The nation's farmland is disappearing at a rate that means higher food prices and serious environmental damage in the future if the trend continues unabated, two high-ranking federal officials warned yesterday.

According to an 18-month federal study of the nation's farmlands, the United States is losing 3 million acres of agricultural land each year to new industrial and residential development, man-made lakes and other uses, Agriculture Secretary Bob Bergland and Council on Environmental Quality Chairman Gus Speth said yesterday.

A nation that has always fancied itself the land of plenty, including plenty of farmland, is approaching a point when nearly every inch of its agricultural land may be used, the "National Agricultural Lands Study" found. To head off that possibility, the federal study group recommended a series of steps, including local measures to encourage developers to use less productive -- and probably more expensive -- parcels.

The study also recommended that federal agencies reexamine their own programs, such as highway construction and housing subsidies, that have abetted the disappearance of agricultural land.

"This question never has been seriously addressed because, for as long as I can remember, all of us thought we had land to spare," said Bergland. Between 1967 and 1975, however, some 23 million acres of agricultural land disappeared into other uses, probably irreversibly, according to the study.

In the top 100 counties in value of farm products, the population grew at nearly twice the national rate of growth from 1970 to 1978, the study said.

The study, directed by Robert Gray, found that the United States has approximately 413 million acres of cropland and about 127 million acres of potential cropland. It would require using 84 million to 143 million more acres in the next 20 years than are in use now to meet increasing demand for agricultural produce, it estimated.

Unless it slows down, the conversion of agricultural land will be "a driving force behind the rate of inflation," pushing both land and food prices higher as concrete laps over cropland, said Bergland.

"We're paving paradise and passing the costs -- higher food prices and destruction of our rural landscape and heritage -- to our children," said Speth. Expected export trends, increased domestic requirements and future gasohol production will "put unprecedented pressure on our nation's best agricultural lands" in the future, he said.

If land continues to disappear, "I think we would see a gradual reduction in our ability to be a major and leading exporter of food products . . . and as more marginal lands are brought into use, there would be a sharply increased environmental impact," said Speth.

Bergland noted that approximately 100 million acres of farmland already are being eroded at a rate of eight tons a year -- "a rate beyond what can be renewed by nature." The future may produce even more pressure on fragile lands such as timber lands, wetlands and deserts, he said.

"The South is where the pressure is greatest and where nothing is going on," said study director Gray. Gray said some 270 municipalities have developed agricultural land protection programs -- often zoning to protect farmland. Several states also are developing programs aimed at preserving farmland, Gray said.

"We've moved out of surplus into a need to maximize the use of our resources," said Gray. The 1950s and 1960s were decades of surplus; the 1980s -- like the 1970s -- will be a decade of transition, he said. Increased yields per acre may help ward off shortage for awhile, however.

The study recommended that local communities move quickly to establish agricultural land protection programs before development begins bidding up the price of land. It also recommended that either the president or Congress announce a policy making clear the national interest in agricultural land.

It also urged federal agencies and local communities to steer development to more marginal lands, which may be more expensive to develop than flat, well-drained prime land.