The pilot was performing aerial application of liquid fertilizer onto a hay meadow. During the third pass, which was south of the previous two passes, the airplane contacted an unmarked power line that the pilot did not observe until "I was on it." The airplane then struck two trees and cart wheeled into terrain. The commercial pilot, who was the sole occupant, sustained minor injuries. Examination of the airplane showed the wings were bent, fuselage was twisted and buckled, and the engine firewall was buckled.