The pilot reported that he was performing a practice soft field landing on a grass runway. During the landing flare, the wind increased, the airplane drifted to the left, and the indicated airspeed decreased by 10 mph. The pilot attempted to correct for the deviation when the airplane developed an excessive sink rate and landed hard on the sand adjacent to the left edge of the runway. The nose landing gear and propeller dug into the sand, the airplane nosed over and came to rest inverted. The airplane sustained substantial damage to both wings, the left aileron, the rudder, the vertical stabilizer, and the right wing strut. Wind at the time of the accident was 210° at 11 knots gusting to 17 knots. The pilot stated that there were no preimpact mechanical malfunctions or failures that would have precluded normal operations.