Agriculture Secretary John R. Block warned yesterday that he will recommend a presidential veto of the 1981 farm bill unless Congress drastically changes its costly approach to dairy price supports.

But, Block added, the dairy support package is only the most objectionable of a series of objectionable provisions the administration intends to battle when the House and Senate take up the pending legislation.

The Senate is not expected to debate its version until next week at the earliest. House action will come later.

During a luncheon meeting with reporters, Block said the diary section alone, if not changed, would be cause for a veto reommendation, which he said he felt "confident" President Reagan would accept.

But the secretary also said a combination of other objectionable provisions could provoke a veto recommendation. Among them he listed the high sugar price support in the bills, continuation of the grain target price payment program and the peanut allotment system.

The present dairy support program could cost taxpayers as much as $2 billion this year through government purchases of excess production. The House Agriculture Committee has adopted a plan that would cost about $1.5 billion; the Senate, about $990 million.

The administration contends that the diary program must be sharply curtailed as part of its effort to bring the federal budget into balance. The diary lobby and its allies on the agriculture committees, ignoring White House appeals for austerity, have confected a new support plan that would require, at a minimum, about half the money allocated for all commodities.

"The bottom line for us is 70 percent of parity, with the option to go below that with an annual adjustment," Block said. The Senate approach is 70 percent of parity with a semiannual inflation adjustment. The House settled on 75 percent with twice-yearly adjustments.

Block made these other points: He has had "no contact" with the White House about its reported acceptance of a sugar support program the administration originally opposed, but he said the 19.6-cents-per-pound support price in the House and Senate bills is "unacceptable." Reagan reportedly promised key southern Democrats he would reconsider his sugar position in return for their votes on the Republican budget reconciliation package 10 days ago.

No date has been set, but he is "hopeful" that U.S. and Soviet negotiators can meet by the end of this month to discuss new grain sales. The Soviets have been given options on meeting times to hold exploratory talks on renewal of the bilateral trade agreement.

He supports recently introduced legislation that would relax food-safety standards the Department of Agriculture and other federal regulators now must observe. "We need to have leeway. . . . Our technology is not perfect in assessing risk," Block said.