Harry Wallner, who sells fertilizer to farmers around Pleasant Plains, Ill., left Washington the other day just the way he arrived--with a quizzical look on his face.

He went around to see congressmen, he went to see Agriculture Secretary John R. Block and anyone else whose ear could be bent, but that look of quandary just wouldn't go away.

What is bedeviling Wallner and thousands of other small business people in rural America who sell supplies to farmers is the Reagan administration's payment-in-kind (PIK) program to give farmers surplus commodities in return for not planting.

Generally, these suppliers support PIK for a very simple reason: when the farm economy falters, the farm-supply business falters. If PIK will help, they're for it. But what bothers them these days is that they can't get a clear idea about the administration's plans for next year.

"Last fall was a disaster," said Wallner, who also is president of the National Fertilizer Solutions Association. "There was indecision on the farmers' part--they didn't know what the administration would propose. We laid in fertilizer, expecting a brisk spring. By January we had fertilizer and equipment on hand. Then in March they got 2 1/2 times the enrollment they had anticipated in PIK. It was devastating. Left a lot of dealers with huge supplies they couldn't sell."

"What we're saying now is that we can live with whatever the Department of Agriculture does. But we just want to know their intentions. And that's what we're having difficulty finding out. Dealers have to know by July so they can do their planning for 1984," he said.

Although it is foregone that an acreage-reduction program will continue next year, Block and other administration officials have been vague about when it will be announced and what it will entail.

The 1983 program could take as many as 82 million acres of wheat, corn, sorghum, rice and cotton out of production as the administration attempts to bring supply in line with lagging demand and to increase farmers' income.

But PIK also means that entrepreneurs such as Harry Wallner will be selling fewer seeds, fertilizers, pesticides, fuel, implements and other agricultural inputs. USDA has estimated that total farm input use will decline between 5 and 7 percent this year.

Fertilizer suppliers would be among the hardest hit, with a 12 to 14 percent decline in sales, USDA economists reported. But a random dealer-survey conducted in March by Wallner's NFSA, based in Peoria, Ill., projected a 25 percent drop in sales this year.

"This is what makes an early announcement on the 1984 program so important," said David K. Murray, executive vice president of NFSA. "The later the announcement, it will force good businessmen out. They can have an adequate cash flow in the fall if the department announces its plans early."

Similar pleas for an early announcement were heard last week by a House Small Business subcommittee reviewing the impact of PIK on suppliers, some of whom urged special federal assistance to help dealers through the hard times.

Fertilizer Institute spokesmen Bill Lohry of Sioux City, Iowa, and Harvey Strothmann of Columbia, Mo., said USDA's decision to allow entire farms to go out of production was a "stunning blow" to suppliers in choice Midwest grain-producing counties. They urged that USDA abandon the whole-farm approach in 1984.

A similar theme was sounded this week by a House Government Operations subcommittee headed by Rep. Glenn English (D-Okla.), which issued a report criticizing USDA for not giving farmers and suppliers a clear idea of 1984.