Since the early 1970s, it has happened every spring in Frederick County. The farmers fertilize their fields with manure, just as their fathers and grandfathers did - and the phones begin ringing madly at the Frederick County Health Department.

That jangling signals the inevitable news: a new crop of refugees has stepped outside of their new homes in Frederick's fields, sniffed the unsavory odor, and realized that there's more to the bucolic, rural existence than theyd bargained for.

"These people that have moved out from the cities," sighed Carl Margrabe, a county health inspector. "As soon as spring comes, they open their windows and get the odors from the fields . . . then they call up and complain."

Other county officials are burdened with complaints about the tractors that new up early in the morning, combines that keep rolling, noisily, after dark to harvest the grain, and livestock that grunts, groans and howls at night.

"What it all amounts to is transition," explained Charles Gillis, the director of the county's environmental health department."Were going from a strickly rural to a residential area . . . when a person moves out of the city, he's not familiar with the rural lifestyle."

But that doesnt keep residents of the Washington area from heading far out on Interstate 270, part the Montgomery County line into Frederick County and new homes 40, 50 and even 60 miles from downtown.

In the 14 years since it was completed, this 33-mile stretch of interstate highway has pulled metropolitan Washington's suburbia steadily northward with it. Frederick County has become its final frontier.

During the decade between 1960 and 1970, 5,035 people migrated to Frederick County, accounting for less than half its population gain during that period. But in the next five years, another 8,200 people moved into the county, accounting for 75 per cent of is growth during that period.

Most of the newcomers, planners say, are expatriate urbanites whose lives revolve around Washington. In part, they move to Frederick County is search of the world painted in farm-country storybooks - undulating green hills where placid cows graze. More importantly, they can find in Frederick County houses with large yards at prices they can afford.

But when the would-be pioneers reach Frederick, they find that they have not escaped from the big city by themselves. New subdivisions have been springing up at the rate of two and three a year in Frederick County. New schools are overcrowded almost at the same time the construction workers hammer the last nail. So many wells have tapped into one source of water in the southern part of the county that some of the wells have already run dry.

At the same time, the new residents are finding that many roads in Frederick County are not paved, parks are few and far between, and the county's pretty, pleasant farms generate unpleasant farm noises and smells.

Still the new exurbanites keep coming. Earl Smith, a stockbroker who moved from Potomac out to the Monrovia area of southern Frederick County eight years ago, distills the couses of this phenomenon into three simple sentences:

"People like the idea of the country. Taxes aren't as high here. And people are willing to put up with some of the inconveniences to live like the people in Potomac do who have money."

The pull of these enticements is expected to increase over the next 20 years, as housing opportunities elsewhere dwindle. "Americans continue to prefer a low-density redential environment, but they now have to move farther out to find it," explained a report released in February by the Council of Governments.

"The counties of the metropolitan fringe," the report added, "once seen as places for a pleasant country drive, are becoming the next suburban frontier." Later, the report noted taht "a new ring of development with land use based on the meanderings of the automobile is being created around Washington."

For Frederick County, these forces mean that 60,000 people are expected to move into the county during the next two decades, joining the nearly 100,000 who are there today.

The aim of the county' planners, explained Mark Horak, a regional planner for the Maryland Planning Department, is to mould Frederick County's landscape into a checker-board blend of urban and rural communities.

"We definitely don't want to be Prince George's." he said. "We don't really want to be Montgomery either. We'd like to have a rural-urban county."

However, for a large section of the county near the I-270 corridor, the planner may be too late. A runaway river of suburbia has swept in an are through southern Frederick County, from Middletown on the West through Frederick City and beyond New Market in the east.

At the heart of this band is Green Valley, an area without firm boundaries, stretching out on all sides from the intersection of Maryland routed 75 and 80. The place names of Green Valley best tell what it was, and what it has become.

Once, Green Valley was dotted only with stolid, literal names left as monuments to old people and old purposes: Monvrovia, Ijamsville, Price's Distillery Road.

Around these have grown up the new subdivisions, with sales brochure names intended to evoke pastoral serenity: Meadowbrook, Rolling Green, Pleasnt Grove. These names have brought the homogenous 1970s to the county where Francis Scott Key was born, where Civil war skirmishes were fought, and where John Greesleaf Whittier set his Civil War poem "Barbara Frietchie."

All of these collections of new homes were onve farms, bought up one by one, with developed piecemeal, one by one, with individual well and septic systems for every house.

Rapidly, these subdivisions are being filled by people like Donald and Shirley Foster. This couple moved from the Boston suburbs into the Loch-Haven subdivision almost three years ago. Foster, a Washington lawyer, leaves from work each weekday at 6:45 in the morning and gets home at 7 in the evening.

"We really wanted to be out in the country," said Shirley Foster. "We wanted to be where it was not built up and overcrowded and a filling station on every corner . . . We wanted to be someplace where our three kids could breathe and run."

But, she added wistfully, "I didn't realize it would be growing so fast."

Even while living that far out, the Fosters fell themselves a part of the Washington area. Occasionally they will go out together in to downtown Washington to see a play or a concert.

But the drive "is a deterrent." said shirley Foster. "I wish the commute wasn't so long." At the moment, the Fosters live about 45 miles northwest of downtown Washington and less than 10 miles south of Frederick City.

