The pilot of the tailwheel-equipped airplane reported that he was landing at a soft, grass-covered off-airport site. He said that just after touchdown, as the airplane's main wheels contacted the 14-inch tall grass, the airplane nosed over during the landing roll. The airplane sustained substantial damage to the wings and vertical stabilizer. He said that winds at the time of the accident were calm. He noted in his written report to the NTSB that there were no preaccident mechanical anomalies with the airplane.