While the aircraft was on base leg the pilot saw a powered aircraft which had been conducting circuits throughout the day on final approach. He considered that he would land too close behind the other aircraft so elected to land on the grass strip beside the runway. During the landing the long grass impeded the left wing slightly. The pilot corrected but the right wing struck the ground and a ground loop ensued. Examination of the tyre marks indicated that the pilot had actually landed outside the strip markers in an area of long grass and rough ground. Witnesses observed that the powered aircraft had conducted a touch and go landing and was airborne again before the glider crossed the threshold boundary fence.