On July 8, 1998, at 2100 central daylight time, an Air Tractor AT-502, N9185C, operated by a commercial pilot sustained substantial damage when it overran the runway on takeoff from runway 17 (2,300' x 100' wet/turf), near Minto, North Dakota, and impacted a drainage ditch. The pilot said that the turf runway was soft and the airplane was slowed too much for liftoff in the last one-third of the runway takeoff length. The pilot reported minor injuries. The 14 CFR Part 137 flight was departing on an aerial application flight in visual meteorological conditions at the time of the accident. There was no flight plan on file. The flight was originating at the time of the accident. The pilot gave a written statement concerning the events surrounding the accident. He said that the first 700 feet of the takeoff runway was concrete and the remaining 1,600 feet was wet turf. He said that during the takeoff roll on the wet turf the left main landing gear tire encountered a "soft rut" and the airplane yawed to the left. He said at that time his head hit the roll-bar. He said, by the time he recovered, the airplane had slowed to the point the airplane impacted a ditch at the end of the runway and nosed over. The pilot did not indicate any mechanical problems with the airplane and said that the engine power remained "full" throughout the takeoff roll.