If you buy bagged potting soil or potting mixes regularly, then you probably fear the dreaded gnat. Fungus gnats lay eggs in bagged soils and then emerge later to both annoy you 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 an essential step in keeping your plants gnat and disease-free and considered vital for successful plant growth and health. Sterilization gives your plants a clean slate and allows you to better control their growing environment.

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 Soil?

There are multiple ways to sterilize soil and keep plant diseases out. They can either be by using chemicals or heat. Depending on the amount of soil you want to sterilize, different methods will work. For example, for small amounts of soil, you can chemical or heat methods. However, for large amounts of soil, you should use solarization.

The first step for a sterilized soil is thus picking a method that you are comfortable with and which you have the materials to do. Ideally this should be done before your fertilize your soil. If you are growing vegetables, then here you can find the best organic fertilizers for vegetables.

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

To give you a range of options, in this article, we will be looking at 6 individual methods for sterilizing soil, which includes 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 make an informed decision about what method is best for you to ensure garden plant growth.

How To Sterilize 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 the use of chemicals that produce gas. When the chemical is applied, the gas it gives off 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 choosing to sterilize with chemicals for gardening purposes. Some gardeners prefer not to use chemicals for soil sterilization to keep things organic.

However, when attempting to sterilize fields or large amounts of soil, it can be the only practical option.

There are other factors to consider as well. Chemical sterilization is not recommended for use in greenhouses as the fumes it gives off can hurt other plants.

Also, different chemicals are toxic to different things, so chemical sterilization will not necessarily make your soil completely sterile.

It is absolutely essential for all of the chemical sterilization processes that you follow the manufacturer's instructions when handling these chemicals.

These chemicals are used because they are toxic, and they are toxic to both animals and plants and the microorganisms they are meant to kill. Always follow safety procedures and instructions carefully!

Here is a breakdown of the pros and cons of choosing to go with a chemical sterilization method.


  • Doable on fields and outdoor gardens
  • Generally less effort


  • Potentially dangerous
  • Can harm other plants
  • Does not necessarily kill everything

A lot of the things that impact sterilizing your soil with chemicals depends on what chemical you use. Let’s look at what the process looks like 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, there is one chemical method that is easy to get the materials for. This can be an excellent method for those new to sterilization because it does not require technical know-how.

Remember that although hydrogen peroxide is not as dangerous as some chemicals, it still needs to be handled with care. Hydrogen peroxide can harm your skin and needs to be diluted to be used safely on 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. 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. 2
    Spray the water mixture onto the soil.

That’s it! You’re done! This 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 in a 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 it can be applied with water. Formalin is an effective sterilization technique against fungus, but it does not do much to deter pests such as gnats.

Sterilizing your soil with formalin is a reasonably straightforward process. You should use formalin which is a solution containing water and formaldehyde. Here are the basic steps.

  1. 1
    Dilute the formalin with water. The ratio should be 1 part formalin 38% — 40%  to 49 parts water.
  2. 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. 3
    You will need to wait for approximately 20 to 40 days for the soil to be fumigated and safe to use.

When sterilizing with this method, you will need to wait until temperatures are higher. The formalin will not fumigate, creating the required gas, in low temperatures.

3. Sterilizing Soil with Other Chemicals

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

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

Test Soil Before Using

The chemicals used to sterilize soil can also be harmful to your plants. If done properly, the chemicals should not continue to 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, then 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, then your soil is safe to use.

How to Sterilize Soil with Heat

The more readily available method for soil sterilization for home gardeners is with heat as you can use simple household items. All of the 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. 

How Much Heat and For How Long?

The biggest questions when it comes to soil sterilization with heat are how hot it needs to get and how long it needs to 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 to 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, and lower will not provide enough heat to kill the various bugs, insects, microorganisms, and diseases infesting your soil. 

Pros and Cons

Just as with chemical soil sterilization, there are various advantages and disadvantages to potting soil mix or soil sterilization with heat. Many people find sterilization with heat appealing because it does not involve adding chemicals to the soil. However, it also requires a lot of effort and time to sterilize with heat.

Sterilizing with heat is particularly difficult the larger the batch of soil. Sterilizing outdoor garden soil with heat is impossible in many cases, although there is one method using solar energy that can work.

Here is a general breakdown of the pros and cons of using heat to sterilize potting mix or soil.


  •  Chemical-free
  • Kills pretty much everything


  • Time-consuming and requires a lot of effort
  • Not suitable for outdoor gardening

How To Sterilize Soil in a Microwave

The fastest way to sterilize soil with heat is to use a microwave that effectively kills gnats.
With this method, you 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 in a microwave.

  1. 1
    Fill either a container or a thick plastic bag with a couple of pounds of moist soil mix or potting soil.
  2. 2
    Put the lid on the container and 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. 3
    Heat the soil in the microwave for around 90 to 150 seconds.
  4. 4
    Using a thermometer, check the soil to see if it has reached the desired soil temperature of 180 to 200 degrees Fahrenheit.
  5. 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. 6
    Allow the soil to cool down naturally to room temperature before removing it from the bag or container.

Sterilizing in the microwave is fairly quick, but it can become very time-consuming depending on how much soil you need to sterilize. You will naturally be limited by the size of your microwave.

Thus, this is a great 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 bigger projects.

How To Sterilize Soil in an Oven

Sterilizing potting soil at home 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 meat thermometer. Here are the steps.

  1. 1
    Preheat your oven to 180 degrees Fahrenheit.
  2. 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. 3
    Wet the soil until it is moist enough to stay in a ball when rolled in your hands. Break up any clumps.
  4. 4
    Cover the roasting pan / baking sheet tightly with aluminum foil. Poke a hole in the top for the meat thermometer.
  5. 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. 6
    Let the soil heat in the oven for at least 30 minutes. To keep the soil at the right temperature during this entire time, you may need to adjust the temperature gauge or open the door of the oven at times.
  7. 7
    Remove your soil from the oven. Let the soil to cool to room temperature before using.

Sterilizing soil in the oven 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, so it helps to 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.

How to Sterilize Potting Soil with Steam

Steam is a popular technique for cleaning because it kills germs, pests and diseases without the use of chemicals. Steam is so powerful that it can not only clean your couch but also sterilize your soil.

You can sterilize soil with steam with a variety of different kitchen tools, including a pressure cooker method, steamer, or plain soup pot. Like the 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. 1
    Place a few cups of water in a soup pot or pressure cooker.
  2. 2
    Put the soil in containers and cover 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. 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. 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. 5
    Turn off the heat of the pressure cooker and allow the soil and water to cool before removing. If you do not plan to use the soil immediately, you should keep the foil on until you are ready.

Steaming is a highly effective way for sterilizing. However, hot steam and pressure can also be dangerous. If you are using a pressure cooker, follow all instructions for proper use and always allow things to cool completely before opening or handling anything.

How To Sterilize Garden Soil in the Sun

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

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

  1. 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. 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. 3
    Wet the soil so that it is moist soil, 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. 4
    Wait several weeks to 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 soil in plastic bags and leave them in the sun for the same sterilization process on a smaller scale.

You should use transparent plastic sheets that are not too thick for the best results as they will heat faster. However, do not use extremely thin sheets as they are prone to rips that will break your seal.

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


Sterilizing your soil gives your plant the best start possible by ensuring your plant does not have any fungi, diseases, bacteria, or pests out to get them and saves you a big problem. There are a lot of different ways to sterilize your soil. For small batches, sterilizing with heat in your kitchen is both effective and safe. Still, for large amounts and an outdoor garden, you will either need to use chemicals or the power of the sun.

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