On September 6, 1999, at 1100 central daylight time, a Cessna 172, N9785T, collided with underbrush/trees and nosed over on the departure end of runway 18 at a private airstrip in Houston, Mississippi. The personal flight was operated by the pilot under the provisions of Title 14 CFR Part 91 with no flight plan filed. Visual weather conditions prevailed at the time of the accident. The airplane sustained substantial damage and the commercial pilot received and his passenger were not injured. The flight departed Houston, Mississippi, at 1055. The pilot reported that he conducted a preflight inspection of the airplane before it was removed from his hangar. Following the preflight inspection, the pilot taxied to runway 18 where he completed a normal engine run-up. During the attempted takeoff, the pilot recalled that the engine did not operate at the normal takeoff performance level. After liftoff from the 1600-foot sod airstrip, the pilot recalled that the airplane would not climb. The airplane collided with 30 foot tall trees and underbrush, and nosed over about 50 feet from the departure end of the runway. During a telephone conversation, the pilot stated that he completed the engine run-up procedure as he taxied to the runway for takeoff. He reported that all systems checked normal, but he believed he forgot to return the magneto switch to the both position before attempting the takeoff. He also said, he and his son had previously completed several takeoffs from their private airstrip and they were usually airborne within the first half of the 1600-sod airstrip. No mechanical problems were reported by the pilot. The post-accident examination of the airplane also failed to disclose a mechanical problem.