Soil sterilization can happen through heat methods including the oven, microwave, steaming, or boiling water.
These processes are very simple, quick and can only be used if you are sterilizing small amounts of potting soil.
To sterilize large where your beds have plants out in an outdoor garden – you will want to use the solarization method which generally takes much longer.
Soil sterilization is important and can be the only method of growing healthy and vigour seedlings.
In this process, a gardener could kill any pathogenic bacteria, insect eggs, fungal spores, and nematodes and protect the plant from all types of disease.
In General – Regardless of which method the gardener chooses, the results are still desirable.
Methods of Sterilizing Potting Soil
1. Steaming
This is a very effective and easy way to sterilize your soil.
Step 1: Add 3 or 4 cups of water to the bottom of a steam pot, which has holes in it and a container that sits up a bit from the bottom. If you don’t have a steam pot, just put a wire holed cook rack on the bottom of a normal soup pot slightly above water depth. It wouldn’t hurt to put a lid on it to boil faster.
Step 2: Add the soil you want to sterilize to a rack above the pan (the pan or container has to fit in your steam pot). Keep your soil no more than 4” deep, do not pack it down or choose soil that is packed down this way you will have enough steam and heat that will penetrate.
Step 3: Cover the pan/container with aluminum foil to help hold in the steam, and put it on the burner and turn on the heat to boil the water. Set your timer for 30 minutes. In 30 minutes or less your water should boil. While the water boils the heat and steam radiates from the pot will circulate around the soil container, killing any pests in the soil mix.
Step 4: After 30 minutes of boiling turn off your stove and allow to cool before working with it (don’t get burned).
2. Boiling Water
Boiling is performed in the exact same way as steaming.
Step 1: Get a large bucket or heat-resistant container. Fill the bucket half full with the soil you want to sterilize.
Step 2: Fill your kettle half full with water and heat until the water boils. The temperature can reach 212 degrees Fahrenheit, which will kill heat-resistant viruses and weed seeds; the amount of water needed is equal to the amount of potting soil.
Step 3: Carefully pour the hot water into the soil bowl until it is damp, but not soaking wet. With a wooden spoon or other suitable tool, mix the water into the soil until it starts clumping together. The hot water destroys insects and their eggs in the soil, leaving it clean and ready for planting.
Step 4: Allow the soil to cool before planting.
3. Microwave
Microwaving soil sterilization has many more benefits than other methods.
It’s more efficient, purely thermal, highly controllable, allows more nutrients available for plants, etc.
Step 1: To start, you’re going to want to make sure the soil you are sterilizing isn’t too dry. If it is too dry, a spray bottle with water can be used to mist the soil until it becomes fairly moist. The soil should be stirred to ensure even distribution of the moisture.
Step 2: Place up to 2 pounds (approximately 1 kg) of this moist soil into microwave safe containers with lids. You don’t need to cover the soil with foil but you should use lids with ventilation holes for the steam to escape from.
Step 3: Place the container with the soil in the center of the microwave oven.
Step 4: Preheat the microwave oven to 200°F (93 C.). When steam appears, cook the soil for another 30 seconds. The duration will depend on the amount of soil, the initial temperature of the soil, the moisture level, and the microwave oven wattage. In most cases, this is likely to take between 1 and 5 minutes.
Step 5: Remove from the microwave oven and let cool slightly.
Step 6: The sterilized soil is now ready for your use.
Note: Be aware you will need a microwave that can evenly heat the soil or you will have soil that is not completely sterilized. A few pathogenic bacteria could later repopulate the rest of the soil with all unwanted life. Find a microwave oven with a turntable.
4. Oven
You can also use an oven to sterilize soil!
Step 1: If the soil is dry, use some water to sprinkle a bit of moisture onto the soil before it becomes muddy. This moisture will steam the soil as it sterilizes. Next, fill a large bucket half full of soil.
Step 2: Preheat the oven to 200 degrees F. Take a oven-safe baking dish and spread the potting soil evenly to 4 inches deep. Do not pack the soil, but rather leave it loose. Cover the baking pan with aluminum foil.
Step 3: Insert a thermometer into the potting soil directly in the center of the pan through the foil. Make sure the thermometer is centered in the soil. Place the pan into the oven and keep watch of the soil temperature. Let the soil reach a range of 160-180 degrees Fahrenheit for a minimum of 30 minutes before removing it.
Step 4: Do not fully uncover the soil. After 30 minutes, remove the foil and set it aside. Use the soil when it has cooled off and is at room temperature.
Sterilizing Outdoor Gardening Soil
Solarization
I simply think the best way to sterilize outdoor garden soil is to use polarization. The process uses heat from the sun to “cook off” soil and kill pathogens. To get the job done well, it does take a little patience because you’ll need to have a couple weeks of hot sunny days.
Step 1: Use a large peice of clear plastic to cover the area you want to sterilize. The plastic sheet needs to be a heavy weight that the sun won’t break through.
Step 2: Remove all the plan debris and rocks that could break your plastic sheet once in place. Also, break up any clumps.
Step 3: Place your soil in an even layer on the top of the plastic keeping it 6 inches away from the edges for securing your border.
Step 4: Water, water, water! Put water on the soil until the top 12 inches is completely wet. This will help with the sterilization process.
Step 5: Place another plastic sheet over the soil and secured the edges down with bricks, rocks, or lengths of wood. Make sure your top sheet does not have ripples so that the heat build up is trapped.
Step 6: Make sure the plastic sheet is sufficiently secured down so that a stiff wind does not blow it away.
Step 7: Leave the plastic in place for four to six weeks. The soil will be sterilized, no pathogens, and ready for planting!
Why You Should Sterilize Soil
Sterilizing soil comes with plenty of benefits including;
It kills fungus gnats and nematodes
Those little dirt bugs can affect your plants some!
There very common and you may even find them in bagged potting soil from a store.
These bugs will actually live in and lay eggs in the dirt and that could be harmful for your plants.
One way to sterilize the soil is to heat it, effectively killing the bugs and supporting better plant growth.
It kills pathogens
Unsterilized soil is home to a multitude of harmful bacteria, viruses, and other microorganisms that can spread between plants, and they can persist in the soil for a long time.
Why risk your precious plants’ health with all this junk in the soil, when you can just get rid of them by sterilizing the potting soil before you plant?
Weeds are less likely
Weeds steal all the nutrition from your soil, leaving plants lacking nutrients.
Sterilizing your soil greatly reduces the potential for weeds to germinate in that soil, maximizing the soil nutrients required for healthy plant growth.
Other advantages include:
- Produces quality produce.
- Improved crop yields dramatically.
- Used soil nutrients blocked from plant uptake often are opened to be used by plants.
Final Thoughts
Soil sterilization is a big deal when it comes to gardening. It will provide your plants with a fresh and clean growth medium.
You are going to eliminate not only weed seeds, but also pathogens and harmful microorganisms.