Potting soil, even bags marked "sterilized," can sometimes contain fungi and bacteria, which are harmful to plants. Generally, a quality soil medium mix does not contain these bad elements. The older soils or generic potting soils are the ones more susceptible to insects, bacteria and mold. Sterilizing the medium before using eliminates the dangers for plants.
Add moist soil to a roasting pan making sure the depth of the soil is no more than 4 inches. If the soil is completely dry, add a little water to moisten.
Spread the soil in the baking dish and cover the pan with the foil. Stick the thermometer directly in the center of the foil and into the potting soil. Do not let the thermometer touch the bottom of the pan.
Turn the oven on a very low temperature. The ideal temperature to sterilize plant soil is 160 to 180 degrees Fahrenheit. Digital oven settings are great, but for a standard oven, the "warm" setting works well.
Maintain the baking process until the thermometer temperature reaches 160 to 180 degrees Fahrenheit. Make certain the soil temperature does not go above 200 degrees because this will cause any plant toxins in the soil to be released into the air.
Cool the sterilized potting soil in the pan. Remove the soil and put in a sterile storage container until ready to use. The pan must be washed well to remove any contaminates from the soil.