If they paid farmers a federal price support for growing USDA Choice Optimism, John R. Block would be first in line with his hand out.

As Ronald Reagan's secretary of agriculture, Block has proved his ability to harvest optimism where most other farmers find only blight, pessimism and depression.

He may be presiding over the shakiest farm economy in 50 years, he may be the only secretary ever to call catsup a vegetable and he may have rocky relations with Congress, but Block thinks he's doing just fine as No. 1 farmer. "In spite of the agricultural economy not being what it should, I am encouraged and confident as secretary of agriculture," he said recently. "I've learned a lot . . . . I'm a smarter, better secretary now. . . . I serve the president well in my capacity."

Some in Washington's agriculture establishment--farmer and commodity groups, legislators, lobbyists--agree with Block's auto-diagnosis. But not many. Almost all think he's a nice guy, but most regard him as an Illinois hog farmer still learning politics.

In 17 months in office, Block has traveled at home and abroad more than any other secretary in modern times; he has been cuffed around brusquely on Capitol Hill over a farm bill and economic issues; he has enraged the nutrition lobby with food policy changes; he created a storm by firing USDA's widely respected soil conservation chief; a number of his personnel appointments evoke sotto voce derision; he still has no assistant secretary for congressional affairs.

Republicans who will talk for the record say, predictably, that he's doing an A-1 job. "He stacks up very well," said Rep. Paul Findley (R-Ill.). "He deserves high marks for ending the grain embargo. Farmers feel they have a champion in him."

Sen. Richard G. Lugar (R-Ind.) said, "He was an excellent choice for secretary. He is a farmer, he is perceived as one of them. He does a good job within the constraints that are there."

Lugar's view was seconded by Joseph A. Kinney, agriculture adviser to the National Governors Association and a longtime Block friend. "Jack Block has more chemistry with farmers than any secretary in recent history," he said.

But Democrats like Rep. Glenn English, an Agriculture Committee member from Oklahoma, think the chemistry is a witch's potion. They think Block and the Reagan White House are directly responsible for current hard times on the farm.

"There is no question Block has the tools to improve the economic situation in agriculture, but he is philosophically opposed to using them. He has to bear the responsibility for a good part of the difficulties farmers are facing," English said. "His solution is to pray for bad weather, which shows you how bad he is for agriculture."

Partisanship aside, "lightweight" is the word heard most frequently to describe Block. Wise-cracking farm reporters who have watched other secretaries call him "Howdy Doody" and "Blockhead." A more serious assessment, by one of Block's closest acquaintances, goes this way:

"The secretary has yet to show he understands the nuances of Washington politics, but he has grown. His speeches are better now. But what's kept him golden is his unswerving loyalty to Reagan. . . . He has tremendous blind faith in Reaganomics, but not much philosophical understanding of it."

The hallmark of it all is Block's boundless optimism. In the face of a continuing decline in the agricultural economy, his speeches and interviews are laden with a central thought: let free markets work, let Reagan's economic recovery plan work, get government out of agriculture and farmers will be in clover.

Trouble is, Block's critics like to say, that ain't the way the world turns. Agricultural markets aren't free, there are doubts about the recovery plan, agriculture couldn't exist without government programs, there is no reason to think farmers' sagging incomes will improve soon.

Roger Clark, a Brady, Neb., farmer and vice president of Farmland Industries, a cooperative, is one of the secretary's constituents who is cooling. "One of my disappointments is that in all of his talks he says, 'Just wait, we'll have a recovery.' Even with recovery in other sectors, agriculture won't recover without other action. We can't continue to pile up bushels and bushels of grain like we're doing."

Clark added, "Every farmer will say he doesn't want government involved in farming, but the fact of life is that government is involved and government has to get into it now to get it straightened out."

That is anathema to Block, which sends some farmers right up the wall. "He's an honest, good man," said an official of a major farmers' organization. "But he's an ideologue. He absolutely believes this free market B.S. When he claims credit for lifting the Soviet grain embargo, that's B.S., too. They came to the right political moment to lift it. He didn't end it."

Block does take credit for overcoming hardline opposition and persuading the president to lift the embargo imposed by Jimmy Carter in 1980 after Soviet troops invaded Afghanistan. Block--as well as many of the country's farmers--continues to believe that the embargo was a severe self-inflicted wound.

"Lifting the embargo was my high point," he said in an interview. "It was a campaign promise and I thought all I had to do was make a motion in the Cabinet and it would be lifted. The minute I brought it up I got beat up good alongside the heat."

