In a twist that underscores the ironies of the effort, Sunday's Farm Aid concert here for financially strapped U.S. farmers will be held in the center of the most fertile patch of grain-growing land in the nation.

While preparations for the 14-hour electronic hoedown to dramatize the Corn Belt credit crisis race forward in this unpretentious university town, the unusually deep, rich topsoil of the Champaign area is producing an extraordinary harvest.

Professors here at the University of Illinois' prestigious School of Agriculture predict that many farms in a five-county section of central Illinois centered on Champaign will reap more than 200 bushels of corn per acre -- almost double the expected national average of 120 bushels per acre for 1985. Scores of other farms in the same black-earth region are expected to exceed the national norm easily.

Darrel Good, an associate professor of agricultural economics, said that even in a time of low commodities prices, such barn-busting results "mean money in the bank for a farmer who has no debt and high yields." Although the data is not precise, about half of Illinois' 95,000 farms appear to fall in that category.

Among the most favored farms anywhere in the nation are those in Vermilion, Champaign, Piatt, Macon and McLean counties, 100 miles south of Chicago and 650 miles west of Washington. There is irony in that, too.

Settlers in the early 1800s shunned the area because it was a vast bog known as the Great Swamp. But in the mid-19th century, farmers drained the marshes to find a soil enriched by eons of decayed vegetation.

"It became a paradise," said Robert G.F. Spitze, a professor of agricultural economics. "There are other areas in the U.S. where the soil is as rich, but none so extensive as here. There's nothing like it. It's the only place I've ever seen where you can dig down and down and never find a change to subsoil."

Huge harvest machines are stirring across this zone as the 1985 harvest begins. The men often pilot the air-conditioned, self-propelled reapers in the field while their wives drive the heavi- ly laden trucks back to town or farm to unload at the grain elevators and storage bins that dot the flat landscape.

Harvest time means 18-hour days and hardly a moment to visit with a stranger. But one leathery-faced farmer, spying an out-of-towner stumbling along the edge of a vast corn field west of here, paused long enough to consider a question about the meaning of Farm Aid.

"Band-Aid, you mean," he said. "I ain't no starving little child," he added before climbing back into his machine's cab.

But even in such a prosperous place, huge yields are a substantial problem for many farmers. In recent years, they have helped drive agricultural commodities prices through the basement, taking thousands of family farms to the brink of financial extinc- tion.

Through elaborate telecommunications hookups now being plugged in here, and some 1,000 reporters and music critics who will be on hand, Farm Aid intends to tell the world about this contrary situation.

Country singer Willie Nelson first raised the Farm Aid idea to Illinois Gov. James R. Thompson, a friend, last month. Thompson, who is seeking a fourth term next year, jumped on the opry wagon, offering the state's assistance.

The university, one of the nation's premier land-grant institutions, became the right place because its Big Ten football team, the Fighting Illini, happened to be playing away this weekend, leaving Memorial Stadium's 76,000 seats available.

Even so, there was substantial nervousness around campus at the thought of thousands of tobacco-chewing, cowboy-booted stockmen and sod-busters stomping their way through 14 hours of country rock and laments. The concern was sharpened because the stadium playing area, Zuppke Field, has just been converted from grass to AstroTurf, at a cost of $1.8 million.

After talks with the university, Farm Aid organizers agreed to lay down 143,000 square feet of plastic sheet covered with plywood to protect the new playing surface. When they had trouble finding that much plywood, they turned to a composition board called Geotec, which is now being installed.

The fuss has raised a few eyebrows. The AstroTurf, observed the Champaign-Urbana News-Gazette this week, "is being treated with more tender loving care than most of the stars attending the charity concert.