The U.S. farm economy is still, to put it plainly, in punk shape. But these are turning out to be the best of times for Secretary John R. Block--at least the best of his two-plus years in Washington.

His payment-in-kind program to cut crop surpluses has had an overwhelming response from farmers who have agreed to idle up to 82 million acres.

The rise in interest rates and farm production costs has slowed. And now there are signs that a break may be coming in the vital export area.

The Soviet Union's announcement last week that it wants to discuss a new long-term grain agreement marked another in a series of recent highs for Block, a result of his incessant haranguing of Cabinet colleagues to take a realistic stance on farm trade with Moscow.

For months, Block had argued that the farm economy could not improve measurably without stronger sales overseas. And, he argued, the United States needed the Soviet Union as a regular customer to make that happen.

But in an administration that takes a notably harsh view of the Soviets on most issues, Block kept bumping into roadblocks: refusal to lift quickly the Carter administration's grain embargo, suspension of the grain sale talks.

The secretary this month called Reagan's offer to reopen long-term talks "the last step on the long road of repairing the damage of the grain embargo . . . . The president knows how I have felt about this issue. It really was more necessary to get other people to agree."

He credited the support of Secretary of State George P. Shultz as a major element in the decision. Block, always a careful team player, declined to provide the name, but he said "there was one person in particular opposed. He never believed the time was right." USDA insiders speculate that he was talking about Defense Secretary Caspar W. Weinberger. SPRUCE UP (AND DOWN) . . . It's not really an extension of the U.S. Forest Service's clear-cutting policy, but they have put the ax to some of the splendid old trees that for decades have stood a stately guard around USDA's main building on the south side of the Mall.

The tree-clearing and additional landscaping that is to follow, it turns out, is one of the few Carter administration ideas that has not been scuttled by Reagan followers at USDA. The plan calls for planting 176 new trees and some 25,000 smaller plants to spruce up the USDA complex. "The plan was drawn up in 1979 or 1980 and it will be a showplace," said John Franke, assistant secretary for administration. He said yearly upkeep will cost about $80,000.

Among other ideas being considered, Franke said, are planting of some traditional American farm crops in small-scale displays and a mini-display of soil conservation tillage practices.

If it's in the nature of government to fix things that aren't broken, there still is a modicum of respect for the past. Will USDA topple the commemorative pear tree planted near the front door in the 1960s by Lady Bird Johnson? "Absolutely not," Franke said. "It's sacrosanct." LIKE GANGBUSTERS . . . Employes grouse about having to wear USDA name tags around their necks, but the security program set up by Block shortly after he took office apparently is paying off. In April, 1981, thefts to the tune of $12,000 were reported, with the loot ranging from electric typewriters to coffee pot kitties. Last month, according to security chief Walter Evanoff, losses of only $195 were reported.

"We're keeping closer checks on the doors and monitoring who comes into the buildings," he said. "The traffic is enormous--in the week of May 1 to May 7, we counted 5,386 visitors coming into our five buildings." NAME GAME . . . A new member of the USDA team that lobbies Congress on soil conservation and forestry issues is Brian Stangeland, 27, a graduate of Moorhead State University in Minnesota. His father happens to be Rep. Arlan Stangeland (R-Minn.), but Tom Kay, congressional liaison director, said that had nothing to do with his hiring. Brian Stangeland was picked over two dozen other candidates because "I was looking for the best man." If the elder Stangeland needs to be lobbied on an issue, Kay said he will do it himself and not sic son on father . . . . Isabel D. Wolf is the new administrator of the Human Nutrition Information Service, moving up from the acting slot she had held since March 1 . . . . Among the 13 recipients of USDA's Distinguished Service Award last week was an unusual one--the Mediterranean Fruit Fly Eradication Team, based in Seattle, cited for its work in California last year.