Before they began farming in 2001, Homer Walden and Dru Peters knew that agriculture was the single largest source of pollution to the Chesapeake Bay. They were aware that raising animals with conventional practices contributes large amounts of nitrogen and phosphorus to area waterways. They understood that the same poisons that kill weeds and pests also find their way into creeks and rivers, where they kill other living organisms.

They wanted to farm differently. They wanted to produce not only high-quality, hormone-free food, but also food that left the land and the Chesapeake Bay as clean as it could be.

After years of experimenting with different techniques, the couple has achieved their goal. Their Sunnyside Farm, nestled in northern York County, PA, is a zero-discharge farm. They use no commercial fertilizer, spray no chemicals and contain and reuse all of their stormwater. Animals — and engineering — do all of the work.

The chickens supply the fertilizers. The couple keeps 450 egg-layers in 100-square-foot mobile pens on wheels. Every day, they move the pens down a wide row, where they plant vegetables. The chickens eat antibiotic-free food and then excrete their waste. Day after day, Walden moves the pen until the whole row is fertilized.

Next come the pigs. They're the weed control. It takes two or three of them about eight hours to clear a 100-square-foot area of all manner of weeds. They eat and dig, plowing nutrients back into the ground. Then, Walden moves them down the row and they clear the next square.

Next come the cattle. They're the mowers. Ten cows graze on the grass, cutting it down. In the process, they become the pasture-fed beef that customers crave.

Last, there are the broilers. They're the bug control. They eat the pests and help the hens fertilize the row.

On top of a 400-square-foot hoop house, where Walden grows radishes and Swiss chard, rain gutters send stormwater into rain barrels. Drip tape and PVC pipes carry this water out to the beds. They're the irrigators, adding extra water to his crops so he doesn't have to use a hose. Walden also collects this water to feed the chickens.

"We're pretty crazy," Walden said as he showed off his contraptions.

If that's true, it's a good kind of crazy. Walden and Peters have been running weekend workshops on how to build the movable chicken pens using parts from local Home Depot stores. Dozens of aspiring backyard farmers have come to learn the ways of the "movable livestock" movement, which Joel Salatin popularized at his Polyface Farm in Virginia. Last year, Sunnyside received both the Pennsylvania' Governor's Award for Environmental Excellence and the national Farmer Heroes Award from Farm Aid.

Walden, who never graduated high school, was a longtime sheet-metal worker. Eventually, he learned how to build models for weapons systems at Johns Hopkins' Applied Physics Lab in Laurel, MD. He did not invent the mobile chicken pens, but he tinkered with the concept and went through eight prototypes until he developed the right one. His pens, he said, are so easy to move that a 6-year-old could do it; his grandson, who lived with the couple at the farm for a while, was able to move them easily. Walden, who entered farming in his 40s and is now 59, said making the pens easy to move was a necessity; though he is strong now, he said, he won't necessarily be able to do the heavy lifting farming requires in 10 years.

Peters, who was a longtime editor at Prentice Hall in New Jersey, does the marketing and the research for the farm. Sunnyside makes its money with a community-supported agriculture model, offering shares of vegetables, chicken, pork and eggs to about 100 customers. Most are in the Baltimore area, where the couple lived for decades before moving to Pennsylvania. Peters also sells her meat and produce at a farmer's market in Hershey every weekend. The couple lives off what they grow, a steady diet of fresh pork, beef and eggs that taste so good they bear little resemblance to their grocery-store counterparts. A carrot pulled out of the ground, its top still on, tasted as though it were dipped in sugar. A radish was crisp and delicious.

"People come here, and they say, 'this is the food that I want to be eating,' " Peters said.

It was the quest for good, fresh and sustainable food that drew the couple into farming. But their low-tech methods have other upsides they have learned to appreciate. They don't have to contend with diseases such as avian flu, and they don't have to feed their chickens hormones to grow to a certain size or antibiotics to keep them healthy. With no fertilizers added besides the chickens' contribution, they don't have to worry about damage to the nearby creek.

That's important to Peters, the daughter of a Smithsonian herpetologist.

"I was really raised to see that there's a connection in all that we do," Peters said. "I wanted to have systems in place to provide food for people with as little environmental impact as possible."

Increasingly, Peters and Walden aren't the only farmers trying to do right by the Chesapeake and fielding customers' questions about their farming practices. Customers still ask what a farmer feeds his cattle, but they also want to know if he fences them out of streams or if he uses manure or commercial fertilizer.

"We spend a great deal of time explaining how we farm," said Tom Albright, a third-generation family farmer in Baltimore County. Albright's operation raises 5,000 broilers a year. Like Sunnyside's owners, he uses the mobile-pen concept. Though he can't claim to be zero-discharge — his farm is too large, with 200 acres and 150 head of cattle – he tells customers about a lot of his environmental practices. Those include a decades-long devotion to no-till and cover crops, and a $300,000 project to put in a manure storage area and covered feeding barn.

Steve Rouse, a former Baltimore radio host who farms six acres in Harford County, said he has made a lot of investments to control runoff that is coming from neighboring farms at his small operation. Customers do ask about how he's protecting the Chesapeake, in addition to asking about when they can come pick blueberries and strawberries.

"It's obviously more and more all the time, the awareness," Rouse said. "I'm sure there are some farmers who still feel they're doing the best they can, and they don't want to spend the money to change things around."

Walden and Peters know that the big chicken companies of the world are never going to raise chicken the way they do. But the idea of food grown without pesticides, chemicals or discharges to local waterways seems to be an appetizing one. Through little more than an e-mail list, Peters has built Sunnyside into a viable business.

"People told us, 'you can't feed the world on little ideas,' " Walden said. "Well, basically, that's how the world eats. It's a lot of people making little changes."