On a day too wet to harvest soybeans, the Russell brothers drove a Ford pickup through military security and went to work repairing their damaged combine. An F-18 fighter jet shot overhead with a sound like the sky ripped open and dropped onto a runway nearby.

That's how it is when the family farm is in the middle of a Navy air base.

The brothers harvest corn, soybeans and milo from fields near the runways and weapons compound of Patuxent River Naval Air Station in Southern Maryland. It's not exactly peaceful, and it looks a little strange to see a tractor puttering through a base where pilots and scientists test out cutting-edge aerospace technology.

But in St. Mary's County, where the base known as Pax River dominates the economy, it makes some sense for the Navy to host a farm: The fields ensure a clear sight line from control tower to runway. The crops minimize the risk of fire around the weapons compound, and they are safer than woods or dry brush in case of crash landings. The fields even help keep deer and birds away from the planes.

It takes just one turkey vulture flying the wrong way to bring down a $40 million jet -- and it's not easy to keep birds away from Pax River, with its 6,800 acres of land jutting into the Chesapeake Bay. Kyle Rambo, director of the base's conservation division, spends much time and energy trying to scare off birds and safeguard pilots.

It saves the Navy about $50,000 a year to have a farm there, Rambo said.

Besides, he said, "it's extremely rich, productive farmland. It's been farmed for more than 300 years."

Nationally, about 85,000 acres on Navy bases are leased by private farmers, generating almost $2 million a year for the federal government and saving about $6 million a year in mowing and maintenance costs, according to the Navy. Those funds can go to conservation programs on the bases, such as a migratory bird study at Pax River.

Andrew Russell said his family has been farming in St. Mary's for centuries. Brian, Andrew, Glen and Leroy Russell grew up on a tobacco farm, in a family with 13 children. As adults, the four of them formed Russell Brothers LLC, and after accepting a financial settlement through the Maryland tobacco buyout program a few years ago, they began looking for new crops. Now they raise grain, cattle and nursery plants.

But it's getting harder to find land, they say. The Navy base keeps growing -- it provides about 20,000 jobs and drives more than 80 percent of St. Mary's economy -- and rural land has become a hot commodity for subdivisions across Southern Maryland. Tobacco farms could be small and profitable, but switching to grain usually means looking for more land, said Donna K. Sasscer, agriculture and seafood coordinator for the county.

From 1992 to 2002, St. Mary's went from 673 farms to 577. Last year alone, 1,500 acres of farmland dropped from St. Mary's tax rolls.

So leasing farmland from the Navy, at Pax River and its nearby Webster Field annex, makes economic sense for the Russells, too.

"It's so much land in one spot," Brian Russell said on a recent day while working on their combine.

"And there's not the development pressure on the base that there would be on a private farm," said his brother Andrew.

The Russells have farms in two St. Mary's communities, Morganza and Clements, and lease a few other spots in the county. But the acreage at Pax River is by far their largest, and it costs only a few thousand dollars a year in rent.

So they put up with some hassles. Like the contract, pages of federal-government fine print that specifies what, where and how high. They can't grow certain vegetables that might attract deer. They can't grow corn too close to the runway. Their milo has to be a certain type that birds hate, not the kind that fills birdfeeders.

They have to bring all their equipment down from sleepy Morganza to the busiest corner of the county, where lines of cars wait at traffic lights. It's only 20 miles or so, but it takes an hour-and-some by combine. As a result, they try to get all their planting, all their spraying, all their harvesting done at one time.

They had to get a map to find their way around the base to all the slivers and patches of land they can farm in between runways and hangars. They had to get military security clearance. And they had to learn how to radio the tower to get clearance to chug across the runways in their combine. Everyone has been very polite and patient with them, they said. But still, "it makes you nervous at first," Andrew Russell said.

"I'm still nervous," Brian Russell said, laughing and leaning back against the big green combine dusted with soybean fuzz. "Just afraid you're going to end up where you're not supposed to be."

They have to make sure no rocks or clumps of dirt are stuck in the tires before they go onto the landing strip -- nothing that could get sucked into a jet engine.

They also learned about the lights along the side of the pavement on the airfield: Blue lights signal a taxiway, where planes roll along the ground. White lights signal a runway -- look out.

Some days, planes take off and land all day long. At first, that got to the farmers. "When you're out here all day, that gut-rattling noise," Brian Russell said. "You hear it and feel it. . . . It vibrates inside."

A big white plane off in the distance, beyond their soybeans, is so top-secret that they've been told not to get anywhere near it or its armed guards.

"We don't know what it is," he said, "and we don't want to know."

Last year, he was hesitant about coming back. But the Russell brothers are getting used to it. They're hoping they'll win another five-year contract when the current one runs out in about two years. Bids are judged not only by amount but also by farming techniques, Rambo said; the Russells recently won a statewide soil-conservation award.

Another plane dropped out of the clouds with a roar. "You kind of forget about that after a while," Brian Russell said. He even wonders what it's like for the pilots, soaring up there over the bay; he's never been in a plane. "It must be beautiful."

Sometimes they're there late at night, he said, and the stars come out over the fields and the river. He thinks about what it was like long ago, before the base was built, when Indians hunted there or the first colonists planted tobacco.

"Some nights," he said, "it's just so quiet and peaceful here."

Staff researcher Bobbye Pratt contributed to this report.

Glen Russell checks the soybeans that he and his brothers grow on the base as an F-18 Hornet roars over land nearby.

The Russell brothers harvest corn, milo and soybeans such as these from fields near the runways of Patuxent River Naval Air Station.Glen Russell looks over a trailer load of soybeans with the Patuxent River Naval Air Station's hangars and runways as backdrop.