On August 20, 2008, at 2015 central daylight time, an amateur built NTBK LLC Wag Aero Sport Trainer, N141ST, collided with the scrub trees while landing on runway 15 at the Green Castle Airport, a private airstrip, in Oxford, Iowa. The private pilot received minor injuries. The airplane sustained substantial damage to the wings and firewall. The personal flight was operating under Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed. The local flight originated from the Green Castle Airport at 1940. The pilot reported the total runway length is 4,800 feet, the northern 1,400 feet of which is grass and the southern 2,400 feet is asphalt. He reported he was practicing touch and go landings on the grass portion of the runway when the accident occurred. The pilot reported he made three touch and go landings, and decided to make one more before it got too dark. He stated that the final approach descent path appeared to be above the low-lying trees. He stated that he did not realize he was too low until he felt a "sharp blow" to the airplane and the airplane rotated to the right descending to the ground. The pilot reported that he should have realized that "...landing as close to the trees as I intended was unsafe" when the runway lights came on while he was on final approach.