Seedlings are more likely to be killed by such diseases as fungus infections in the soil than are established plants. Sowing seeds in sterilized soil is the primary way to prevent this. You may purchase sterile-seed starting mixes, but they are only sterile until they come in contact with something else. If you are reusing soil, using a bag that has been open for some time or mixing your growing medium, it is best to sterilize it first. No special tools are necessary, and the entire process takes less than an hour to complete.
Mix one part potting soil or compost with one part peat moss. Spread the mixture into a roasting pan so that it is no more than 3 inches thick. The peat moss prevents the soil from becoming compact and hard while sterilizing.
Sprinkle water on the surface of the soil until it is just damp but not soggy. Cover the roasting pan with a sheet of foil.
Preheat your oven to 200 F. Push a meat thermometer into the center of the pan and into the soil through the foil.
Place the roasting pan in the oven. Watch it until the thermometer reads 180 F, then set a timer for 30 minutes.
Check the temperature of the soil when the timer rings. Set the timer for an additional 30 minutes if the temperature reads between 180 and 200 F. Remove the soil from the oven if the temperature has gone over 200 F.
Remove the soil from the oven at the end of the second 30-minute period. Allow it to cool down on the counter to room temperature before using. If you do not use the soil right away, leave it covered in the pan.