While landing on a dirt airstrip, the airplane's left main wheel encountered soft dirt and the airplane subsequently exited the side of the runway. Shortly thereafter, the nose gear sunk into soft terrain and the airplane nosed over coming to rest in an inverted position. The pilot and two passengers were able to exit the airplane unassisted. An examination of the airplane after the accident revealed that the vertical stabilizer sustained structural damage. There were no reported problems with the airplane's flight controls.