How to Sterilize Soil: The ultimate guide for every treatment

Soil is teeming with life.  Unfortunately, not all that life is what you want growing in your garden beds. Sometimes your soil is full of nematodes, fungus, insects, and weed seeds. These pests can damage the plants you are working so hard to grow. 

Luckily for you, we’ve created the ultimate guide to sterilizing your soil. 

We are passionate about soil health. So, we’ll explain not just how to sterilize your soil but, more importantly, when to sterilize it. That way, your garden is as healthy as you can make it. 

Chemical Sterilization Methods

work with soil
Photo credit: Depositphotos

There are two primary means of sterilizing your soil, chemically and organically. Let’s start by talking about chemical sterilization methods and move on later to more organic options.

Pesticides and Herbicides

Commercial pesticides and herbicides can be applied to your soil to kill pests and weeds. By their very nature, these chemicals are toxic. While they are designed to kill specific weeds and pests, they can have a negative impact on the environment around your home. 
Large scale farms tend to rely heavily on chemical sterilization methods. That’s because when you grow just a single crop, or monoculture, you find that crop is at a much greater risk for pests than a more bio-diverse farm (1). Chemical methods are touted as a cheap, quick fix, but the reality is there are more significant problems affecting commercial farms.

Creating a more biodiverse farm can be an effective way of combating pest problems. So, in general, we don’t recommend the use of chemical sterilization methods.

If you choose to go in this direction, keep in mind that specialized safety equipment should be used when applying any kind of chemical product. And it is essential to follow the manufacturer’s directions.

However, there is one chemical method that we can get behind, which leads us to hydrogen peroxide.

Hydrogen Peroxide

We all know hydrogen peroxide. When you were a kid, and you scraped your knee, your mom would pour it over your wound. If you were like me, you’d grit your teeth while it bubbled away. My mom always told me that’s how I would know it was working. But what did working mean? 

Hydrogen peroxide is an antiseptic and is antimicrobial. That means that it kills microbes. So if you are looking for a safe way to get rid of the microbes in your soil, there are worse ways to go. 

While hydrogen peroxide occurs naturally (you can find it in raindrops), most of what you will encounter has been processed in a factory, so we consider it a chemical compound. It is water with an extra oxygen molecule added to it. So rather than H2O, its chemical formula is H2O2. 

How To Sterilize Your Soil With Hydrogen Peroxide

It’s fairly easy to sterilize your soil with hydrogen peroxide. 

What you need

  • 3 % Hydrogen Peroxide
  • 1 gallon of water
  • A large container
  1. First, measure ½ cup of 3% hydrogen peroxide
  2. Add the hydrogen peroxide to 1 gallon of water
  3. Mix well
  4. Add the mixture directly to your soil, make sure you saturate the soil thoroughly
  5. Use all the mixture directly, the solution will not stay potent

Organic Methods

While chemical sterilization methods may seem the cheap and easy way to sterilize your soil, the truth is that using harmful chemicals always comes with a price. That is why we always prefer organic methods when working in our garden. 

The organic methods discussed all involve using heat to kill off fungal spores, nematodes, and harmful microbes. We promised a comprehensive list, and that’s what we are providing- the good, the bad, and the stinky! So, while all these methods will work to sterilize your soil, we can’t say that we recommend them all. 

Microwave

You may have heard you can use your microwave to sterilize your kitchen sponge. But did you know you can use the same technique on your soil? If you are looking to sterilize a small amount of soil, people suggest you can put it in the microwave just like your sponge. 

Microwaves have been shown to kill a variety of pathogens, like bacteria, viruses, and protozoan parasites in sponges. And it is thought that it is the heat rather than the microwave radiation that is effective (2).

“Two minutes of microwaving on full power mode killed or inactivated more than 99 percent of all the living pathogens in the sponges and pads…”

The same idea applies to the dirt from your yard. By microwaving moist dirt, you can use the heat to kill your soil pathogens. 

You want to make sure there isn’t any metal in your soil. Metal and microwaves don’t mix. (Unless you want a lightning show going on in there.) 

