On August 16, 2001, at 1115 central daylight time, a Piper PA-25-235, N6794Z, collided with the terrain during an aerial application flight in Kenmare, North Dakota. The pilot was not injured and the airplane sustained substantial damage. The 14 CFR Part 137 flight was operating in visual meteorological conditions and no flight plan was filed. The flight originated in Kenmare, North Dakota, at 1100. The pilot reported he was applying Roundup RT on a flax field which contained kochia weeds which were about 4 feet tall and the flax was about 18 - 20 inches tall. He reported that this was the second load that he was spaying on this field and he had been dropping flags to mark the spot where he turned on the chemicals. He reported that he flew in over wires and noticed what he thought was a flag directly in line with the pass. In addition, the pilot reported, "I use a Satloc GPS marking system with the light bar attached between the chemical tank and the fuel tank. If the spray is turned on after being turned off while on the same pass, the GPS light bar will blink indicating that the airplane is on a pass that has already been sprayed." The pilot looked at the light bar which was blinking at which time he felt the "...airplane lurch as the wheels touched either the weeds or the flax crop. At the same time as the plane lurched, the chemical in the tank sloshed out of the top of the tank and covered the windshield." The pilot reported that he kept the nose of the airplane up and the airplane settled into the field.