The pilot said that when he turned onto final approach for runway 13, he noticed two rows of round cornstalk bales in front of him and this became a distraction. The airplane touched down short of the runway and after rolling about 50 feet, the nose wheel hit a hole in the ground and the airplane nosed over.A Federal Aviation Administration inspector who went to the scene said the ground was soft from a previous rainfall. His examination of the airplane revealed the firewall was buckled.