The takeoff was attempted on a grassed sandy strip. The pilot reported that the aircraft seemed to accelerate slower than normal. At 50 knots he lowered half flap, and as the indicated airspeed approached 60 knots he realised that insufficient runway length remained to effect the takeoff. The pilot pulled back on the control column but the aircraft did not rotate. He then closed the throttle but was unable to stop the aircraft in the remaining runway. The aircraft overran the runway and overturned. No fault was found with the aircraft that may have contributed to the occurrence. It is probable that the soft, sparsely covered, sandy strip surface contributed to the reported poor acceleration. The pilot's delay in abandoning the takeoff attempt resulted in there being insufficient strip remaining in which to stop the aircraft.