6 Easy Ways for How to Sterilize Soil Indoor and Eliminate Diseases and Bugs

If you buy bagged potting soil or potting mixes regularly, you probably fear the dreaded gnat. Fungus gnats lay eggs in bagged soils and then emerge later to annoy and kill your plants.

Besides gnat eggs, bagged soils can also harbor harmful microbes, fungi, and other nasty creatures. Unfortunately, you cannot see these creatures, so how do you get rid of them?

Sterilizing your soil before planting is essential in keeping your indoor plants gnat and disease-free and is considered vital for thriving plant growth and health. Sterilization gives your plants a clean slate and allows you to control their growing environment better.

This article will outline the steps a garden should follow to safely and effectively sterilize soil before planting. Keep reading for everything you need to know!

How to sterilize potting soil?

There are multiple soil sterilization methods that keep plant diseases out. They can either be by using chemicals or heat. Different methods will work depending on how much soil you want to sterilize.

For example, you can use chemical or heat methods for small amounts of soil. However, for large amounts of soil, you should use solarization.

The first step for sterilizing soil is picking a method you are comfortable with and having the materials for.

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Each way to sterilize potting soil has its own advantages and disadvantages. There are many more specific methods under these broad categories of chemical and heat.

To give you a range of options, in this article, we will be looking at 6 methods of soil sterilization, which include sterilizing with

1. Hydrogen Peroxide
2. Formaldehyde
3. Microwave
4. Oven
5. Steam
6. Solarization

Knowing how the sterilization process works for each of these will allow you to decide the best method to ensure healthy plant growth.

How To Sterilize Potting Soil with Chemicals

Soil sterilization with chemicals is the process that many commercial companies use because it is far easier and more economical to use chemicals when sterilizing on a large scale.

Although different chemicals work in different ways, the basic principle behind chemical sterilization is using chemicals that produce gas. When the chemical is applied, its gas can penetrate the soil and kill the targeted pests, bacteria, and insects to secure plant health.

Pros and Cons

There are several advantages and disadvantages to sterilizing with chemicals for gardening purposes.

It is essential for all chemical sterilization processes that you follow the manufacturer’s instructions when handling these chemicals. These chemicals are used because they are toxic and harmful to animals, plants, and the microorganisms they are meant to kill. Always follow safety procedures and instructions carefully!

Test Soil Before Using

The chemicals used to sterilize soil can also be harmful to your plants. If done correctly, the chemicals should not linger in the soil after the sterilization process and waiting period. However, if you are unsure and do not want to risk killing your plants, the best method is to test your soil before use.

A simple way to test your soil is to fill a jar with the soil and scatter cress seeds. If the seeds germinate and sprout, your soil is safe to use.

Let’s look at the process for some of the major chemicals used to sterilize the soil.

1. Sterilizing Soil with Hydrogen Peroxide

For home gardeners who want to sterilize the soil, using the Hydrogen Peroxide chemical method is easy to get the materials for. This can be an excellent method for beginners because it does not require technical know-how.

Although hydrogen peroxide is not as dangerous as some chemicals, it must be handled carefully. It can harm your skin and must be diluted to be safely used in your soil.

There are several different solutions of hydrogen peroxide available for purchase. The higher the concentration of hydrogen peroxide, the less you will need. Both 3% and 35% solutions work well.

Here are the steps for sterilizing with hydrogen peroxide:

1. Add hydrogen peroxide to water. The exact amount of hydrogen peroxide you need will depend on the amount of water and the solution’s concentration. You can look online for recommendations.
2. Spray the water mixture onto the soil.

That’s it! You’re done!

The hydrogen peroxide method definitely gets points for simplicity. It can also be used for both small and large jobs.

You can prepare the mixture in a spray bottle or large many gallon sprayers from your local gardening center.

2. Sterilizing Soil with Formalin

Formalin is a mixture of water and formaldehyde, commonly used to sterilize outdoor soil as you can apply it with water. It is an effective sterilization technique against fungus but does not deter pests such as gnats.

