One of the characteristics of a good potting mix is that it should be free of weed seeds, insects, and diseases. We know that even just a scoop of soil contains billions of microorganisms that can possibly benefit or harm the plant you’re growing. To avoid the risk of the damage caused by these, gardeners should use sterile soil in their potting mixes.
What is Soil Sterilization?
Soil sterilization is a method used to eliminate harmful organisms such as pathogens, weed seeds, and soil-borne pests in the soil that can potentially harm the plant being cultivated. The goal of sterilization is to disturb the unbalanced population of harmful organisms in the living soil. That way, the soil will remain healthy and suitable for growing healthy plants.
Sterilizing soil is a form of a control method. While it may significantly reduce the chance of developing diseases among plants, other factors may lead to the same problem. Poor conditions such as waterlogging, excess moisture level, and exposure to diseased plants can still encourage the growth of harmful organisms.
Types of Sterilization
One way to kill all forms of life in the soil is through heat. It’s the most common type of sterilization. In heat sterilization, moist heat is more effective than dry heat.
Different organisms have varying thermal death points. Here’s a table summary of the temperatures needed to kill the specific organisms according to Baker, K. F. (1957).
|Moist soil, 30 minutes at||Organisms killed|
|120°F (49°C)||Water molds (oomycetes)|
|145°F (63°C)||Most plant pathogenic fungi, bacteria, and viruses, worms, slugs, centipedes|
|160°F (71°C)||Plant pathogenic bacteria, soil insects|
|180°F (82°C)||Weed seeds|
|212°F (100°C)||Heat resistant plant viruses and weed seeds|
There are two popular methods in heat sterilization: one is using steam and the other is using solar energy.
This method utilizes the use of steam to kill unwanted organisms. Once the hot steam that fills the pore spaces within the soil reaches the required temperature, the organisms will be killed.
Also known as soil solarization, this method uses solar energy to kill bacteria, insects, and weeds present in the soil. The process involves covering the soil with a transparent polythene cover to trap the intense heat from the sun underneath which creates an unsuitable environment for weed seeds, organisms, etc. This method works best on heavy soils like clay or loam.
Herbicides are chemicals applied to the soil to kill and prevent weeds from competing with plants for water and nutrients.
Soil fumigation is another form of chemical sterilization. They are applied to the soil to reduce the population of organisms. Fumigants can either target specific pests or kill a broad spectrum of living organisms in the soil.
Nowadays, there are a lot of pre-sterilized potting soil or soilless mixes being sold in the market. These products have already undergone the process of sterilization. You can use them directly after purchase.
But, if you wish to sterilize your own soil, that should not be a problem. We will discuss a step-by-step procedure on how you can do such.
Soil Sterilization at Home
For a small-scale gardener who wants to sterilize a small amount of soil, these two methods of heat sterilization can be done:
- Fill a saucepan with ½ pint of water. Using a gas range or an electric cooker, heat the water rapidly until it boils immediately.
- Pour some previously sieved soil to fill the pan to about ½ inch on top. Cover the pan.
- Heat the soil for another 7 minutes.
- Take the pan out of the cooker and let it stand for another 7 minutes. Keep the pan covered.
- After that, scoop the soil out of the pan. Spread it thinly on a clean surface and let it cool.
- Once cooled, you can mix the sterilized soil with other amendments to create your desired potting mix.
- Prepare the oven and preheat it to 180oF.
- Spread the moist soil on the pan at most 4 inches thick. Cover the pan with aluminum foil.
- Put the pan inside the oven and leave it there for 30 minutes once the temperature reaches 180oF.
- After 30 minutes is over, remove the pan and let the soil cool down.
- Once the soil’s temperature went back to normal, you may use it in mixing your potting media.