Whether you are starting seeds for your summer vegetable garden or your flowerbeds and borders, the right potting soil ensures they will get off to a disease-free, pest-free beginning. Because the soil from your garden contains weed seeds and other undesirable elements, is unsuitable for starting seelings. Garden soil also compacts easily; because it does not supply proper aeration to the roots, your seedlings can suffocate. Making and sterilizing your own potting soil solves these problems, and it's easy to do.
In a roasting pan, evenly spread a 4-inch layer of mature compost---either store-bought or from your own compost heap---making it level with the flat with your hand.
Cover the top of the roasting pan with aluminum foil. Seal it tightly around the edges.
Poke a meat thermometer through the foil in the center of the pan. Place the roasting pan in an oven preheated to 200 F.
Bake the soil until the thermometer reaches 180 F. Set a timer for 30 minutes and continue baking until the timer rings.
Remove the roasting pan from the oven and allow the soil to cool to room temperature. Use right away or store covered until needed.
Mix 1 part sterilized compost with 1 part peat moss and 1 part vermiculite to make a sterile seed-starting mix. Store covered until needed.