At 10 a.m. the temperature outside the Goodman Milling Co. on Main Street stood at 10 degrees and Donald Goodman had some bone-chilling news for David Eggers, corn farmer.

"We aren't buying. We aren't selling," said Goodman, a robust man wearing a red wool stocking cap. "And I don't know when we will. Or at what price."

This was of more than passing interest to Eggers, a sophisticated young farmer. Corn is his livelihood. He has 40,000 bushels of it in storage, waiting for sale. A 20-cent drop in the per-bushel price, for example, would cost him $8,000.

The price of corn has become of more than passing interest to the rest of the nation, too. When President Carter last week announced a partial embargo on grain sales to the Soviet Union, feedgrain prices became interwined in the politics of 1980 and the president's fate in the Jan. 21 Iowa precinct caucuses, the first formal test of the campaign.

The grain cutback may or may not hurt Carter politically, depending on what moves the administration makes beyond today's decision to spend $2.25 billion to buy from exporting companies much of the grain ordered by the Soviet Union.

Eggers, though, has made up his mind.

"I think farmers in general have lost trust in any politician now," he said in a slow, sure voice as a harsh wind buffeted Main Street. "If you go back to 1976 you'll find films of Jimmy Carter promising the farmers of Iowa that he'd never put on an embargo.

"Now, you see him doing exactly what he promised he wouldn't do. So my trust in him is out the window."

Goodman nodded in agreement. "I want to go to the Democratic caucuses more to vote against Carter than for anything else," he said.

There was frustration in the words of Eggers and other farmers in this town of 1,200 today.

They realize their political clout has eroded. Even in Iowa, farmers make up only 12 percent of the electorate.

"Sure he's going to hurt his farm vote," said Eggers. "But there just aren't enough of us to do much good."

It would be hard to find a more typical Iowa town than State Center, a collection of wood frame houses and brick storefronts clustered on the now snow-covered lands northeast of Des Moines.

Germans were the first to settle here. They were a sturdy, thrifty lot who prospered from the rich, black earth. The town took its name from its location near the geographical center of the Hawkeye State.

When the Des Moines Register reported last year that the center of the state actually was on a hill near a neighboring town, one wag suggested that State Center change its name to "Off Center."

"When I came here 43 years ago, State Center wasn't much different than it is today," said William Haesemeyer, president of the Central State Bank. "There are maybe 200 more people now. It's always been a somewhat wealthy retired farmers' town."

State Center has a single row of businesses on Main Street, two grain elevators, six churches, a library, three doctors and two taverns. There's one water tower, painted silver and red, but no stoplights or fast-food franchises.

The town's survival depends on the surrounding farms. Grain dealers here, as in most of Iowa, quit buying late Friday. And with the nation's grain market shut down today, they still weren't buying.

This has a practical meaning for David Eggers. Like most farmers, he harvests his corn in the fall and sells it bit by bit over the rest of the year, whenever the price looks right or he has bills to pay.

He has a $20,000 bank note due next week. "I was going to sell some corn last week to pay it, but I just didn't get around to it," he said.

Depending on what additional measures the Carter administration comes up with, grain dealers are telling farmers to expect some drop in the price of corn, which sold for $2.20 a bushel last week.

"People were losing money the way it was. Everyone was holding on, hoping the prices would get better. Now everything is totally up in the air," said Derald Merrill, who found 20 farmers waiting when he arrived at State Center Grain and Milling Co. at 7:30 a.m.

What farmers will do is to delay selling. Those with bank loans due will refinance. This will be costly. Most borrowed a year ago when the interest rate was 10 percent. Now it's 15 percent.

"Politically, this embargo is not going to do Jimmy Carter any good around here," said Haesemever.

This is Repulican country and there are perhaps better political barometers in the state. But Jimmy Carter, then an obscure former governor of Georgia, pulled off a surprise here in 1976 and carried the State Center percinct caucus.

It was largely through the efforts of Ed Davis, a farm implement dealer, and his friends. Davis drove Carter around central Iowa. He and his wife were invited to the inauguration and a special White House dinner. The Carter's son Chip has stayed at the Davis home.

The embargo will hurt his business and may damage Carter, Davis said, "but things look good for him around here. I have a gut feeling about it. Besides, most people had their minds made up before this."