A D.C. Superior Court jury yesterday awarded $400,000 to a Georgia farmer who lost the use of his left eye when a D. C. policeman fired a tear gas gun into his face and then wrongly arrested him during the farmers' massive February 1979 tractorcade protest here.

The award is apparently the first in a civil suit stemming from the demonstration in which more than 100 farmers tied up D.C. traffic in snarls by repeatedly driving their tractors and equipment through the city.

"What's one of your eyes worth?" farmer Johnny H. Colston, 29, asked yesterday after he heard the jury make the award. "I don't think it's enough. I'm glad to be headed back home."

Colston, a tall, bearded man who raises soybeans, corn, wheat and oats in Kite, Ga., said his tractor was part of the Feb. 5, 1979, traffic jam at the intersection of 7th Street and Independence Avenue SW. He testified that D.C. Police Officer Charles R. Aldridge smashed the door of his tractor and fired powdered tear gas into his face, leaving Colston unconscious and permanently impairing the vision in his eye.

Colston also alleged in the four-day trial that he had been wrongly arrested and charged with assault and reckless driving by D.C. police officers who had stopped his tractor. The assault charge was dropped the next day, according to the lawsuit, and Colston was acquitted by a Superior Court jury in March 1979 of the reckless driving charge.

J. Gordon Forester Jr., Colston's attorney, played a telvision tape of the incident during the trial, which the lawyer said bolstered the farmer's account.

Forester also said that during the trial he asked Aldridge whether he had been carrying a shotgun the day of the incident. The officer said no, but added that had he been carrying one, he might have used it, according to Forester. Aldridge could not be reached for comment and a spokesman for the D.C. Corporation Counsel's office, which defended the policeman and the city in the case, declined comment.

Colston's attorney also presented evidence and testimony provided by a tear gas expert who said that the kind of riot gun that Aldridge allegedly shot into Colston's face from a distance of three feet should not have been used in a confined area such as the cab of a tractor, but only in large, open areas. Another Georgia farmer, Jimmy Sumner, who had been driving his tractor directly behind Colston's corroborated Colston's account of the incident in other testimony.

After the jury rendered its verdict and left the courtroom, Judge Fred L. McIntyre, who had presided over the trial, noted that he thought "it was pretty clear" there was liability in the case. "Of course, damages are always hard to compute," the judge said, but he did not indicate whether he plans to reduce the amount.

The city can appeal the verdict or ask McIntyre to reduce the award.

Forester declined to say what his fee in the case would be, but lawyers traditionally get about one-third of civil damage awards, which in this case would amount to about $133,000.