The pilot stated that he departed a private airstrip the morning of the accident and flew to a nearby public airport. After performing several touch-and-go takeoffs and landings in the traffic pattern, he departed the traffic pattern to continue the flight. He opted to perform an off-airport landing in an open dirt field at a state park. After one reconnaissance pass, he verified that the intended touchdown area appeared flat and smooth except for a few obstacles, one of which included a dead tree stump on the left side of the rollout path. During the three-point touchdown, the airplane drifted left. As the pilot attempted to correct the drift, the airplane aligned directly with the stump. The left main gear struck the stump, shearing the wheel assembly at the axle, which dug into the dirt. The airplane nosed over and came to rest inverted, facing the opposite direction, which resulted in substantial damage to the rudder and fuselage. The pilot stated that there were no pre-impact mechanical malfunctions or failures that would have precluded normal operation.