How to Sterilize Soil

Sterilizing plant-soil ensures it’s free of bacteria, fungi, viruses, and pests that can spread from plant to plant and decimate your indoor plants. If you’re not sure how to sterilize your potting soil or aware of the best methods I’d recommend these 7 ways to sterilize soil.

Here are 7 ways to sterilize soil:

  • Boiling water
  • Hydrogen peroxide
  • Sunlight
  • Microwave
  • Oven
  • Steam
  • Chemicals

Yes, you really can use natural methods such as sunlight and steam for soil sterilization. In this informative guide, I’ll explain how the above sterilization options work so you can disinfect your potting soil successfully. I’ll even tell you what to do if you have large quantities of soil you want to sterilize, so keep reading!

Best Methods to Sterilize Soil

How to Sterilize Soil With Boiling Water

The first soil sterilization method involves nothing more than some boiling water. 

This is one of the easiest ways to sterilize potting mix, but it can be among the riskiest since you’re working with high-temperature water. Make sure you take your time so you don’t get burned!

Here’s how to sterilize soil with boiling water.

Transfer the potting soil to a bowl. Then, in a pot, simply heat the water until it begins to boil. The amount of water you use should be equivalent to the quantity of potting soil.

In other words, if you have only a moderate amount of soil, don’t fill the pot all the way with water. For larger quantities of soil, you’ll need more water.

When the water comes to a boil, turn off the stovetop burner and carefully transport the pot to the bowl of soil. Slowly tip the boiled water into the bowl. The soil should be moist but not saturated if you used the right amount of water.

Water boils at approximately 212 degrees Fahrenheit. At such a high temperature, if your plant’s soil had pest infestations, both the insect eggs and the adults will die. 

The key is to work the water into the soil a bit. You can use a spoon for this. For any instrument that goes into the soil, please disinfect it with isopropyl alcohol or even bleach when you’re done. 

How to Sterilize Soil With Hydrogen Peroxide

Hydrogen peroxide is an antiseptic, bleaching agent, and oxidizer. You probably have some in your bathroom pantry right now. 

Well, it’s time to find your bottle, as hydrogen peroxide is an excellent soil sterilizer.

You can use hydrogen peroxide if your houseplant has developed signs of root rot or if it has another fungal infection. Critters such as aphids also will not stick around once you deposit hydrogen peroxide in the plant soil. 

First, before we go any further, check the label of your bottle of hydrogen peroxide. If its concentration is higher than three percent, you shouldn’t use it for soil disinfecting. It’s too strong. 

Is your hydrogen peroxide safe to use? Then it’s time to dilute it with water. Here’s how much three-percent hydrogen peroxide you should use per quantity of water:

  • 1 cup of water: 1 ½ teaspoons of hydrogen peroxide
  • 1 quart of water: 2 tablespoons of hydrogen peroxide
  • 1 gallon of water: ½ cup of hydrogen peroxide
  • 5 gallons of water: 2 ½ cups of hydrogen peroxide
  • 10 gallons of water: 5 cups of hydrogen peroxide

Pour the two ingredients into a bowl or container, then wet the soil with the mixture. To prevent premature evaporation, sterilize plant soil with hydrogen peroxide later in the day when the sun has begun going down. 

Once you pour the water and hydrogen peroxide into the soil, walk away and leave your plant until morning. The mixture should have evaporated by then and your soil will be far cleaner. 

How to Sterilize Soil In the Sun

Here is what I think is probably the simplest and cheapest soil sterilization option you have: sunlight. 

Unlike the boiling water method, you can’t get burned or spill water on the floor. You don’t need to run to the store and buy something like you do for the hydrogen peroxide method. Sunlight is free and (hopefully) infinite.

So how do you harness the power of the sun to sterilize your potting soil? 

Take your soil and cover it in thin plastic such as plastic wrap or plastic zippy bags. If you only have a little bit of soil to sterilize, I’d suggest dividing it and filling zippy bags with the soil. 

If you have a lot more soil than that to sterilize, then you can use industrial plastic sheets. Put the soil on one sheet and then cover it with another. 

