6 Ways How To Sterilize Potting Soil

Fertile soil in the baking tin at the top of the oven.

Soil sterilization is an inexpensive technique that will help you eliminate pathogens and control weed-growth in your garden. Most expert gardeners practice soil sterilization, whether it be small or large scale gardening.

But, plant disease like damping off is the gardener’s number one problem, when it comes to compost-enriched soil.

Soil sterilization is another option you can try if you wish to deal with fungus gnats, larvae, bacteria, pests, viruses and damping-off in your soil.

This method is effective through both physical and chemical means. Physical measures include steaming and solar energy, while chemical control includes herbicides and fungicides.

Sterilized soil is often used in the greenhouse operations, specialty crops, production of high-value crops, and weed control.

The use of the mushroom composting approach is to sterilize organic materials, but not for soil sterilization only.

Soil by nature contains harmful bacteria, fungi, and pathogens. Insects and larvae are also a problem to  gardeners  that can kill vulnerable plants when neglected.

Store-bought soil is also susceptible to contamination due to bag punctures, mishandling, and many other more reasons.

This is why sterilizing potting soil is essential when executing gardening tasks like sowing seeds, transplanting seedling, and cuttings.

Sterilizing potting soil is simple and can provide ample benefits to the plants’ overall health. It also helps prepare the few household items needed along with the various soil sterilization. The process is efficient and will only take 45 minutes.


Benefits of Soil Sterilization

  • It generates ideal conditions for plant growth and development in the soil.
  • It helps the release of the nutrient like nitrate to encourage the plant growth potential.
  • Less harmful compared to chemical applications.
  • Soil sterilization promotes high-quality produce.
  • It will lessen the population of pathogens that is harmful to the plants.
  • It helps control the spread of disease in the soil.
  • Inexpensive
  • Non-toxic
  • Quick cycle time
  • Reduce manual labor

6 Methods On How To Sterilize Soil At Home


The  Oven Method

Note: Soil sterilization using the oven method is ideal for small batches.

You’ll need:

  • Fertile soil
  • Oven
  • Roasting Pan/ Baking Sheet
  • Mixing Spoon
  • Meat thermometer
  • Aluminum Foil
  • Water

Caution: Sterilizing potting soil can release a strong, unpleasant earthy odor when you do it indoors. Try to open windows and turn on the ceiling fan to allow sufficient ventilation before the sterilization process begins.

Prep For Soil Sterilization

The right consistency

  • The fertile soil should be moist enough to form into a ball when you pack in between your hands.
  • In addition to that, it should easily crumble when you remove the pressure.
  • If your potting media does not contain the right consistency, gradually add water to the soil and thoroughly mix until you achieve the right consistency.

Fill the soil into the pan

  • Fill the soil and distribute evenly into the roasting pan, and make sure to break any clumps if you find one.
  • Do not fill soil up to the edge of the pan, instead keeping it under four inches deep is ideal.
  • Cover the pan tightly with aluminum foil to prevent moisture loss and soil burning.
  • Create a hole in the middle of the aluminum foil that is big enough to fit the meat thermometer.
To Sterilize The Soil

The process

  • Preheat the oven to a low temperature and carefully place the tray of soil inside.
  • Ensure to keep the oven temperature no more than 180 degrees Fahrenheit when you start sterilizing as a temperature of more than 180 degrees Fahrenheit can cause burning.
  • Burning can generate chemical toxins to the soil and can harm the plants.

The time duration

  • Keep the soil in the oven for about 30 minutes, maintaining the temperature at 180 degrees Fahrenheit.
  • Monitor the temperature to ensure the soil sustains the sufficient warmth it requires.
  • Address the temperature fluctuation promptly to prevent the burning of the soil and achieve the output you desire.
Cooling And Using The Soil

Cooling

  • Remove the soil from the oven after baking for 30 minutes.
  • Let it sit until it reaches room temperature.
  • Carefully lift the foil from the tray to allow the remaining heat to escape.
  • Be careful not to burn your skin because the steam will be extremely hot.

Using the soil

  • Once the sterilized soil reaches room temperature, it is now safe to remove from the tray.
  • Begin to sow about four times deeper the size of the seed.
  • Pour water onto the sterilized soil using the sprinkler to avoid possible over-watering.
  • Protect the seeds sown into the container by covering with plastic wraps, and then place in the shade with room temperature.
  • Avoid placing the container under direct sunlight until germination.
  • Once the germination begins, remove the plastic wraps and gradually introduce light for several different periods of the day.

