Agriculture Department officials are putting the last touches on plans to carry out a new soil conservation program, the most stringent ever adopted in this country, that will take millions of erosion-prone acres out of production and penalize farmers who mistreat their land.

The program, established by the 1985 farm bill, is aimed at reducing the soil erosion that carries away fertile topsoil, pollutes and clogs waterways and causes annual off-farm sediment control costs estimated at more than $2 billion.

Unlike other federal conservation efforts in the past 50 years, which have poured billions of dollars into marginally effective schemes, this one has an important new wrinkle. Government farm-program subsidies will no longer be available to farmers who allow erosion to continue on their farms.

"The program represents a turnaround in the direction of the country," said Agriculture Secretary John R. Block before he left office last Friday. "There is an appreciation for conservation and a willingness to take action. The conservation reserve is the heart and soul of this thrust, but we've finally come as a nation to the point that we're not going to spend government resources on people who abuse the soil."

USDA officials are predicting heavy signup next month in the reserve, the first part of the program, which will pay farmers to retire their most erodible land for at least 10 years and maintain it in soil-preserving grasses and trees. The government also will pay half of the farmer's cost of establishing his grass or trees.

Peter C. Myers, assistant secretary for natural resources and environment, said that the assurance of steady income for 10 years on erodible land of questionable productive capacity would be a strong incentive to farmers to enroll in the reserve.

Pilot studies found farmers willing to put land in long-term reserve for less than $30 an acre. But USDA officials think per-acre costs under the new program will be considerably higher as farmers weigh potential crop income against the benefits of long-term lockup of their acreage. Farmers will bid to participate accordingly. No more than 25 percent of any county's cropland will be allowed into the reserve to avoid adverse impacts on farm-related businesses.

The first-year goal is to lock 5 million acres of the most fragile land into the reserve, with 10 million more targeted for 1987. By 1990, if money is available, the department intends to have at least 40 million acres -- that is, about 80 percent of the nation's most vulnerable 53 million acres -- in the reserve.

The Reagan administration is proposing to spend $190 million on the reserve this year and $550 million next year. But it also wants to cut other USDA conservation programs by $224.5 million in 1987 -- a move expected to meet heavy resistance on Capitol Hill.

Block reversed administration opposition to the reserve last spring by arguing that the long-term retirement of erodible land could save the government up to $4 billion a year in surplus grain storage costs and production subsidies for farmers. As acreage in the reserve mounts, it is also expected to slow overproduction.

The administration's change of heart cleared the way for Congress to make the reserve a centerpiece of the conservation section of the farm bill, which includes these key elements:

*The Conservation Reserve. A program that will pay farmers to remove land from production for 10 years. Hay harvest or livestock grazing will be banned on these lands and any farmer who breaks his contract will be required to repay all of the government's costs, with interest.

*Sodbuster. A provision that bars farm program payments, crop insurance and federal loans to farmers or speculators who plow up fragile grasslands without approved conservation plans. Millions of acres of these erosion-prone lands have been plowed up in the West in the last decade.

*Swampbuster. A similar ban on payments to farmers who drain wetlands for conversion to crop production. Millions more acres, mainly in the Southeast, have been stripped of hardwoods, drained and put into production over the last 20 years.

*Compliance Plans. A ban on federal payments after 1990 to anyone farming without an approved conservation plan. Farmers will have until 1995 to come into full compliance with their plans.

*Conservation Easements. A provision that will allow farmers to cancel part of their unpayable debt to the Farmers Home Administration (FmHA) by granting the agency easements on wetlands and highly erodible sections of their farms. The provision could ease the threat of foreclosure or bankruptcy for many farmers.

Robert Gray of the American Farmland Trust, one of a number of conservation groups that lobbied heavily for these provisions, predicted that farmers will respond heavily next month and again in the fall when the USDA calls for more bids on entries into the reserve.

"There is a lot of interest in this in Iowa, Missouri and Minnesota, and I think we will have a good signup this spring. The department is doing a good job training its people and getting the word out about the program," he said.

The reserve and the other portions of the program that tie conservation compliance to farm program benefits will be administered by the Agricultural Stabilization and Conservation Service, which has offices in every farming county in the country.

Milton Hertz, acting administrator of the ASCS, said the department is aware of wide enthusiasm for the reserve among farmers, but that there still is no indication how high farmers' bids will run as their price for participation; nor does the department yet know what it will consider acceptable.

"After the first go-around of bidding, we can look at the most erodible land and see where we can get the biggest benefit for the buck," Hertz said. "The sad thing is that some of our [production-oriented] farm commodity programs have actually been contributing to the erosion problem."

But as Block and Myers said, the new farm legislation decrees an end to conservation policy working at cross purposes with production policy.

"To me," said Myers, a Missouri farmer before he joined the administration, "the real turning point is when we tie together the reserve, the sodbuster and the compliance requirement. The government is through financing soil erosion. We're saying, 'if you want to abuse your soil, do it on your own.' "

"I think the worst of our land will go into the reserve," Myers continued. "Most of our erosive land can be brought into compliance by changing our way of farming . . . . The biggest thing is that farmers will have to change their attitude. Part of the problem is economics, but more of it is attitude. They all think that erosion is bad, but they don't think the problem is on their land."