THE PILOT HAD COMPLETED CHEMICAL SPRAYING, AND WAS RETURNING TO THE AIRPORT FOR REFUELING AND CHEMICAL RELOADING. WHILE ON A TWO MILE BASE LEG, THE ENGINE QUIT. A FORCED LANDING WAS MADE IN A TOBACCO FIELD, RESULTING IN A BENT WING SPAR. IN HIS REPORT OF THE ACCIDENT, THE PILOT INDICATED THAT HE WAS RELYING UPON THE FUEL GAUGE TO DETERMINE THE REMAINING FUEL QUANTITY, RATHER THAN ELAPSED TIME. ABOUT TWO GALLONS OF FUEL WAS FOUND IN THE AIRPLANE DURING THE POST ACCIDENT INSPECTION. UNUSABLE FUEL IS PLACARDED ON THE INSTRUMENT PANEL AS TWO GALLONS.