If, for economic reasons or for the sake of recycling, you choose to reuse your previous season's potting soil, you must properly sterilize it before you use it again. Old soil carries all the dangers of fungi and soil-borne pathogens that may have been present with the old plant. Placing soil in an oven or microwave removes the potential of transferring these pathogens. After sterilization, the soil will need a quick boost with a water-soluble, complete fertilizer.
In the Oven
Moisten the soil lightly, place it into an oven-proof container, and cover it with a lid. Extension.org suggests placing a piece of foil over the container if no lid is available. Do not spread the soil more than 4 inches deep, suggests Laura Pottorff of Colorado State University Extension.
Place a thermometer into the soil, and set the oven between 180 and 200 degrees F. Do not allow the temperature to go over 200 degrees, as this may produce plant toxins, says the Colorado State University Extension.
Cook at 180 to 200 degrees for 30 minutes, and then remove from the oven. Allow the soil to cool, and do not uncover until ready to use, as cooking the soil creates a stench, says extension.org.
In the Microwave
Fill a plastic microwave container with moist soil and cover with foil. Poke holes in the foil for ventilation.
Cook the soil at 200 degrees using the microwave's temperature controls, and cook for 20 minutes.
Remove from the oven, and tape over the ventilation holes to keep the soil sterile until used, suggests the Colorado State University Extension.