On February 26, 1998, at 1432 hours Pacific standard time, an Air Tractor 301-300, N8828S, lost engine power and nosed over during a subsequent forced landing in a soft field near Woodland, California. The aircraft sustained substantial damage and the pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed at the time of the accident. The flight was operating under 14 CFR Part 137 and no flight plan was filed. According to a Federal Aviation Administration inspector from the Sacramento Flight Standards District Office, the pilot reported that he was spraying ammonia nitrate on a field. He was making an approach to the open field when the engine quit and started to windmill. The pilot stated that he made an emergency landing and touched down in soft dirt. The aircraft nosed over and came to rest inverted. In a telephone conversation with the aircraft owner/operator, he stated that he drained approximately 1 1/2 quarts of fuel from each tank at the accident site. He also explained that the aircraft has two fuel tanks and each tank holds 35 gallons of useable fuel. With the fertilizer onboard, the aircraft burns about 40 gallons per hour. The owner further stated that it is their general operating practice to refuel each hour, in one tank only, out of convenience. The tanks cross-feed into each other. He also explained that at a low rpm, the tachometer time will match the time lapse on a wristwatch, but at a higher rpm, the tachometer lapses at a quicker rate. For example, at a high rpm, 1 hour on a wristwatch will equal approximately 1 hour 10 minutes on the tachometer. After the accident, the pilot reported to the owner that he thought he still had 15 minutes left before refueling. The owner opined that pilot had used his wristwatch to calculate the time, instead of the tachometer, which led to fuel starvation. During the engine teardown inspection, no mechanical defects were noted with the engine.