How to Sterilize Soil
Why Sterilize Potting Soil?
Sterilizing potting soil is very important when sowing seeds, seedlings, and cuttings. Soil naturally contains pathogens, harmful bacteria, and fungi that can harm or kill susceptible plants. Insects and larvae may also be present, which can harm plants and become a nuisance. Store bought soil that claims to be sterile may be contaminated via bag punctures, old shelf life, mishandling, and other numerous reasons. Sterilizing potting soil is easy and very beneficial to plant health. Only a few household items are needed, and about 45 minutes of time to prepare and sterilize the soil.
- Large Roasting Pan/ Baking Sheet
- Aluminum Foil
- Meat Thermometer
- Fertile Soil
- Mixing Spoon
Preparing Soil for Sterilization
Be forewarned - sterilizing potting soil indoors will cause an unpleasant odor that is extremely earthly. Some people can tolerate it, but others may be panicking to open the windows for fresh air. Open the windows and turn on a ceiling fan before sterilization begins. The odor will not linger very long if indoor air is circulating with outdoor air.
The soil needs to be moist enough to compact into a ball, but should crumble when pressure is removed. This can be done by simply packing a ball of soil between the hands. Add a little water and mix until consistency is met.
Placing Soil into Pan
Place the soil into a roasting pan or baking sheet. Evenly distribute the soil in the pan and break up any clumps. Do not fill to the edge of the pan, and keep depth under 4 inches deep. Cover the pan or sheet tightly with aluminum foil. The foil needs to be tight and secure to prevent moisture loss and burning of the soil. Poke a hole in the center of the foil that is large enough for a meat thermometer to fit through.
Place the tray into an oven and set the oven to a low temperature. The ideal temperature for sterilization is 180 degrees Fahrenheit. Temperatures over 180 degrees Fahrenheit may cause burning. Burning the soil will chemically change the composition of the soil. Soil that is burned or overheated will harm plants.
The soil needs to remain in the oven for 30 minutes, while the temperature needs to remain closely around 180 degrees Fahrenheit. Monitor the thermometer and adjust the oven temperature as needed. Paying close attention to temperature fluctuation is a must.
Cooling & Using Soil
After baking for 30 minutes, remove soil from the oven and let stand until the soil reaches room temperature. Carefully lift a corner of foil to allow heat to escape. The escaping steam will be very hot and can burn skin.
The soil can be removed from the pan and used once it has reached room temperature. Sow seeds no deeper than 4 times the diameter of the seed. Water the sterilized soil with a mister to avoid flooding the soil. Seeds sown into a container should be covered with plastic wrap and placed in a cool room. Avoid direct sunlight until germination. Gradually introduce light over a several day period and remove the plastic wrap once germination begins.
Containers need to be sterilized before the newly sterilized potting soil can be added. Soil-borne diseases, fungi, and insects can live in small amounts of soil and debris remaining in previously used and uncleaned containers.