The pilot was en route to spray a field when he received a call over the company radio to return to the airport. He returned for landing with the full application load. The airstrip consisted of gravel and asphalt; grass areas bordered the runway. The main wheels touched down near the runway threshold; the initial portion of the runway was gravel. The tail wheel settled onto the runway as the airspeed dissipated, and the pilot brought the propeller into reverse. However, the tailwheel encountered a “bump” which started some pitch oscillations; this might have occurred at the transition from gravel to asphalt. The pilot reported no elevator or rudder authority, possibly due to the reduced airflow over the empennage resulting from the reverse thrust. He noted that a go-around was not an option at that point. The airplane yawed to the right and departed the paved area, proceeded into the grass area, and finally into the soft ground adjacent to the runway causing the airplane to nose over. The pilot stated that there were no mechanical anomalies with the airplane that would have precluded normal operations.