Important Tip: 

It is essential to disinfect the containers before filling the newly sterilized potting soil. This is to prevent soil-borne diseases like fungi, and to prevent several insects that can thrive in small amounts of soil and debris in previously used and uncleaned containers.


The Microwave Method

Note: Soil sterilization using the microwave method is ideal for small batches.

Another option to sterilize the soil is through the use of a microwave. The process is almost similar to the oven method except for the use of a plastic bag.

  • To use the microwave, fill the thick plastic bag up with two pounds of moist soil.
  • Leave the top open and carefully place it in the center of the microwave.
  • Heat the microwave to two and a half minutes in full power. (650 watts)
  • Check the temperature of the soil using the meat thermometer to ensure you keep the soil at 180°F to 200°F.
  • Close the plastic bag carefully after the time duration, and put it in the cooler to retain the heat.
  • Allow the soil to cool down before using it.

The Stove-Top Method

Note: Soil sterilization using the stove-top method is ideal for small batches only.

  • Pour 3-4 cups of water in the steamer pot.
  • The steamer should be the type that has a perforated container suspended inside a larger pot.
  • If a steamer pot is not available, you can use the standard soup pot by placing a wire rack at the bottom to allow the perforated container to be placed just above the water level.
  • Pour the soil into the perforated container and keep it no deeper than 4 inches.
  • Avoid pressing it down to allow sufficient heat penetration during the steaming process.
  • Place the soil container in the steamer pot or set it on the wire rack inside of your soup pot if you’re using the alternative method.
  • Close the lid of the pot and place it on top of the stove.
  • Bring the water to a boil, and once the water begins to boil, set the timer for 30 minutes.
  • Allow the heat and steam inside the pot to completely sterilize the soil container.
  • Once 30 minutes of boiling is over, turn the stove off and allow the pot to cool down completely before removing the soil container.
  • You can also try directly filling the pot with soil, saturate it with water and place it on the top of the stove. Then follow a similar fundamental procedure to complete the process.

The Pressure Cooker Method

Note: This method is ideal for small batches only.

  • Add 2 cups of water into the bottom of your pressure cooker.
  • Pour 4 cups of soil into the shallow heat-safe container and set it on the rack inside of your pressure cooker.
  • Close the lid of the pressure cooker and open the lids steam valve.
  • Turn on the pressure cooker and let the water boil. Close the valve once the steam starts to escape.
  • Set the timer for 15 minutes to allow the pressure to build-up inside the cooker, which sterilizes the soil.
  • You can turn the pressure cooker off and allow the soil to cool down completely before using it.

The Solarization Method

The soil solarization is an eco-friendly method that uses the sun’s intense power to eliminate fungi, bacteria, insects, and weeds in the soil.

The process involves covering the ground with a transparent polyethylene cover to trap solar energy.

The sun heightens the soil temperature enough to kill pathogens, making it free from contamination.

Note: Soil sterilization using the solarization method is ideal for large batches.

You’ll need:

  • Clear plastic sheets
  • Sprinkler/ drip irrigation hose
  • Water
  • Rocks

Procedure:

  • Choose a spot in your backyard that receives at least 8 hours of direct sunlight during the day.
  • Prepare the area by removing any existing debris.
  • Lay the clear plastic sheeting and fill with soil that is about four inches deep.
  • Water the soil to keep it moist but remember not to over water.
  • Cover the soil with other plastic sheets and secure the plastic edges by tucking rocks in the border.
  • Leave the soil to bake under the sun for at least four weeks in the hot sunny weather.
  • It usually takes 6-8 weeks for the dirt to be ready to use, particularly in colder weather. Allow the soil to cool down properly before using it.

Solarization works best on soil like clay and loam or a mixture of the two. These types of soil hold enough moisture to produce adequate steam every day,  compared to the loose type.

Steams are the primary components to eliminate bacteria, weed seeds, and the insects’ eggs in the soil.

However, this method is less effective in sandy soil as it produces less steam due to its fast-draining ability. Laying a drip irrigation hose under the plastic cover to add water regularly will allow you to maximize the benefits of Solarization in sandy soils.

Water beads usually appear on the under-surface of the plastic early each morning. It will gradually disappear by noon as the water slowly turns to steam.

