On August 12, 2005, at 1330 central daylight time, a Grumman G-164A, N9632, operated by Pro Air Service, Inc., as an aerial application airplane, received substantial damage on impact with terrain during a forced landing about two miles northwest of Marshalltown Municipal Airport (MIW), Marshalltown, Iowa. The pilot reported a loss of engine power. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 137 aerial application flight was not operating on a flight plan. The commercial pilot was uninjured. The local flight originated from MIW at 1315. The pilot stated that his preflight inspection of the airplane revealed no discrepancies. Following his preflight, he taxied to the chemical loading area to load the airplane with his first load of the day and then taxied to the fuel pumps to obtain 50 gallons of fuel. After refueling, he taxied to the run-up area where he performed a run-up which he described the engine and controls as "all operating properly." The pilot stated that he performed a "normal" take off and about one mile from the departure end of the runway, the engine began to lose power and the airspeed began to decrease. He nosed the airplane "slightly" down and added a little power to correct the problem; however, the engine continued to lose power. He applied maximum power, but the engine did not respond and continued to lose power. The airplane's altitude and airspeed were lower than he "cared for." He continued to fly the airplane straight ahead to avoid a stall and was certain that he could not keep the airplane in the air. He dumped the chemical on-board and "shortly thereafter" attempted to land on a soft and wet corn field in a three-point attitude. The airplane flipped over upon touchdown. Examination of the airplane by the Federal Aviation Administration revealed that all airplane control surfaces and their connections throughout the airplane were intact. All of the engine cylinders were intact with no visible cracks or other damage noted. The engine was attached to the airframe and was rotated through the crankshaft; no anomalies were reported. A copy of a refueling invoice from the departure airport's fixed base operator lists 53.5 gallons of fuel was pumped into the airplane prior to the accident takeoff.