How to Sterilize Your Soil In the Microwave

This method works best for creating small amounts of potting soil.

What you need

  • Microwave oven
  • 1 pint of soil
  • Microwave safe container
  1. Make sure your soil is moist (remember it is the heat that kills, so you need plenty of water)
  2. Place one pint of soil in a microwave safe container
  3. Make sure you leave it open for steam to escape
  4. Place the container of soil in the microwave 
  5. Microwave on High for 2 minutes 30 seconds

Check out this video to see more about how to microwave your soil:

Oven

If you are concerned that you may have metal in your soil, or you simply don’t have a microwave, you can use the same theory to cook your soil in your oven. Now using your oven will take significantly longer than your microwave. But you can probably fit more dirt in your oven than in your microwave.

Another thing to consider is that this process is stinky. I mean super duper stinky. So make sure you have plenty of ventilation. 

How to Sterilize Soil In the Oven

What you need

  • Oven trays
  • Aluminum Foil
  • Meat Thermometer
  • Soil
  1. Preheat your oven to 250 degrees Fahrenheit
  2. Place your soil in an oven tray or casserole dish
  3. Cover the dish with aluminum foil
  4. Place in your oven 
  5. Cook for approximately 30 minutes
  6. Check your temperature, you want it to be at least 180 degrees but less than 200 degrees Fahrenheit

Steam

steaming compost
Photo credit: Geograph

If you are looking to sterilize an object, steam can be very effective. It’s how you sterilize jars if you are looking for how to can beets or if you are using jars for fermenting purposes. But steam can sterilize more than just jars.

You can use steam to sterilize your soil either with a pressure cooker or on your stovetop using a steamer or stock part. The process will be similar no matter which method you choose to steam sterilize your soil. Like sterilizing in your oven, the heat from the steam will help kill off all those pesky pests.

The amount of soil you can sterilize will be limited to the size of your pressure cooker or stockpot. Chances are you won’t be able to fit a very large tray in there. So, this method is best for small projects like growing lettuce indoors.

How to Sterilize Soil With Steam

What you need

  • Stockpot with a lid, steamer, or pressure cooker
  • Metal containers for soil that fit your steaming pot
  • A rack if you are using a stockpot
  • Aluminum foil
  • Water
  • Soil

Without a pressure cooker

  1. Place your soil in your containers
  2. Cover with aluminum foil
  3. Add several cups of water to your stockpot
  4. Place a rack or steamer tray in the bottom of the pot
  5. Place your containers with soil on top of the rack
  6. Place the lid on with a bit of room for steam to escape to avoid pressure build up
  7. Bring the water to a boil
  8. Boil for 30 minutes 
  9. Allow to cool 
  10. Keep covered with foil until you are ready to use your soil

With a pressure cooker

  1. Place your soil in your containers
  2. Cover with aluminum foil
  3. Add several cups of water to your pressure cooker
  4. Place a rack or steamer tray in the bottom of the pot
  5. Place your containers with soil on top of the rack
  6. Place the lid on your pressure cooker and open the steam valve 
  7. Turn on your pressure cooker
  8. When steam begins to escape, close the steam valve
  9. Heat at 10 pounds of pressure for 15-30 minutes
  10. Turn off your pressure cooker
  11. Allow the soil to cool 
  12. Keep covered with foil until you are ready to use your soil

Solarization

Solarization of soil
Photo credit: Felix Wong

Rather than using your microwave, oven, or pressure cooker to heat up your dirt, solarization uses the heat from the sun to kill the harmful organisms that may be harming your plants. So, solarization is an excellent method to use if you are looking to sterilizer large areas in your garden.

The idea is to use clear plastic tarps to heat up your soil, ultimately achieving the same goal as the other methods. It just happens more slowly and on a larger scale. The folks at Texas A&M have not only perfected the process but also proven its effectiveness (3). This is the process they recommend.  