Block's early inability to move the president and outgoing Secretary of State Alexander M. Haig Jr., who wanted to go slowly for fear of sending the "wrong signal" to the Soviets, became a celebrated source of frustration for him.

He said he felt then, as he feels now, that the embargo had hurt the U.S. farm economy more than it hurt the Soviets. But worse, perhaps, it interrupted a steadily growing and lucrative trade connection that American farmers have not regained.

"We have 25 to 30 percent of the Soviet market now," Block said. "We had 75 percent of that market when the embargo was put on, and we could have retained it. We could have an additional 350,000 people working in this country if trade had stayed at that level . . . but there's no use in dwelling on history. I think we can get back to 50 percent of the Soviet market if we demonstrate we are a reliable supplier."

The idea of export expansion, not just with Moscow, is a key in Block's hope for better times on the incredibly productive U.S. farm. "One of my disappointments is that we have not been able to expand our exports as fast as I wanted," he said. "But other countries are aggressively seeking new markets, too. We have no monopoly on agricultural production and exports."

While Block talks frequently on this subject and while he has traveled to Europe several times, to Mexico, to China, ostensibly to promote more commodity sales, congressional and other agribusiness critics fault him and the White House for not following words with action.

The administration has resolutely opposed putting up money for an export revolving fund that Republicans and Democrats in Congress strongly supported in last year's farm bill as a means of stimulating overseas sales.

"We are not satisfied with the performance on exports," said Allen Paul, president of the Agriculture Council of America, a farmer-agribusiness trade promotion group. "Trade missions aren't going to do the job. We are in a cutthroat competitive situation. . . . We should be working to fund the revolving fund created by Congress."

The Block-Reagan resistance to that spending program, however, is not unique. The administration's budget reductions in USDA's nutrition programs--food stamps, school lunch, child-care feeding, diet and food safety protections--have created bipartisan criticism and resistance.

The height of the uproar came last year and left egg splattered all over Block and Reagan. That was when Block's bureaucrats came up with the idea of counting catsup as a vegetable in school lunches, a money-saving scheme that was laughed out of town.

Block, who had gone along with the proposal, apparently never envisioning the flap that would follow, later admitted privately that he had goofed by not keeping closer track of what was happening at lower levels of the labyrinthine USDA.

Ellen Haas, of the Community Nutrition Institute, is an unsparing critic. "I think he's blind to the needs," she said. "There's no excuse to live in the simple world that he does. When he makes a decision to cut a program or put out a regulation, he tries to sell it, unaware of the needs of children or rising unemployment."

CNI also gives the Block team poor marks for its failure to win House approval last fall of its plan to cut down the $2 billion federal dairy price support program after pushing it through the Senate.

Block's position is that the plan simply wouldn't fly in the Democratic-controlled House. But the administration lobbying effort, such as it was, appeared to many to be a series of bobbles which led finally to full-scale desertions by GOP legislators.

"They left Paul Findley high and dry. There was no leadership. Couldn't get the White House behind it," Haas said. "There was a real urban-conservative coalition, with consumers and processors, and it fell apart. The administration blew it . . . . And the result is that they're spending $2 billion on that program again this year."

Findley, a leader in the dairy fight, agreed somewhat. "I'm disappointed," he said, "but I must admit that the administration's Hill lobbying work was not good at that point. Secretary Block was overseas and he had a team that needed seasoning."

Under the general operating scheme, Block makes the public appearances and rules on policy. His closest aides, always protective, have roots in Illinois. Deputy Secretary Richard E. Lyng handles day-to-day nuts and bolts; the White House decides on key political appointments.

The milk issue, however, was only the more prominent of issues that seemed to raise questions about Block's control of the department.

Last spring, for example, when USDA published controversial new forest-management regulations, Block said in an interview that he hadn't kept up with the issue and wasn't conversant with the proposals.

Block agreed that his inability to get an assistant secretary for congressional affairs appointed has created some operational problems for him on Capitol Hill. He has told at least two potential nominees that the job was theirs, only to be thwarted by political hardball players and leaving trade lobbyists shaking heads in dismay.

More recently, after press reports that his staff had begun screening agricultural research applications for political purity, Block canceled the decision, indicating that he wasn't aware such a potentially controversial step had been taken without his approval.

Block is aware of the sniping, but he's not particularly moved. He feels he is in control and he feels the president thinks so, too. "This White House doesn't set its own agriculture policy without counseling with the secretary of agriculture," he said.

And, as he recently told Congress, "Our programs are looking to the future, to greater and more prosperous days for American agriculture."

The optimist lives.