On June 26, 1998, at 1415 central daylight time (cdt), a Grumman- Schweizer 164B, N48532, operated by a commercial pilot, sustained substantial damage when on takeoff roll, the airplane went off the end of the runway, struck a road shoulder, skipped over the road, and nosed over in a field. Visual meteorological conditions prevailed at the time of the accident. The aerial application flight was being conducted under 14 CFR part 137. There was no flight plan on file. The pilot reported no injuries. The local flight was originating from a private airstrip at Hatton, North Dakota. In his written statement, the pilot said that the airplane was on takeoff roll approximately three-quarters of the way down the runway, and just at flying airspeed when the airplane "hit a soft spot that slowed the airplane." The pilot attempted to dump his load of chemical, but the dump valve would not open. At the departure end of the runway, the pilot tried to lift the airplane into the air so as to clear a road. The airplane's main landing gear struck the road shoulder causing the airplane to skip over the road. The airplane impacted into the field and flipped over onto its top, approximately 50 feet south of the road. A Federal Aviation Administration (FAA) inspector examined the wreckage at the accident site. The airplane's engine was broken off at the engine mounts and firewall. Both propeller blades were bent aft and twisted. The cowling and forward fuselage were crushed downward and aft. The vertical stabilizer and rudder were bent left, forward and crushed inward. Flight control continuity was confirmed. The dump valve on the chemical hopper could not be opened. No anomalies were found with the engine, engine controls or other airplane systems. The FAA inspector determined that the airplane's gross weight at the time of the accident was approximately 6,654 pounds. Weight and Balance Data taken from the G-164B Pilot's Operating Handbook indicates that the maximum gross weight for the G-164B with the R-1340 engine is 5,200 pounds. Civil Aeronautics Manual (CAM) 8, paragraph 8.10-4 (b) "Maximum Weight" states that establishing a maximum weight for an agricultural aircraft is not required. "In lieu thereof, maximum weights for the special purpose loads (e.g. hopper or tank capacities) should be selected by the applicant and demonstrated in a flight check."