At noon today Secretary of Agriculture Bob Bergland looked out at a crowd of worried farmers at the Princess Steak House here. "We're being tested, you and me," he declared in a stern, measured voice. "We're being tested by the Russians to see what we're made of.

"They invaded a country, and while they did it they were buying corn from Iowa, millions of tons of it," he continued. "The president made the only choice he could make. He suspended trade with them."

Bergland was giving the hard sell. When he started, the crowd of farmers, packed shoulder to shoulder into the room, looked like it was ready to eat him alive. But by the time he was finished 30 minutes later they treated him like a pet pussycat.

The same thing happened at stop after stop today as Bergland, making his first trip into the nation's heartland since President Carter sharply curtailed the sale of grain to the Soviet Union last Friday, moved across Iowa. Only once, in his first four appearances of a four-day tour through the state, did a single person raise his voice in a tone that approaches anger over the curtailment, which has become the most politically sensitive issue in this state that will kick off the 1980 campaign with its Jan. 21 precinct caucuses.

"How can you justify bailing out exporters and big grain companies and doing nothing for the farmers and small grain dealers of Iowa," a heated Bruce Anderson, an Albert City grain elevator operator, asked Bergland in a meeting at Storm Lake, Iowa. "It seems unfair."

But such disparaging remarks were rare today. In part, the reactions was a tribute to Bergland's presentation and the ability of Carter supporters to pack the house in places he appeared. Bergland farmed in the cold plains of northwest Minnesota. He talks farmer's language.

But the reaction also has something to do with the nature of the Iowa farmer -- something that Carter's political opponents, who have savagely attacked the curtailment, may have overlooked.

The farmers are genuinely incensed at the partial embargo. But they are reserved, highly patriotic people, who work in a high-risk business. They accept uncertainty as a part of their life. "I think you'll find the people in Iowa as patriotic as they are anywhere," said Mike Schutz, a Pomeroy, Iowa, farmer who listened to Bergland here. "If this embargo is what it takes to show the Russians we'll go along with it. We might not like it. But we'll go along with it."

Bergland's message to farmers he met in Des Moines, Pocahontas, Storm Lake, Cherokee and Le Mars was a simple, no-nonsense one. "I'm urging people not to panic," he said here in the heart of some of the nation's richest farmland, a place where land routinely sells for $3,000 an acre. "Don't do anything you'll regret. Just hang onto your hats for a while. We'll ride this thing out."

There were still plenty of questions for Bergland and a strong undercurrent of uneasiness over the "suspension of trade," the term Bergland used instead of embargo. Most were practical ones on how the embargo and related programs would affect each farmer and what it would do to corn and wheat prices over the long term.

The agriculture secretary had little definitive to offer on the price question. The prices of both wheat and corn felltheir legal limit today, 10 cents a bushel for corn and 20 for wheat.

Bergland said he expected prices to fall this week. "What I'll be watching is what happens during the next week," he said. "If it goes down next week and stays down, and we have a panic, we'll be in trouble. Carter will be in trouble."

Bergland, who routinely travels with a security guard for protection, will be campaigning for Carter in this state until Saturday. His trip is being paid for, he said, by the Carter reelection campaign.