Conventional wisdom advises that you should avoid using used potting soil because it could be contaminated with diseases, pests and weed seeds. While you can discard or toss the used soil into your compost pile, you can also restore it -- through heat -- back to its original, sterile state. While there are several methods that effectively sterilize or pasteurize soil, the oven or microwave is the easiest method for most gardeners. Turn on the fan vent and open the windows, because the heated soil can let off a foul odor.
Preheat the oven to 180 degrees Fahrenheit.
Spread the potting soil in oven-safe containers so the soil is no deeper than 4 inches. Break up any clumps and remove any old plant debris, including roots.
Moisten the soil with water if it is dry. Cover the soil tightly with aluminum foil and insert a candy thermometer into one of the containers, going right through the foil. You can sterilize several containers on different racks in the oven as desired.
Place the containers in the oven. Check the temperature every 10 to 15 minutes. When the soil reaches 180 degrees, check the other containers of soil, if applicable. Once all the soil reaches 180 to 200 degrees Fahrenheit, let the soil bake for 30 minutes.
Remove the containers of soil from the oven and set them on cooling racks to cool. Keep the foil in place until you use the soil or store the soil in sealed containers or bags.
Break up any clumps and remove old plant parts, such as leaves and roots, before sterilizing used potting soil in the microwave.
Fill a large polypropylene bag with 2 pounds of moist potting soil. Do not seal.
Set the bag in the center of the microwave with the top open. Set the microwave on high -- about 650 watts -- and turn on for 2 1/2 minutes, recommends Colorado State University Extension.
Seal the bag after the timer goes off. Wait until the soil cools enough to hold it to remove it from the microwave. Keep the soil in the sealed bag until it cools completely before using it. You can store the soil in the sealed bag, if desired, until you need it.