N81799

The pilot reported that he had been soaring in his glider for approximately 3 hours. While returning to the airport, the glider encountered a sink and had descended 2,500 feet before the pilot was able to find a more favorable air mass. At that time, the glider was too low to fly to the airport and the pilot elected to land in a field. While on final approach, the pilot noticed that the field had deep ruts; however, he was committed to landing at that point. During the landing, the left wing struck a rut and the glider ground-looped, which partially separated the tailboom. The pilot reported no preimpact mechanical malfunctions with the glider.