You can try sterilizing the potting mix by putting 2 pounds in a microwave-safe container for 90 seconds.
Or, you can do it in a traditional oven by spread soil in a cookie sheet and wrap aluminum foil around it. Insert a meat or candy thermometer into the center of the soil and preheat the oven to 190F. Bake it for 30 minutes.
---Justin, Senior Garden Editor, BHG.com