On May 7, 1995, approximately 1420 Pacific daylight time, a Grumman G-164B, N6895Q, registered to/operated by Trans Ag, Inc., and being flown by an airline transport pilot, was substantially damaged when it nosed over during the landing roll following a loss of power while initiating an aerial spray run near Stayton, Oregon. The pilot sustained minor injuries. Visual meteorological conditions prevailed and there was no flight plan in effect. The flight, which was to dispense fertilizer, was to have been operated under 14CFR137. The pilot reported that upon arriving at the destination field and entering the spray pattern at an altitude of approximately 80 feet above ground the engine began to "quit and restart and backfire." The pilot rolled wings level, however, the surging/ backfiring continued and he elected to execute a forced landing due to power lines in his flight path. Upon touchdown in a soft dirt field the aircraft rolled approximately 300 feet after which the wheels sank into the soil and the aircraft nosed over. The operator reported in a telephone conversation that subsequent to the accident the magnetos were checked and found to operate satisfactorily. Additionally, the air induction system was examined and found to be unobstructed. Uncontaminated fuel was found within both wings following the accident. Fuel is delivered to the engine by a gravity feed system. The Operator also reported that rapid accelerations of throttle may result in backfiring and a tendency for the engine to be slow to "spool up" with the corrective action being to reduce throttle as appropriate.