How to Sterilize Soil With Solarization

What you need

  • 1 to 4 mil clear painter’s plastic (NOT WHITE OR BLACK)
  • A hose
  • A garden bed
  1. Clear your garden bed
  2. Water the garden bed well with your hose
  3. Cover the area well with clear plastic
  4. Bury the edges of the plastic. This will help keep the heat in
  5. Leave the plastic in place for at least 4 weeks during the hottest month of the year
  6. Remove the plastic

This method works best with clayey soil that will hold the water. WIth sandier soil, you may need to use drip irrigation beneath the plastic to keep your soil moist.  

Hot Compost

soil hot compost
Photo credit: Flickr

Did you know that you can kill most weed seeds, plant diseases, and insects in your compost pile? Well, you can if you heat it up enough. Using the Berkley Method of Rapid Composting, the temperature of your compost should reach 160 degrees Fahrenheit, which will kill the pests but not the beneficial microorganisms (4). 

Now, the Berkley Method is just one way to speed up your compost but is very effective.

Here’s what you need to do:

The Berkely Method of Rapid Compost

What You Need

  • A compost pile 36” X36” X36”
  • Compost Material ½”- 1 ½” in size
  • Compost Material equal parts green and brown
  • Water to keep the pile moist
  • A pitchfork or something to turn the pile
  • A tarp to cover your pile
  • A compost thermometer (optional)
  1. Create your compost pile using the appropriate material
  2. Make sure your pile is damp but not wet (it should feel like a wrung out sponge)
  3. Turn your pile regularly
  4. Within 24-48 hours, the pile should begin to heat up. You can measure this with a compost thermometer. The ideal temperature is 160 degrees Fahrenheit. Temperatures above 180 degrees will kill the beneficial microorganism as well and slow down or stop the composting process
  5. Keeping your pile covered will help keep the heat in
  6. As the composting material begins to breakdown, you will see your pile shrink
  7. The process should be complete within 2-3 weeks 

When Should I Sterilize My Soil?

Now that you know all the ways you can sterilize your soil, let’s talk about when you want to do this. Because let’s be clear- when you are sterilizing your soil, you are killing off all life in your soil, both beneficial and harmful microorganisms.

In general, you don’t want to be growing plants in dead, lifeless soil. However, if you are starting seeds or bringing potted plants into your home, you want to make sure isn’t anything in your soil that could negatively impact your plants. 

In the case of starting seeds, sterile soil can help give them a better chance not only of germination but also in those early days of life. Seedlings are particularly vulnerable to pests and disease.

In the case of indoor potted plants, you want to make sure you aren’t bringing fungus or insects inside your home. There is nothing worse than introducing a pest accidentally and then trying to get rid of it. By sterilizing your soil you can guarantee nothing will be coming inside uninvited.

FAQs

Seedlings grow better in sterile soil, but overall, plants grow more slowly. Seedlings grow better as there are fewer pathogens for them to fight off while they are vulnerable. However, once they are no longer at risk, the lack of beneficial microorganisms in the soil can slow their growth.

You can sterilize soil in as little as 90 seconds in the microwave. Other methods will take longer. And the more soil you wish to sterilize, the longer it can take. For example, if you are sterilizing a large garden area, we recommend using solarization, which can take up to four weeks.

Sterilized soil is not organic if you use chemical pesticides and herbicides to sterilize it. However, other methods of sterilization that do not use chemicals can be considered organic, particularly methods that simply use heat for the sterilization process.

  1. Monocultures in America: A System That Needs More Diversity. Retrieved from: https://blogs.umass.edu/natsci397a-eross/monocultures-in-america-a-system-that-needs-more-diversity/
  2. Researchers: Microwave oven can sterilize sponges, scrub pads. Retrieved from: https://news.ufl.edu/archive/2007/01/researchers-microwave-oven-can-sterilize-sponges-scrub-pads.html
  3. Soil Solarization. Retrieved from: https://agrilifeextension.tamu.edu/library/gardening/soil-solarization/#:~:text=Soil%20solarization%20is%20an%20environmentally,and%20weeds%20in%20the%20soil.&text=The%20sun%20heats%20the%20soil,%2C%20weeds%2C%20and%20weed%20seeds.
  4. The Rapid Composting Method, Retrieved from: https://vric.ucdavis.edu/pdf/compost_rapidcompost.pdf

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