Agriculture Secretary John R. Block said yesterday that he plans to brief President Reagan, perhaps this week, on the deteriorating economy of the Farm Belt, but he warned that there is "no new grandiose scheme to save the day."

The secretary, responding to farm-state complaints the administration has turned its back on thousands of farmers so strapped they may not be able to get loans for spring planting, agreed there is a "serious problem" in the agricultural economy but said rural America should not look entirely to Washington for solutions.

Block added that the farm financial situation "has heated up" since he last briefed Reagan about a month ago. Some banks are in trouble as well as farmers. But, Block continued, "I don't want farmers to think all of the answers are here in Washington . . . . It is a big problem and everyone has to work on it."

Meanwhile yesterday, an official of the Independent Bankers Association of America, which represents about 4,100 country banks, said an expanded federal loan guarantee program would be the quickest and easiest way to deal with the growing credit crisis in the farm belt.

"In the final analysis," Weldon Barton said, "it is a question of whether the Republican senators who represent those areas, particularly those who face reelection in 1986, become committed to taking significant action to alleviate this situation. That's basically where the initiative is going to have to come from."

A group of Republican senators has called on Reagan to form a presidential task force to look at the farm-credit situation, but Barton said yesterday that "there is little demonstrated need for more study . . . . Time is very short."

Barton charged that the administration's farm debt restructuring package, announced by the president six weeks before the November election, had been made "unworkable" by adding conditions that were not attractive to country bankers.

Under the Reagan plan, the banks must agree to forgive at least 10 percent of a loan to a farmer to have the transaction guaranteed by the federal government. They are not going along with that plan, Barton said, because "many have reached their bottom line on ability to absorb loan losses . . . and we are having an escalated degree of farm-bank failures."

"We don't mind some saving of face," the IABA official continued, "but we hope there is some way the administration could disentangle itself from the forgiveness feature and provide loan guarantee authority, expanded in a way that would be workable."

Barton said that of 40 banks that failed since last June 15, 22 were farm banks, lenders with at least of one-fourth of their portfolios in farm loans, in rural communities.

"What this says is that during the last six months, a larger number of agricultural banks were reaching the bottom . . . . Now we have a situation in the farm belt with several hundred agricultural banks that have reached the limit in their ability to absorb losses from farm loans," Barton said.

In other activities related to the farm economic crunch:

* An unusual coalition that includes the American Farm Bureau Federation, the National Farmers Organization and the Grange -- often at odds -- plans to mount a lobbying effort here this week to pressure the administration and Congress to move more rapidly in dealing with the credit issue.

* Sen. John Melcher (D-Mont.), denied a chance to hold a full-scale Senate Agriculture Committee hearing, has called a "public hearing" of his own for Wednesday to take testimony from lenders, farmers and others on the credit problems facing agriculture as the 1985 planting season nears.

Block said yesterday that some changes in the president's debt-relief program are "the subject of review within the administration," but he gave no hint of what they might be. "There is a chance that adjustments will be made," he said.

However, he conceded that the situation, compounded by falling land values, high interest rates and low farm prices, is certain to send many farmers into bankruptcy.

"I don't think we'll lose as many as those who are sounding the alarm would suggest," Block said. "I would hope that not only the federal government is prepared to make accommodations to minimize the rural upheaval. All the lending institutions have a responsibility, too."