones. A separate industry of government-approved organic certifiers has emerged to inspect farms and food handlers to ensure they conform. Some advocates don't think the rules go far enough and are asking for a requirement that dairy cows be pasture-fed, not raised on feedlots.

The latest USDA survey in 2003 found that 2.2 million acres of farmland and ranchland had been certified organic, but that number is believed to have risen substantially since then, said Jake Lewin, of California Certified Organic Farmers.

Concerns about the increasing commercialism of organic farming reached a new level when Wal-Mart announced it was joining other major grocery chains in ramping up organic sales.

Some small farmers worried that the world's biggest retailer, notorious for squeezing suppliers to get the lowest price, would push them out of business.

Other advocates welcome the news, saying growers would benefit from rising demand and consumers would see prices drop. In the past, organic food has been associated with high-end retailers like the Whole Foods Market supermarket chain.

"It will bring organic to a whole new economic stratum that our farmers' markets and natural food stores have been unable to reach," said Bob Scowcroft, of the Organic Farming Research Foundation.