The commercial pilot reported that before the aerial application flight, he added Jet A fuel to both wing tanks for a total of 150 gallons of fuel onboard. While flying low-level over a field on the third load, the pilot noticed a change in engine power. He maneuvered the airplane out of the field and the engine steadily lost power. The pilot tried to advance the power forward with no change and the airspeed and altitude decreased. The pilot performed a forced landing to a bean field. During the landing, the airplane bounced, slid across a road, and then came to rest upright. The airplane sustained substantial damage to the fuselage and the right wing. The property owner of the bean field reported that he helped the pilot add about 9 gallons of red die automotive diesel fuel to the left fuel tank and about 5 gallons of the fuel to the right fuel tank after the accident. The pilot reported that the low fuel warning light illuminated about five minutes before the partial loss of engine power, yet the pilot continued with the low-level aerial application maneuvers. When the low fuel warning light illuminates, the airplane flight manual states to, “fly straight and level and monitor fuel gauges” and “land as soon as practical.” The operator reported that there were no preimpact mechanical malfunctions or failures with the airframe and engine that would have precluded normal operation.