The pilot stated that the glider was at 2,500 feet as he neared the completion of the second leg of his three-leg cross-country flight. He searched for lift and a suitable off-airport landing site simultaneously, in case he was unable to find lift. The glider encountered sink, and the pilot completed a landing to a soybean field. The glider touched down one-quarter mile prior to the point of intended landing and ground-looped, which resulted in substantial damage to the left wing. The pilot did not report any pre-impact mechanical anomalies that would have precluded normal operation.