Cessna 305A, N5300G, was being used to tow a glider (Schweizer SGS-1-26C, N8612R) from a 3102 ft grass runway. The glider pilot estimated the tow line was less than 150 ft in length. He reported the Schweizer was very sensitive on tow and that the manufacturer's recommended towing speed was 50 to 60 mph, but the tow pilot preferred a higher tow speed. On several previous occasions, the glider pilot had requested a 'conservative' speed. Reportedly, they compromised on a slow tow to 1000' AGL, then a speed of 80 mph to the release point. The glider pilot stated the wind was gusty, but the initial climb after takeoff was normal to about 150' AGL, then the glider encountered turbulence. Slack developed in the tow line and the glider pilot corrected by using yaw. Witnesses reported the glider moved out of position to the left, then right, then back left and high, raising the tail of the tow plane. Subsequently, the tow plane entered a steep descent and crashed, though both pilots had released the tow line. The glider landed safely. No preimpact mechanical problems were found.