The private pilot and mechanic were performing at flight following the completion of an annual inspection. After takeoff the pilot flew in the airport traffic pattern, departed the area, then elected to return. While on final approach to land, when the airplane was about 30 ft from the runway threshold, the pilot stated that the airplane was struck by a downdraft. He reported briefly hearing the stall warning horn but did not recall if he applied power and stated he likely froze and did not respond. The airplane struck the ground short of the runway in a bean field, collided with a ditch, then nosed over coming to rest inverted on the runway. The vertical stabilizer was substantially damaged. The pilot reported there was no preimpact mechanical malfunctions or failures of the airplane that would have precluded normal operation.