The Interior Department yesterday adopted a compromise on how much longer companies may strip-mine prime farmland without returning it to its former productivity.

Under the 1977 surface mining law, companies already mining the farmland were told that after a certain date they would have to restore the land to full productivity.

Following the approach of Illinois, the Carter administration set August, 1982, as the date. But the Reagan administration suspended that rule. Interior Secretary James G. Watt told a congressional subcommittee that the rule had been based on a "strict, inflexible interpretation of the law" and would be revised to promote coal production.

But Agriculture Secretary John R. Block and at least two dozen Democratic and Republican members of Congress urged Watt to adopt the Carter date, arguing that it would help protect 48 million acres of midwestern farmland that lie atop coal estimated at 22.2 billion tons.

In a notice in the Federal Register yesterday, Interior announced it would set April 3, 1983, as the cutoff date.

Environmentalists, who had pushed for an Aug. 3 deadline, said they were not happy with eight months' delay but were pleased a deadline had been set. A spokesman for the American Mining Congress said his group would seek to have the decision overturned in court.