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How to Sterilize Soil with Boiling Water (Plus 3 Alternative Methods to Try)

How to Sterilize Soil with Boiling Water (Plus 3 Alternative Methods to Try)

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If you enjoy gardening, at some point you’re going to want to plant new plants and seeds in soil. The problem is that soil isn’t always ideal for planting. In fact, soil can contain all kinds of bugs and pathogens that can negatively affect the growth of your plant.

You might be thinking that soil is supposed to be dirty, but the truth is that using clean soil is best for your plants.

How to Sterilize Soil with Boiling Water (Plus 3 Alternative Methods to Try)

The Problem with Soil

You could go out and buy new clean potting mix every time you want to plant some seeds or transplants, but this can be a pretty expensive exercise. The problem is that if you just grab some soil from your yard, it’s likely to be full of the following:

  • Fungus Gnats and Nematodes: These common tiny insects love to hide in the soil and cause problems. In fact, they are so common that you’ll likely even find them in fresh potting soil from the store. These insects will live in soil and lay their eggs there too. This can cause issues for plants.
  • Weeds: The seeds from weeds are commonly found in soil. When you plant a new seed, the last thing you want is for weeds to grow alongside it and take out all of the soil nutrients.
  • Fungus: Have you ever wondered why potting soil from the store comes in bags with ventilation holes? This is because soil can easily grow a variety of molds and fungus in humid and moist conditions.
  • Pathogens: There are thousands of different kinds of bacteria and viruses that live in regular soil. Many of them can harm your growing plant. In fact, pathogens like this can remain in the soil for a long time and may even be transferred from one plant to another, unless you eliminate them.

As mentioned earlier, even fresh potting soil can contain these kinds of problems. This is why it’s always best to look for sterilized potting soil.

Of course, the cost can really build up if you need to do a lot of planting. This is why it’s a good idea to learn how to sterilize your own soil at home.

If it’s too late and you already have bugs, check out my guide for dealing with pests on your plants.

How to Sterilize Your Own Potting Soil Using Water

Potting Soil

The best way to kill pathogens and get rid of other soil pests is to sterilize the soil before you plant. There are a number of good ways to do this, but one of the most common is using boiling water.

Whether you’ve just bought yourself some seed starter or potting mix from the store, or you have a handful of soil from your garden, it’s important to sterilize the soil before using it for planting. The easiest way to do this is to use boiled water.

Place the amount of soil that you want to sterilize into a bowl and then pour boiling water over it. Once the soil is moistened, you can then work it around with a spoon or other utensils. The hot water will kill the insects and their eggs in the soil and leave it fresh for planting.

There is also another easy way that you can sterilize the soil and kill pathogens, nematodes, and fungus gnats. By placing the bag of soil in your freezer for a few days, you can interfere with the insect life cycle and kill pathogens.

In fact, some people prefer a double method of freezing the soil first and then using boiling water just to make completely sure that the soil is good to use.

Are There Any Other Sterilization Methods?

There are also other methods of sterilizing soil if you don’t trust the boiling water. Here are some that may work for you:

1 – Using Steam

Like boiling water, this is another effective and easy method of sterilizing soil. Just fill a pan with water and then place the layers of soil to be sterilized in a rack above it. You can then close the lid and bring it to a boil.

Once boiled, allow the steam to escape through a small opening, and then leave the water boiling for 30 minutes. This will be enough to kill any pests in the soil mix.

2 – Using an Oven

Another easy method to sterilize the soil is to use your oven. Just get an oven safe container and layer about four inches of soil into it. Cover this soil layer with foil and place in the oven at between 180 and 200°F.

The only issue with this method is that overcooking the soil can produce toxins that won’t be ideal for planting. To avoid this, use a baking thermometer and place it in the soil.

Once it reaches a temperature of 180°F, you can turn off the oven and remove the soil. Leave it covered with foil until you are ready to use it.

3 – Use Your Microwave

Microwave

Likewise, you can also use your microwave to sterilize the soil. You’ll need to moisten the soil before doing this.

Just place the soil into microwave safe containers with lids. You don’t need to use any foil to cover the soil, but you will need to make sure you use lids with ventilation holes for steam to escape. Heat the soil on a high setting for 90 seconds per two pounds of soil.

Once they are all done, you can remove and tape over the vent holes with some tape until you’re ready to use the soil for planting.

Final Thoughts

Most people plant seeds and young plants into soil without ever thinking that the soil contains pathogens, fungus gnats, and nematodes. Even fresh potting mix from a store can contain these nasty little pests and they can affect the growth of your plant.

The best way to deal with these pests is to sterilize the soil using one of a number of methods, or even multiple methods in serial. Boiling water is one of the easiest, but you can also freeze the soil, put it in an oven, or use your microwave.

Natasha Dowdell

Tuesday 3rd of May 2022

After pouring the boiling water on the soil and stirring it,does the water need to be drained off?

rose

Thursday 24th of March 2022

will using boiling water kill rat droppings by pourin the boiling water sever times in flower pots. then when plenting use gloves.

Bob Sloat

Wednesday 2nd of March 2022

Hi, Some of the seed starting mixes that I use include what is referred to as "wetting agents" and silicon materials. Can the boiling water technique reduce any of the benefits that those ingredients provide?

Yen2417

Monday 22nd of March 2021

does sterilizing soil (have a 1-sack of soil from backyard area, its normal soil) then using boiling water will also kills the good bacteria in this soil?

E

Friday 16th of October 2020

I thought you might like to correct the following typo:

2 – Using an Oven ... Cover this soil layer with soil and place in the oven at between 180 and 200°F.

I believe you meant to say: ... Cover this soil layer with foil and place in the oven at between 180 and 200°F.

Have a great day! Thank you for your post!

Kirsti

Sunday 6th of February 2022

@Lisa, for sure do not put foil in the microwave! 😜 But thank you for the info! I’ve had a bag of dirt in my car that was frozen solid since Christmas, it’s early February now. Im going to put some boiling water over my soil JUST to be sure! I bought a bunch of succulents and other plants that had come with gnats 😟 from the same store as the dirt. Time to give these babies some fresh soil!

Lisa

Friday 16th of October 2020

Thanks so much for pointing that out! You have a great day as well!