The pilot stated that he fueled the airplane for two flights with skydivers and thirty minutes of reserve fuel. He further stated that on the second approach he had to adjust his intended flight path for other airplane traffic. Then, as the pilot decreased the pitch of the airplane on final approach, the engine "sputtered" and lost power. The pilot landed the airplane in a field; however, during the landing roll the airplane nosed over and came to rest inverted which resulted in substantial damage to the left wing spar. The pilot reported to a Federal Aviation Administration inspector that the airplane had experienced fuel starvation and that he did not maintain the required reserve. The pilot indicated that more fuel would have been his recommendation to avoid this accident.