I have 8 to 10 Cu. Ft. of potting mix that needs to be sterilized. Too much to bake or microwave. This morning I partially filled one of the new BIG Ziplock bags (2 X 2.7 Ft.) with enough moist potting mix to be about 4 inches thick when laying on it's side. Placed it on my concrete driveway in direct sun. After only two hours a meat thermometer indicated 120 degrees about 2 inches from top and 110 degrees about 3 inches down. It was only 11:00 AM and driveway will get another 8 hours of direct sun with high of 95 forecast. I'll check temperature again mid afternoon.

Question: what temperature and for how long is required to sterilize the potting mix ?