The protection of about 48 million acres of prime farmland from damage by coal strip miners has become the subject of a heated debate for the Reagan administration, Congress and environmentalists.

Secretary James G. Watt's mining-oriented Interior Department and the soil-protection aims of Secretary John R. Block's Agriculture Department are clashing.

Agriculture aides tell of pressure by Interior to modify its position. But they say that Block will notify Watt by a Monday deadline that he continues to support strict regulation of mining on the best farmlands.

Meanwhile, at least two dozen Democratic and Republican members of Congress are urging Watt to adopt a regulation he earlier voided, requiring mining companies to assure that prime farmland will be restored to full productivity before they can get coal-stripping permits.

At stake are roughly 48 million richly productive acres, more than half in the Midwest and Great Plains, under which lie an estimated 22.2 billion tons of coal, much of it within easy reach of modern stripping equipment.

Concern over the mining issue is heightened among farm and conservation groups by the annual loss of about 1 million acres of prime land to housing and commercial development, highways and federal projects.

Despite mining industry claims, soil conservation experts at the USDA are skeptical that much of the land could be returned to equal productivity once it has been mined.

Congress thought it had put the issue to rest in 1977 when it passed a strip-mine-control law. Environmentalists and some farm-state legislators wanted to ban stripping on prime farmland because of fears that it could not be reclaimed adequately.

In a bow to industry, Congress allowed existing stripping operations to continue on prime lands. But it required mining firms to show, after a "grandfather" period, that such stripped land could be brought back to full productivity.

Using an approach devised in 1979 by the GOP administration of Gov. James R. Thompson in Illinois, the Carter administration proposed that the grandfather period end this August, five years after passage of the law.

Block, then Illinois agriculture secretary, strongly supported the five-year cutoff as a means of providing protection for some 9.5 million acres of prime farmland in that state. Last year, as chief federal agriculture executive, Block reiterated his position in a message to Watt.

But Watt contended that Congress had not been specific in defining the grandfather period and ordered that the Carter-era regulation be suspended and more public comment obtained.

Dean Hunt, an official at Interior's Office of Surface Mining, said, "Everybody was in a quandary over this ....It was a midnight rule-making by the Carter administration. But there is no date in the law and we felt the legislative record did not support the August, 1982, cutoff."

The protesting legislators contend that there was no doubt of congressional intent on the five-year exemption period. In 1980, led by Sen. Dale Bumpers (D-Ark.), the Senate voted 84 to 1 to impose the August, 1982, cutoff. No House action was taken on the issue.

Bumpers and Sen. Charles H. Percy (R-Ill.), in a letter to Watt this week, said they were "disappointed" in the secretary's handling of the cutoff. A similar letter from a bipartisan group of more than 20 House members, led by Reps. Jim Jeffords (R-Vt.) and Ed Jones (D-Tenn.), urged OSM Director James Harris to stick with the August cutoff.