Only about two dozen officials were there to bear witness, but a momentous event occurred yesterday at the Agriculture Department: Organic farming got a foot in the door again.

The bureaucrats listened raptly as Dick Thompson, one of the country's best-known organic farmers, described the nonchemical techniques that have made his Iowa farm a profitable environmental showcase that has taken him out of debt.

"Twenty years ago, this wouldn't have happened. It probably wouldn't have happened one year ago," said Thompson, expressing some surprise over having been asked by Peter C. Myers, an assistant secretary of agriculture, to give his lecture and show the color slides of his farm.

Officially, the USDA looks down its institutional nose at organic farming, which rejects the expensive manmade fertilizers, pesticides and herbicides that characterize mainstream American agriculture.

Former secretary John R. Block fired the department's only organic specialist in 1981 and described organic research as "a dead end."

But Block is gone, and the department, in spite of itself, is being forced increasingly by Congress and farmers to focus on farming practices that stop soil erosion, cut costs and reduce off-farm pollution problems caused by the chemicals that wash into streams and public water supplies.

The "seat-of-the-pants" experimentation that Thompson has carried out in his organic operation achieves all of those goals. It draws thousands of visitors each year to his farm near Boone, Iowa, and keeps him on the winter lecture circuit.

Myers, a Missouri farmer who headed the Soil Conservation Service before he was promoted last year, met Thompson some months ago at the Rodale Research Center in Pennsylvania and was intrigued with his stories of high yields and minimal operating expenses achieved with organic practices.

"Peter Myers is the key in this," Thompson said yesterday. "Two farmers get together and they start farming right away. That happened with us. He said he would line up a meeting if I came to Washington and I took him at his word."

Myers told the audience that "the thing that is interesting to me is that Dick Thompson is not theoretical. He is a practical farmer. He knows what he is doing . . . . So unload on him. But he knows all the answers."

Until 1967, Thompson farmed as he was taught at Iowa State University and as his neighbors around Boone farmed -- heavy on the chemicals, heavy on the capital outlays, heavy on getting bigger.

"I caught the disease of expansion . . . . I was a good friend of the banker under that system," he said. "Now the question is, if I am going to spend $5 an acre, how do I spend it?"

Thompson described in detail the trial-and-error experiments that have led him into new tillage techniques and crop rotations aimed at holding down weeds while bolstering soil fertility and pushing his grain and forage crop yields to more profitable levels each year.

The secret, he explained, lies in establishing a rotation cycle that helps regenerate soil while blanking out weeds and in using mechanical cultivators at the right time instead of chemicals to combat the weeds that survive.

"This thing is happening everyplace," Thompson said. "Both sides -- we on the organic side and those on the chemical side -- are bending. The ideas of how we have perceived each other are changing."