The pilot was flying at 700 feet over a field in order to judge whether it could be sprayed. The engine lost power and the pilot attempted to make a forced landing in a nearby field. The airplane landed hard in wet grass, damaging the landing gear and vertical stabilizer. Upon reflection the pilot reported that he had lost track of time and ran out of fuel causing the loss of power. The pilot reported that there were no preimpact mechanical malfunctions or failures with the airframe and engine that would have precluded normal flight.