Answer: Solarization is probably the best way to sterilize potting soil. It has vermiculite or perlite in it so you wouldn't want to bake it in the oven or in the microwave. Just place it in a large garbage bag and set the bag out in the sun for a few weeks this summer. Fungus gnats don't come in packaged soils but are attracted to moist potting soils because that's where they feed on decaying organic matter and breed. The gnats come indoors through the screens or through open doors and are attracted to the potting soil because it's moist. The are annoying, but won't kill your plants. The best way to get rid of them is to place a barrier between the moist potting soil and the gnats. Use a layer of sand or gravel over the potting soil surface, or cover it with plastic wrap. When the gnats cannot lay eggs, they will die off and no new gnats will hatch. It takes a few weeks but you can eventually get rid of the gnat population if you place barriers to their feeding and breeding sites.
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