The pilot was unable to maintain airspeed and altitude after taking off from a wet and muddy runway. He closed the throttle and landed in a cotton field, but was unable to stop the aircraft before hitting a ditch. Reportedly, the previous (first) takeoff required an excessive amount of runway and the pilot had instructed the helper to reduce this load by 400 lbs. The owner said that some spray material (sulfur) had remained in the hopper after previous flights and that the helper had not looked into the hopper before adding sulfur for this flight. Also, the pilot said that he did not visually check the quantity of the load before starting his takeoff. Reportedly, the engine RPM and manifold pressure were normal during the takeoff roll, and the aircraft continued to the end of the runway where it hit a berm and became airborne in a nose high attitude. After the accident, the hopper was found to be nearly full.