How to Sterilize Potting Soil
You Can Do It at Home!
Pathogen-free soils are available in garden centers, but knowing how to sterilize potting soil will help assure you of safe, chemical-free mixes for your plants.


Things to Note Before Sterilizing
  • Thoroughly clean everything associated with your sterilization process before and after treatment. Your purpose is to kill contaminants and to avoid re-infection.
  • Remove any large pieces and debris from the potting soil prior to sterilization. To play safe, you may want to use a one-quarter inch sifter.
  • Potting soil must be moist, not wet. It should crumble easily; it should not be compacted.
  • Dry soil is very difficult to sterilize. Add some water, as necessary.
  • Add whatever amendments the potting soil needs prior to sterilization.
  • The length of sterilization time depends on the soil volume and its moisture content.
  • Sterilizing soil develops a strong odor. Provide adequate ventilation during treatment.
How to Sterilize Potting Soil at Home
The amount of soil you need to sterillize will determine if you need the oven or the microwave method.

Oven Method
Have ready a candy or meat thermometer, some shallow clay pots, glass or aluminum pans, etc. for containers. If you do not have tightly fitting lids, use aluminum foil to cover the containers. Pre-heat the oven at 200°F. Evenly spread the potting soil up to four inches deep in the container. Cover with aluminum foil carefully. Press the edges for a tight seal. Create a tiny opening in the middle of the foil so that you can place the thermometer in the potting soil through the foil. Watch the thermometer reading so that you can adjust the oven temperature as necessary. To sterilize the soil, the thermometer should register up to at least 180°F but definitely below 200°F and for a period of 30 minutes. At too high a temperature, you will have an accumulation of minerals, salts, and/or ammonium/soluble organic compounds in the potting soil that are toxic to your plants. When done, remove the covered containers from the oven and let cool naturally. Keep the aluminum foil on until you are ready to use the potting soil

Microwave Method
For small amounts of potting soil, you can sterilize in your microwave. Do not use any container that is foiled or has any metal on it. You can microwave up to two pounds at a time. Polypropylene bags are excellent containers for sterilizing potting soil using the microwave method. Put the soil in the open bag and place in the microwave. For a microwave with 650 watts, you will need to treat the potting soil for two-and-one-half minutes. Otherwise, do not microwave for longer than two minutes to avoid toxins from developing. You can then close the bag and let cool naturally before taking it out of the microwave.

Testing for Toxicity
If you suspect your sterilized soil of toxicity, plant a few seeds in it and a few of the same seeds from the same packet in untreated soil. Lettuce is a good try, as it has a high germination rate. If the seeds in the untreated soil germinate, but the ones in the treated soil do not, you know that there are toxic compounds in the latter. You do not have to do both, if you do not want to. You can simply try the sterilized soil. If the lettuce seeds do not germinate, discard the potting soil.

Learning how to sterilize potting soil will help you control the quality of the medium for your plants.


   l 
  l   Back to Top  | 

Custom Search