If you are tempted to save money by using dirt from your garden as potting soil, instead of purchasing sterile potting mix at the gardening center, it may not be a bargain. Soil fresh from the backyard doesn't drain as well as potting mix, and it typically contains weed seeds. One solution is to make your own potting soil by combining equal parts compost, perlite and peat moss. After making the potting mixture, sterilize the soil at home to help prevent plant disease.
Heat the oven to 250 degrees F.
Spray the soil with water to moisten.
Fill a heat resistant pan with the moist soil, no more than 4 inches deep.
Cover the pan with aluminum foil.
Insert an oven meat thermometer into the center of the soil, through the foil, so that you can read the gauge.
Place the pan in an oven at 250 degrees F. Heat the soil to 180 degrees F, keeping it at that temperature for 30 minutes. Do not allow the temperature to exceed 200 degrees F. Allow the soil to cool before using.