Agriculture Secretary John R. Block, in a major reversal of administration policy, will propose a 10-year, multibillion-dollar "conservation reserve" today that could retire as much as 20 million acres of the nation's most erodible cropland.

The Agriculture Department and farmers would share the cost of planting protective grasses and trees on designated land. The department then would pay farmers to keep the land out of production.

According to USDA, about 53 million acres -- that is, one of every eight acres of U.S. cropland -- would be eligible for the reserve. Participating farmers would not be allowed to use the land for haying or grazing, a prohibition Block said would add to wildlife and conservation benefits.

He said the estimated $11 billion cost of the program over 10 years would be offset by lower federal spending on crop loans, direct subsidies and grain storage costs.

"It is cost-efficient and agriculturally efficient to farm the best land," he said.

Block's proposal, along with a groundswell of support on Capitol Hill, virtually assures that the 1985 farm bill being crafted by Congress will contain significant soil conservation provisions.

A House Agriculture soil conservation subcommittee has already approved a title in the new farm bill that would allow creation of a reserve as large as 20 million acres, with payments to farmers based on bids.

The House bill also includes, with administration support, a "sodbuster" provision that would limit the payment of federal farm program benefits to farmers who convert erodible lands to crop production.

Block said yesterday that he favored a tougher sodbuster plan -- a ban on federal aid rather than the limited approach in the House bill. "With a tough sodbuster provision," he said, "we'll have a strong conservation plank."

But the administration consistently has opposed the idea of a conservation reserve, largely on cost grounds. Block said support is now possible because of the Senate-White House budget resolution compromise that would make more money available for agriculture.

The reserve idea has increasingly drawn support from conservation and farm organizations, which see it as a way to protect fragile soil and at the same time reduce surplus crop production that depresses prices and increases federal farm-support program costs.

"This approach makes soil conservation and farm-program objectives more consistent," Block said yesterday before departing on a six-state trip to announce his proposal. "We haven't been very consistent over the years."

Peter C. Myers, assistant secretary for natural resources and environment, said the USDA estimated that per-acre payments to farmers could be between $40 and $50 for putting land into the reserve. Pilot programs in South Dakota and Alabama have cost less than $30 per acre, he said.