What happens here is solarization, which is a solar-powered means of killing soilborne pathogens and pests. Not all pathogens might necessarily die, but those that don’t will be a lot weaker.

The reason you need the plastic is that it makes the layers like a little greenhouse. They heat up to a high degree. 

You can sterilize soil with sunlight indoors by placing the soil in front of a large window. You can also do this outdoors on your balcony or patio, but you need a clear day with low winds. 

How to Sterilize Soil In a Microwave

Although I don’t recommend putting non-food items in the microwave any other time, soil can safely go in the microwave.

No, you’re not flinging dirt in your micro, so don’t worry about that. Put the soil in microwavable containers instead. The soil should be moistened with water before you microwave it to prevent it from drying out too much.

Don’t use foil wrap in the microwave (it’s a fire risk!) and please puncture a few holes in the container lid to allow steam to release. 

Depending on the size of your microwave, you can fit one container at a time or several. 

On its highest power setting, run the microwave in 90-second increments. When the soil has adequately heated up, let the containers come down to room temp and then take them out of the microwave. Tape the vent holes and store the soil containers if you don’t use them right away. 

How to Sterilize Soil In the Oven

If you’re not quite comfortable with the idea of stuffing containers of soil in your microwave, your oven is another viable option for disinfecting potting soil. 

This time, you need an oven-safe container or sheet. A metal baking pan suffices, as does a glass baking dish. Line the oven-safe pan or dish with foil and then fill it with soil. 

You’ll need a candy or meat thermometer to test the soil temperature, which should be between 180 and 200 degrees to properly sterilize the soil when using an oven.  

Turn your oven timer on for 30 minutes and put the soil in. Don’t watch the timer though, but rather, the candy or meat thermometer. As soon as that hits 180, it’s time to take out the soil, even if it hasn’t been 30 minutes. 

How to Sterilize Soil With Steam 

If you want guaranteed clean soil for that new pothos or philodendron you just brought home, steam-cleaning the soil is another way to go. This method will again involve boiling water, so be cautious as you proceed.

You should have a steamer pot for this. This cookware includes two pots. The smaller one has bottom perforations so it can go inside a larger pot. 

If you need to sterilize your soil ASAP but you don’t have a steamer pot, then use a soup pot and put a wire cooking rack underneath. 

Pour the potting soil in the soup pot or the smaller of the two pots of the steamer pot. The max depth of the soil should be four inches for your safety. Avoid pressing the soil down to make it fit or the heat won’t be able to get through.

Lock the smaller pot in the larger one or position the soup pot on the wire rack. Begin boiling water in the larger pot, putting a lid on the pot. The water should be left to boil for 30 minutes.

As this happens, the steam produced from the heat of the boiling water will penetrate through the soil, sterilizing it.

After the half-hour elapses, turn off the heat, let both pots cool, and then feel free to use your fresh, clean soil.  

How to Sterilize Soil With Chemicals

Few houseplants like chemicals outside of fertilizer; even too much fertilizer can be deleterious to plant health. That’s why I recommend resorting to chemical treatments only if you’ve exhausted all your other means.

Formalin is one such chemical you can rely on. This is a combination of formaldehyde and water. Yes, formaldehyde is the ingredient used to embalm dead people. It’s in a lot of everyday products though, from shampoo to nail polish.

To use formalin to sterilize potting soil, make sure it’s adequately diluted with water. For every one part formalin, you need 49 parts water. Then dump the mixture in the soil, allowing it to soak in. 

It can take between 20 and 40 days for the formaldehyde to fully fumigate and the soil to be safe to use. 

I would recommend treating potting soil with formalin in an isolated area. It’s not a bad idea to wear a face mask and other safety equipment. Formaldehyde might be a carcinogen. 

How to Sterilize Large Amounts of Soil 

Here are two of the best ways to sterilize large amounts of soil:

  1. Lay out the soil you’d like to sterilize on a large plastic tarp and heat it in the sun. I’d suggest doing this outside so you can spread the soil out as thin as possible on the tarp. This way you can expose as much of the soil to the heat & sunlight at the same time. 
  2. Pouring boiling water on the soil. While this method takes a lot of boiling water it allows you to sterilize alot of soil at once.

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