How to Sterilize Soil With Boiling Water!

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We all know that high heat kills germs of various kinds. 

After all, our very own bodies raise temperature to kill pathogens.

We boil, roast, or bake food not only to make it more digestible and tastier but to make it safe and sterile as well.

Lush coconut coir compost for the organic garden.

Can the same logic be applied to the potting soil?

Let’s dig in to find out.

Why Do You Sterilize Soil?

fungus gnat invading soil
Once you have a negative experience with pests and diseases, you realize the beauty of soil sterilization! Sterilized soil is fresh, clean, and pest-egg-free.

The soil needs good microorganisms – the bacteria and fungi that create nutrients and make them more available to the plants. However, in some cases, it is better to sanitize the substrate. To kill whatever lives inside!

Let’s see some examples.

  • Your soil gets contaminated with parasites or pathogens that cause diseases; if you had sick plants in a pot, soil from that pot is very likely contaminated. 
  • In line with the previous point, used all used substrate from old pots is best sterilized; fungus gnats commonly colonize moist substrates and can wreak havoc on your new plants.
  • You may get hold of gardening soil from an unreliable source, and you should sterilize out of precaution.
  • If you’re preparing for starting seeds, having a sterile medium is advantageous since young seedlings easily succumb to fungal and bacterial infections, as well as invertebrates; in fact, fungus gnat infestation is the most common reason behind seedling die-out.
spider mites infesting strawberry plant
Look at how these pesky spider mites invade the strawberry plant. Sterilizing the soil beforehand kills spider mites – and their eggs!

As you can imagine, there are numerous chemical treatments for soil sterilization – from pesticides and herbicides to general biocides. 

However, all these products carry environmental and health risks, and most are a big no-no in organic gardening. 

That is why conscious gardeners are always looking for clean, natural, environmentally-friendly damage-free sanitization methods.

That particular need brings us back to the story’s beginning – using heat to help us kill the germs.

Read More – The Beginner’s Guide to Composting! How to Make Surprisingly Simple Super Soil!

Can Heat Be Used to Sterilize Soil?

pot with boiling water on stove
If you have unwanted fungal problems or seeds in your soil – boiling water will almost certainly do the trick.

Like other applications, we can utilize heat to sanitize a limited quantity of soil.

A commonly recommended method is to bake or microwave the potting substrate. 

However, some people don’t own the microwave. Others dislike the idea of putting the soil in the same oven they use for food prep – plus, baking soil will release weird odors.

If your soil is rich with wood chips, your entire house will unavoidably smell like you have a mini-forest fire in there! 

That is why many homesteaders wonder, “How can I sanitize my soil without baking it?”

What about good ole’ boiling-hot water?

How Do You Sterilize Potting Soil With Hot Water?

Mentioning boiling water as a soil disinfection method can trigger a lot of skeptical questions and remarks.

Some argue even at its hottest (100 degrees C or 212℉), the boiling water is not hot enough to sterilize the soil; what is more, the water will probably be even colder by the time you get to pour it over the soil.

overview temperature soil pathogens bacteria fungi virus weed seeds
To dispel the myth, let’s look at this handy table of organisms killed at certain temperatures (thanks, Herbs at Home)!

As you can see, hot water will take out problematic tiny creatures across all groups. It works exceptionally well for insects and other invertebrates.

The pesky gnats and even peskier spider mites are doomed as soon as they come into contact with the hot water or steam as they have no protection against it.

It is no wonder that many gardeners have been successfully using the method for years.

The only thing you need to ensure is that the hot water has the time to do its work.

Setting aside enough time means you can’t count on simply pouring the boiling hot water over the soil and hoping for the best.

Read More – The Best Worms for Vegetable Garden Success! Make MAGIC Soil!

You Have to Find a Way to:

  1. Keep the soil thoroughly soaked or submerged in hot water. 
  2. Prevent the water from cooling off too much during the half-hour process. 

Another important general rule is to use distilled water, rainwater, or softened water. If you have hard tap water, mineral salts will build up in your soil, eventually affecting plant growth – or even killing them.

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06/12/2024 05:11 pm GMT

How to Sterilize Soil With Boiling Water

Let’s get to work!

Here are the two methods of boiling water soil sterilization:

1. Steaming the Soil on a Stove

Steaming water on your stove will ensure a consistent temperature during the entire process.

Here’s a general instruction on how to do it:

  • Get a large cooking pot – an old one from your kitchen, or buy a cheap used one.
  • Put the substrate inside and saturate it with water. Make sure there is enough water to have some floating on the top of the soil.
  • Bring the water to a boil. It is unnecessary to get the bubbles – a lot of steam also signals that the temperature is high enough.
  • Keep it up for at least half an hour.
  • Let the soil cool completely (best until the next day) and proceed with using it or amending it with slow-releasing fertilizers or compost.

For convenience, you can do this outside if you have a summer kitchen or purchase a cheap hot plate for the purpose.

Steaming Potting Mix to Avoid Spider Mites and Fungus Gnats
Here’s a video showcasing how to sterilize your soil with hot steam.

2. Pouring the Hot Water Over the Soil

You don’t have to boil or steam the soil the entire time actively if you ensure appropriate insulation.

  • Take a sufficiently large bucket made from thick metal; some people even use plastic buckets or boxes, but I’d abstain from that since the insulation is not sufficient, plus the plastics can release all kinds of nasty chemicals into the soil when exposed to heat.
  • Put the soil into the bucket.
  • Bring the water to boil on your stove. Make sure that you prepare a sufficient amount – many substrates can soak in a lot of water. 
  • Pour the boiling water over the soil and mix. The soil needs to be completely saturated and wet.
  • You can pour the soil in there and saturate it with pre-boiled water.
  • Cover the top of the soil with aluminum foil or with a metal lid and leave for at least half an hour.
How to Prevent this #1 Seed Starting Mistake - Boiling Water & Fungus Gnats: Two Minute TRG Tips
Fungus gnats are one of the worst things your soil can harbor! That’s why sterilizing soil may be a wise precaution.

Additionally, you can control the temperature of the soil with an appropriate thermometer during the “cooking” period.

To Sum Up – Can Boiling Water Sterilize Soil? Or Not?

Despite the naysayers, many successful examples prove that boiling-hot water sterilizes soil.

The secret is to ensure that the soil remains hot enough for long enough to let the heat do its magic. 

Bear in mind that the “cooked” soil will be sterile and devoid of most usable nutrients. If you’re planning to plant mature plants in it, you will need to amend it with compost or organic fertilizers – but that’s an entirely different story

What are your experiences with the boiling water soil sterilization method? Let us know in the comments!

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06/13/2024 12:17 pm GMT

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  1. After letting the soil sit in the boiling water for 30 minutes, does the water need to be drained off?

    1. Hi Natasha, thank you for your question. During the sterilisation process, the soil is steamed rather than cooked – you’ll be adding enough water to soak it entirely, but not to create a soup-like consistency. Thus, there won’t be much water left at the end of the process. However, if you happen to add more water than necessary, there will be no issues – and you can undoubtedly drain off the excess since soggy soil is messy and undesirable to work with. Other than that, there is nothing wrong with the remaining water – after cooking, it will be sterile, the same as the soil. If you’re still in doubt, I suggest checking out the videos embedded in the article. Good luck!

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