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.

Materials Needed

  • Oven
  • Large Roasting Pan/ Baking Sheet
  • Aluminum Foil
  • Meat Thermometer
  • Fertile Soil
  • Water
  • 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.

Soil Consistency

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.

Sterilizing Soil

Beginning Sterilization
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.

Baking Duration
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

Cool Down
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.

Important Tips

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.

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Comments 10 comments

angela_michelle profile image

angela_michelle 4 years ago from United States

I actually had never heard of steralizing potting soil. It makes sense, but it had never occurred to me. Thanks for the great information.

Sage in a Cage profile image

Sage in a Cage 4 years ago

Interesting hub. I'm just starting to put some seeds down this week so it was really helpful.

Janis Goad profile image

Janis Goad 4 years ago

Do you sterilize potting soil even when you buy "sterile mix" from the garden store? I never worried about it before.

GoodLady profile image

GoodLady 4 years ago from Rome, Italy

Looks good and we will try it next year! Too late for now. All our potting has been done for the next season. Oh, except for an area we call the hospital where plants that look as though thy are on their last legs get put....and they grow. Perhaps we need to sterilize that soil to make sure they really will be OK now.


Janis Goad profile image

Janis Goad 4 years ago

When do you start potting in Tuscany, Good Lady? We are just getting started in Kamloops, and the soil won't be warm enough to plant out until the end of May. There is still snow in the high areas.

seh1101 profile image

seh1101 4 years ago from Wisconsin, USA Author

Janis - Honestly, I usually don't sterilize bagged potting soil, but I know some who have gone through the work to make sure it is sterile (usually for specialty plants/crops). I generally sterilize when I don't know the source of the soil, such as a municipal mulch/dirt pile. Anything and everything gets dumped into municipal soil piles.

chrissieklinger profile image

chrissieklinger 4 years ago from Pennsylvania

I started growing vegetables a few years ago and this is very helpful information that I never knew about. Thanks for an easy to follow guide. I can't wait to try this!

Brainy Bunny profile image

Brainy Bunny 4 years ago from Lehigh Valley, Pennsylvania

I don't know the first thing about gardening, so it never occurred to me that you would have to (or even could!) sterilize dirt. Very interesting!

denisemai profile image

denisemai 4 years ago from Idaho

Oh, dear. I'm afraid I have two black thumbs. I also never knew about sterilizing soil. My lack of knowledge is most likely what's causing my plant growing to fail. Great, informative hub!

sofs profile image

sofs 4 years ago

I have never heard about soil sterilization. Now that I know.. I will be trying this. I am glad you added those words of caution. Useful information. have a great day!

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