Here are the basic steps for Sterilizing Soil with Formalin:

1. Dilute the formalin with water. The ratio should be 1 part formalin 38% — 40%  to 49 parts water.
2. Thoroughly soak the soil you want to sterilize with the diluted formalin. It needs to soak through the soil to work properly. The recommended number is 5 gallons per square yard.
3. You will need to wait approximately 20 to 40 days for the soil to be fumigated and safe to use.

When sterilizing with this method, you must wait until higher temperatures. In low temperatures, the formalin will not fumigate, creating the required gas.

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3. Sterilizing Potting Soil with Other Chemicals

There are a variety of other chemicals that you can use to sterilize the soil. Still, most of them are not available for purchase by the average consumer.

If you find another chemical soil sterilizer available, carefully follow the instructions. Never try to use a chemical that does not have clear guidelines and is approved for soil sterilization.

How to Sterilize Soil with Heat

The more readily available method for soil sterilization for home gardeners is heat, as you can use simple household items. All heat sterilization methods rely on the fact that sufficient heat levels can kill just about anything. The difference comes with how you choose to apply the needed heat to the soil. 

Pros and Cons

As with any method, there are advantages and disadvantages when using heat.

How Much Heat and For How Long?

The biggest questions about soil sterilization with heat are:

• How hot does it need to get?
• How long must it stay at that temperature to effectively kill off any critters hiding in the soil?

When using heat sterilization, you generally need to get the soil between 180 and 200 degrees Fahrenheit and leave it there for around 30 to 45 minutes.

If you heat the soil to higher temperatures than this, you risk burning it. Lower temperatures will not provide enough heat to kill the various bugs, insects, microorganisms, and diseases infesting your soil. 

1. How To Sterilize Soil in a Microwave

The fastest way to sterilize soil with heat is using a microwave that kills gnats. This method can sterilize a few pounds of soil in a few minutes. You also do not have to monitor the sterilized soil as carefully as with some other heat methods.

To sterilize potting mix or soil in the microwave, you will need either a microwave-safe bag that can be sealed or a microwave-safe container with a lid. Do not use aluminum foil as a lid!

Here are the steps for sterilizing potting soil in a microwave:

1. Fill either a container or a thick plastic bag with a couple of pounds of moist soil mix or potting soil.
2. Put the lid on the container, poke some ventilation holes, and place them in the microwave. If using a bag, put the bag in the microwave with the top open to allow for ventilation.
3. Heat the soil in the microwave for around 90 to 150 seconds.
4. Using a thermometer, check the soil to see if it has reached the desired soil temperature of 180 to 200 degrees Fahrenheit.
5. Take the soil out of the microwave. If in a bag, seal the bag. If in a container, seal the ventilation holes with tape.
6. Allow the soil to cool down naturally to room temperature before removing it from the bag or container.

Sterilizing in the microwave is pretty quick, but it can become time-consuming depending on how much soil you need to sterilize. The size of your microwave will naturally limit you.

Therefore, this is an excellent method to get sterilized soil for a single bag of soil or enough for a small potting medium. But this is usually not a go-to for gardeners with more significant projects.

2. How To Sterilize Soil in an Oven

Sterilizing potting soil at home by baking it in the oven allows you to do much more at once, but it also takes much longer. Sterilizing soil in the oven requires an oven-safe container, aluminum foil, and a meat thermometer.

Here are the steps for sterilizing soil with an oven:

1. Preheat your oven to 180 degrees Fahrenheit.
2. Fill your roasting pan/baking sheet with soil. It should be around 3 inches deep. Do not fill it more than four inches deep.
3. Wet the soil until it is moist enough to stay in a ball when rolled in your hands. Break up any clumps.
4. Cover the roasting pan/baking sheet tightly with aluminum foil. Poke a hole in the top for the meat thermometer.
5. Put the soil in the oven. You want the soil to reach 180 degrees Fahrenheit. However, the soil must not get hotter than 180 degrees. Temperatures over 180 degrees will cause burning, which creates toxins. Use the meat thermometer to monitor the temperature.
6. Let the soil heat in the oven for at least 30 minutes. To keep the soil at the right temperature this entire time, you may need to adjust the temperature gauge or open the oven door at times.
7. Remove your soil from the oven. Let the soil cool to room temperature before use.