Some of the same thoughts wemt though the minds of Jackie Anthony and her husband Marvin when they decided to move out to Frederick. "We lived in an apartment in Rockville, she said. "We moved out her 4 1/2 years ago because we started having children and we figured that the country was a better place to raise them."

Marvin Anthony leaves at 6:15 evening morning for the 70-minute drive to his job as a aerospace engineer at Sytems Consultants Inc. in Rosslyn.

A large part of the Anthonys' reason for moving to the Green Valley area was also financial. They paid $38,000 in 1973 for a three-bedroom house on a half-acre lot. Similar houses in the Rockville area were selling for more than $50,000 at the time.

"We could afford this" Jackie Anthony said. "There easn't that much we could afford in Montgomery County." But there was a little more that to it.

"We have this fantastic view of three mountain ranges from our home," she added, her voice softening with affection for the area. "We see the sunset from here every day. That's a major drawing card."

She and her husband can also see new homes going up around them as each month passes. She estimated that there were about 50 to 80 homes in the immediate vicinity when they arrived, and now "there are about 500 houses in the area."

This growth has put a great strain on the available resources. At the moment, the twin shortages are schools and water.

"The only unpleasant thing is the water conditions," said Mrs. Anthony. "We haven't had any problems, but we live on the brink of them. Three of our neighbors had to drill new wells. That keeps you on edge." Estimated cost of well-drilling: $1,000.

The overcrowding of the six-year old Green Valley Elementary school, she said, "is no-one fault. That's just the way it goes."

There were 759 children enrolled in the Great Valley Elementary schthis year, after about 200 were transfered to Urbana Elementary last summer. The projected enrollment for next year is 890. Portable classrooms have been a fixture in the school for three years, it was already overcrowded a year after it opened.

The tax rate in Frederick County is $2.40 per $100 of assessed valuation, as compared with rates of $3.41 per $100 in Prince George's and a base rate of $3.83 in Montgomery, where tax rates may vary in separate taxing districts.

This means that the owner of a home assessed at $50,000 in Frederick would pay $1,000 in real estate taxes. The owner of the same home in Prince George's would pay $1,705, in Montgomery County, the owner of the same home would pay at least $1,915.

The shared desires of the new Green Valley residents for roads, parks, and uncluttered class rooms for their children led to the formation of the Green Valley Citizen's Association a federation of the various subdivisions whose members kept in touch - sometimes vociferously - with elected officials about their community's needs. In some cases, these citizen activists become local officials themselves. Jackie Anthony's husband Marvin Anthony has just become a member of the Board of Education.

But, often, migrants into Frederick County are not ready for that kind of problem. They bring with them urban assumptions about the kinds of services thath government should provide. Overcrowded schools and the occasional dirt road often come as a shock.

"You're talking about an essentially rural county, but (the newcomers) expect essentially urban services," said Horak, the regional planner. "The people come from metropolitan areas with high tax and service levels."

Frederick County Planning Board Director Lawrence Johnson chuckled when he recalled the woman who appeared last year before the County Commision. "I've been in this county for 30 days," she said angrily, "and I've noticed that my road is not paved. When are you going to do something about it?"

But the present officials, like Johnson, say there is nothing much they can do about the new residents' demands until the pace of development abates somewhat, giving them a chance to plan without a crisis hovering over their heads.

The main problem, Johnson said, has been the absence of strong growth management policies. I-270 brought rapid new development before the county really knew how to channel its own growth, he said.

"There was nothing we could do to stop Green Valley. Not really. We just came in afterwards with band-aids," he said.

Johnson is trying to see that there won't be any more Green Valleys. At the time those subdivisions sprang up in the early '70s, development was first permitted - on hald-acre lots, then, later, on one-acre lots. There were few other restrictions governing the A-1 "Agricultural" zone.

There was nothing anyone was willing to do about it politically then," said Johnson. That willingness finally came last year and this year. In January, about 60 per cent of the county was rezoned.

Now, if a developer wants to build on farmland he has bought, he must first apply for rezoning.

"It'll be a little more difficult for someone to drop a group of houses in the county and walk away," Johnson explained.

That does not mean, however, that growth will be dramatically slowed, at least for the time being. "All we can do is guide it," he said.

They must pay for it, too - or find state or federal funds to pay for it. "By our estimation, we will beed another $24 million worth of school facilities by the year 2000," said Johnson.

There are other, less visible costs, as well. As Donald Dare, of the county community services department, explained, 10,000 of the county's 40,000 workers now work outside the county - in places like Montgomery, where they generally earn incomes high abouve Frederick County norms.

As a result, the county's per capita income has risen 30 to 40 per cent in the last seven years, from about $9,000 to about $12,000. And tagging along behind the rise in average income has been a rise in prices for everything - housing, food, consumer goods.

"The new people aren't raising the Frederick County wage base, but they are raising the Frederick County cost of living," said Date. The squeeze on the people who have lived and worked here all this while is great."

As the Council of Governments report puts it: "It would require an action major proportion, such as the doubling of gasoline prices, rationing, or a blanket restriction on new residential construction . . . to reverse this outward flow."

With the road feeding more and more people into the county, Frederick County planners don't know if the migration's end will ever come. "Right now, hald of Frederick wouldn't be there id I-270 weren't there," said Horak.