Most garden centers and nurseries sell commercial, pre-sterilized potting soil. Gardeners, however, generally have lots of soil sitting around in old pots and containers that can be reused to save the expense of buying new soil. Home gardeners can use several methods to sterilize potting soil. Some use their oven, which can take up to an hour and stink up the house. The quickest and less malodorous method is to use your microwave.
Add any amendments, such as compost or peat moss, to the soil before sanitizing.
Moisten 2 lbs. of soil. To test for correct moisture content, squeeze a small amount of soil in your hand. When you open your hand, the soil should crumble slightly. If it remains in a ball, it is too moist and you must allow it to dry somewhat before sterilizing.
Pour the soil into the roasting bag, leaving the top unsealed.
Place the bag into the microwave oven and heat it at full power (650 watts) for 2 1/2 minutes.
Remove the bag from the microwave oven and seal the top. Allow the soil to cool completely before using.