Baking dirt is a popular method also for homemade compost. However, you should be aware that it will fill your kitchen with the smell of cooked soil, which helps keep your kitchen well-ventilated during this process. Because of the time involved and the limited capacity of your oven, this way is best for small or medium batches.

3. How to Sterilize Potting Soil with Steam

Steam is a popular technique for cleaning dirt because it kills germs, pests, and diseases without chemicals. Steam is so powerful that it can clean your couch and sterilize your soil.

You can sterilize soil with steam with various kitchen tools, including a pressure cooker method, steamer, or plain soup pot. Like an oven, sterilizing with steam takes a chunk of time and can only handle a small batch at a time. It is not recommended for sterilizing large amounts of soil.

Here are the basic steps to get sterilized potting soil with steam:

1. Place a few cups of water in a soup pot or pressure cooker.
2. Put the soil in containers and cover it with aluminum foil. Place the soil-filled containers on a rack above the water and within the pot or pressure cooker. The lid should still be able to fit.
3. Allow the water to boil and begin releasing steam. If using a soup pot, place the lid on, leaving enough room for steam to escape and prevent pressure buildup. If using a pressure cooker, close the steam valve when steam begins to escape.
4. For a soup pot, let the water boil for 30 minutes. For a pressure cooker, let it cook at 10 pounds of pressure for 15 to 30 minutes.
5. Turn off the heat of the pressure cooker and allow the soil and the boiling water to cool before removing them. If you do not plan to use the soil immediately, you should keep the foil on until you are ready.

Steaming and boiling water are highly effective ways of sterilizing. However they can also be dangerous. If you are using a pressure cooker, follow all instructions for the proper use and always cool entirely before opening or handling anything.

Here is a great video on another way to sterilize soil with steam:

4. How To Sterilize Garden Soil in the Sun

You may have noticed that the sterilization way involving heat is not great for sterilizing large amounts of soil. They will do you no good if you need to sterilize outdoor garden soil. Does this mean you are stuck with chemicals, then? There is one source of heat that you can easily access when gardening outdoors and use for large quantities: exposure to the sun.

This process uses the sun’s solar energy to produce steam to sterilize your soil. It is a longer sterilization method, typically taking 2 to 6 weeks to complete. You can use this method for any size of soil batches, from bags of soil to entire gardens.

Here are the steps:

1. Find an area of your garden that gets at least 8 hours of direct sunlight a day. If you are sterilizing an outdoor garden, your area would simply be the garden itself.
2. Place a large plastic sheet in your designated area. Place your soil on the sheet and spread it out evenly. If you are sterilizing an entire garden, you can skip this step.
3. Wet the soil so it is moist, and cover it with a layer of plastic. Use rocks to hold the plastic down and create a seal. In a garden, you can also bury the corners of the plastic to create a better seal.
4. Wait several weeks for the solarization process to end and sterilize your soil. The hotter the daily temperatures are, the less time the solarization process will take.

You can also put a small amount of moist soil in plastic bags and leave them in the sun for the same sterilization process on a smaller scale.

It would help if you used transparent plastic sheets that are not too thick for the best results to heat faster. However, do not use fragile sheets as they are prone to rips that break your seal.

Solarization sterilization works best on clay or loam soil. Sandy soil does not retain moisture and thus will not produce as much steam.

Conclusion

Sterilizing your soil gives your plant the best start possible by ensuring it has no fungi, diseases, bacteria, or pests. There are a lot of different ways to sterilize your soil. Sterilizing with heat in your kitchen is effective and safe for small batches. Still, for large amounts and an outdoor garden, you will need to use chemicals or the sun’s power.

Whatever method you choose to sterilize your soil, do your research and stay safe! With some experience, you will soon be a sterilizing pro, and your plants will thank you.