On September 18, 1999, about 0650 hours Pacific daylight time, an Arrow Falcon Exporters, Inc., OH58A, N5800K, landed hard from an autorotation 1 mile south of the airport at Salinas, California. The autorotation was precipitated by a tail rotor strike with terrain and a subsequent loss of antitorque control. R & B Helicopters, Inc., operated the helicopter under the provisions of 14 CFR Part 137. The helicopter sustained substantial damage. The commercial pilot was not injured. The agricultural aerial application flight departed Salinas about 0620. Instrument meteorological conditions prevailed and no flight plan was filed. The pilot stated he was on a westerly pass over a field approaching wires that he planned to fly under. A berm 4 feet high was just in front of the wires. He applied left pedal to avoid the power line pole to his left. He heard a loud noise to his rear and the helicopter began rotating clockwise. As he passed under the wires, he increased pitch on the collective. He climbed to approximately 100 feet and tried to streamline the helicopter but felt he was turning too fast to control it. He commenced an autorotation and pulled collective pitch 10 feet from the ground. The helicopter hit hard and the left skid dug into soft dirt. After it came to rest, the pilot secured the systems and engine. The helicopter suffered damage to the vertical fin, skids, tail boom, and frame. The Federal Aviation Administration (FAA) accident coordinator interviewed a farmhand who thought the helicopter's tail contacted the berm. The coordinator inspected the helicopter after the accident. The tail rotor blades had fractured and separated; the fracture surfaces were irregular. He also observed the lower section of the vertical stabilizer, starting at a point about 6 inches below its mounting point, bent inboard about 20 degrees. He noted the long tail rotor drive shaft fractured midway between the aft short shaft and the tail rotor gearbox. The fracture angled aft along a 45-degree plane from the bottom to the top of the shaft and exhibited torsional twist in a clockwise direction as viewed from the rear. He turned the tail rotor blades by hand and noted the tail rotor gearbox, and the portion of the long tail rotor drive shaft that remained attached, rotated freely. He did not hear or feel any binding or grinding noises in the gearbox. The manufacturer supplied a drawing that indicated the long tail rotor drive shaft rotated clockwise as viewed from the rear.