It should be the daily cycle as beads are getting fewer and fewer each morning.  When this happens, you can turn on the irrigation hoses and replenish the water in the soil.

You can solarize any area as long as the plastic covering is large enough to cover the target area.

To get the most benefits of solarization, you have to provide weed-free soil, plenty of water, and transparent plastic sheets where you can bury edges that will last up to four weeks under the full sun.


Hydrogen Peroxide Method

If you are new to gardening and want to sterilize your potting soil, but don’t have much technical knowledge about soil sterilization, then the hydrogen peroxide method is appropriate for you.

This method is simple and does not require many skills. Practicing at home can be very beneficial.

The use of hydrogen peroxide to sterilize soil will only take two simple steps.

Step  #1 – Provide a bucket big enough to mix Hydrogen peroxide with water. It should be a large container as the soil needs to be thoroughly sprayed.

Step #2 Spray the mixed solution thoroughly using the sprayer device into the soil. There is a readily available sprayer device in many sizes at all garden centers near you.

The sprayer that can accommodate 20 gallons of water is the most popular. However, you also have to determine how much and what type of hydrogen peroxide you need to use that is ideal for 20 gallons of water.

Hydrogen peroxide shouldn’t be applied to the soil directly. This eco-friendly chemical is a highly concentrated and aggressive oxidizer that can destroy several materials, and ruin the human skin.

Therefore, it is always best to follow the right percentage of hydrogen peroxide and water mixture.

Hydrogen peroxide that is 3% and 35% are the best type to use when it comes to sterilizing your potting mix.

Here, we’ve provided an informative chart to help guide you and show you how much hydrogen peroxide is sufficient enough to sterilize your soil correctly.

VOLUME OF WATER 3% OF HYDROGEN PEROXIDE 35%  OF HYDROGEN PEROXIDE
1 cup 1 and ½ teaspoons 7 to 10 drops
1 quart 2 tablespoons ½ teaspoon
1-gallon ½ cup 2 teaspoons
5 gallons 2 and ½ cups 3 tablespoons plus one  teaspoon
10 gallons 5 cups 6 tablespoons plus two teaspoons
20 gallons 10 cups ¾ cup plus one tablespoon plus one teaspoon
Bathtub (approximately 25 to 35 gallons) 12 to 17 cups 1 to 1.5 cups.

Note: Soil sterilization using the hydrogen peroxide method is ideal for both large and small batches.


Final Thoughts On How To Sterilize Potting Soil

If you prefer to reuse your soil continuously rather than replacing it, it is most beneficial for you to sterilize it. As we already know, sterilized soil provides clean and contaminant-free growing media for your plants.

Soil sterilization is especially helpful in conditions where you need to use the soil numerous times to germinate seeds, transplanting, and propagate cuttings.

However, there are a few factors you must keep in mind to make your sterilization process hassle-free if you opt for any of the following sterilization methods we’ve shared above.

  • Make sure to disinfect potting containers before reusing to prevent harmful microbes hiding in the old used receptacles.
  • It is always in your best interest to sterilize your soil right before using it to avoid the risks of contamination in any form. 
  • Avoid using soil taken directly from your garden as a potting medium.
  • Break hard and compacted dirt to make it lighter and more beneficial by mixing a small amounts of loose ingredients. 
  • It does not matter much if you reuse potting soil with mature plants. However, it is always best to require a sterilized potting mix for new seedling and bedding plants to have the best chance to thrive.

I hope you have found this article informative and if you have any questions or concerns, please leave a comment below and I will be more than happy to respond.

About Benita Abucejo

Hi there! My name is Benita Abucejo. What can I say? I truly love spending my days in the outdoors, specifically in the garden. Gardening has always been a strong passion of mine since I was a little girl. It has brought me so much joy and happiness that it is definitely safe to say that I will be a gardener for life. For a period of time, I was able to work with people who are into home gardening and I found it to be quite beneficial to my physical health, as well as my mental well-being. Here at Seasonal Preferences, I am going to share with you my experience and ideas so that you can fulfill yourself with the same satisfaction and happiness. Of course, if you have ideas, I would love to hear those as well! Being creative in the garden can really be quite fascinating so let's share our experiences and be the best gardeners we can be. With that being said, thank you for dropping by and please leave me a comment on one of my posts if you would like to get in touch!

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