The agricultural biplane collided with an unmarked pole while maneuvering during an aerial application flight. The 9,500-hour commercial pilot reported that he had been contracted to spray an empty cattle feedlot that had not been used for some time to eradicate weeds. During the last application pass, while attempting to clean-up a stockpile of dirt, the pilot failed to see a wooden pole with a light fixture located about 50 to 100 feet beyond the dirt stockpile. The underside of the right lower wing made contact with the light fixture on a pole. The pilot added that the fuel system was not compromised and he was able to fly the agricultural airplane to his home-base airport located approximately 8 miles west of the accident site. The operator confirmed that the pole was masked by the lack of visual contrast. Examination of the right lower wing revealed structural damage to the front wing spar.