THERE IS a gem of information buried in the argument over how many thousands of people may have been left out of the U.S. Census Bureau's preliminary count of people who live in this area. Even if the numbers are slightly off, they clearly indicate that people are moving from the city and its suburbs to the country -- actually to areas that were once used for farms -- in record numbers. The age-old parade of people moving from the country to the city in search of opportunity and modern comfort is now reversed; people are going back to the country, despite the higher cost of gas and cars.Alexandria, Montgomery, Arlington and Prince George's counties, the counties closest to the city, generally lost about 10 percent of their populations in the last 10 years, according to the Census Bureau. But in Loudoun County, on the fringe of the metropolitan area, the population leaped upward 55 percent; in Prince William, the increase was 49.5 percent and in Charles County the population was up 43 percent.

Why are these people moving to the country? According to a study by the Greater Washington Research Center released in March, the main reason is money. You can get more for your dollar the farther you get away from the high-priced metropolitan area. Land and housing are cheaper in the fringe counties, and taxes are less. For example, realtors indicate that a house with three bedrooms that might sell for about $125,000 in the District and close-in suburbs could be bought for $80,000 or less, and would have more land around it, in the fringe counties.

But money is not the only reason. A 1978 study of people moving to Anne Arundel County, cited in the Research Center's report, said that the second most popular reason for moving out of the city and the near-in areas is the desire for a better quality of life. People are seeking peace and quiet, clean air, no busing of their children and a large amount of private land. The people who are moving out of urban areas, according to that survey, are usually families of whites. Some retirees are also going to the country.

There are some long-range implications for the fringe counties that demand thought and planning. First, some effort must be made to prevent in these counties haphazard development that could forever strain sewer lines, roads and schools. Second, there must be some caution taken with businesses that accompany the ever-larger group of newcomers. And finally, concern must be shown for preserving some farmland in an effort to keep food prices in this area from going ever higher.

While the distant counties were increasing in population and the near-in suburbs were losing people, the center city area, the District, lost 15 percent of its population; that left the city with its smallest population in 50 years. This shift of people away from the city points to some future problems for the District. It does not have to worry about losing its principal industry, the federal government, but as the work force moves away there may be a substantial loss of smaller businesses to the suburbs. That loss will cost the city tax dollars and will take away jobs that could go to the hard-core unemployed. The population shift also appears to reinforce earlier forecasts that the city would fast become a preserve of the rich, the poor, single young people and childless couples. That combination of people would leave the public schools without a middle-class base, and could make for social tensions. Sharp, unpleasant conflicts between the very rich and the very poor could come about if the city government and business people who care for the community do not step in and try to establish some comprehensive effort to change the trend.