Potting soil, when used, loses its nutrients and becomes contaminated with a host of parasites and other microorganisms that are potentially dangerous if used to pot a new plant. Although many gardeners throw out old potting soil after it is used or add it to the compost pile, it is possible to sterilize the old soil with heat. Sterilized soil is perfect for starting new seedlings in the home.
Spread a layer of soil inside your oven-proof container. Do not make the layer any deeper than 3 to 4 inches, recommends the Colorado State University Extension. This ensures the entire depth of soil is heated quickly.
Wet the soil so that it is moist but not dripping, says the Colorado State University Extension website, and cover the soil with a piece of aluminum foil.
Stick a meat or candy thermometer through the foil and into the center of the soil mixture, and set the oven to between 180 and 200 degrees F, says Laura Pottorff of Colorado State University.
Heat the soil to 180 degrees F and keep it at this temperature for 30 minutes. Do not allow the temperature to go over 200 degrees F, as this may create plant toxins.
Allow the soil to cool. Do not remove the foil until the soil is ready to use to prevent contamination and to prevent the release of odor from the baked soil.