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Aspiring to do sustainable* container gardening, I'd like to reuse the soil in my balcony pots each year instead of throwing it away. A fascinating idea is that watering a balcony pot with boiling water kills the evil things in it. Certainly, it doesn't kill everything. Very probably us gardeners don't even know of many of the diseases ravaging our plants.

What is the spectrum of expected harmful organisms permanently destroyed by such treatment? How often should it be done? Are there scientific articles on the subject?

* - as this avoids the damage caused by removing peat, packaging it in plastic, transporting it to the place of sale; and when discarded transporting it to a waste dump mixed with all the glass and plastics people have also discarded

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    Watering a flower pot with a plant in it will almost certainly kill the plant. Do you want to know if watering an empty flower pot with boiling water will sterilize the soil in the pot for reuse?
    – Jurp
    May 15, 2022 at 11:31
  • @Jurp obviously! I refer to re-using the soil, not keeping the plant alive. But can't figure how to express it in the question right now - edit away!
    – Vorac
    May 16, 2022 at 7:42
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    I have sterilized soil by spreading it on a tinfoil-lined baking sheet with a built-up edge all around at 200 degrees for at least half an hour. May 19, 2022 at 6:39

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Your question led me to do a little research into my own approach (using a diluted hydrogen peroxide solution). I hoped to find research-backed information from university extension programs, and did find one article from the Penn State Extension.

The article mentions that sterilization or pasteurization requires keeping the soil at 180f (80c) for 30 minutes, or raising the temperature above 212f (100c).

Adding boiling water may sterilize some of the soil, but I would expect the heat to drop below boiling fairly quickly and likely below 180/80 before the 30 minutes is elapsed. Additionally it likely would be inconsistent.

The paper suggests baking the soil on a tray, and ensuring the 180f/80c temperature is met and maintained for 30 minutes. Then removing it and of course allowing it to cool before use.

So far I haven't found similarly reliable information for using boiling water or peroxide for soil sterilization.

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  • H2O2 sounds dangerous but fascinating. It is beyond the point of the question, but might be more than what I asked for! However you would need to at least provide the time period after it is safe to plant seeds or transport a plant into this soil following application.
    – Vorac
    May 16, 2022 at 16:14
  • I'd originally answered with more H2O2 info, but decided to delete that question in favor of a more direct response. Since you're interested I went ahead and undeleted the original H2O2 response: gardening.stackexchange.com/a/62343/931
    – STW
    May 16, 2022 at 18:07
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Boiling water added to a pot will at best possibly pasteurize the soil somewhat. It will be uneven and not terribly dependable as a result.

Boiling water added to a pot with a plant in it will certainly kill the plant.

How often should it be done - never is good. If you are dealing with a disease problem, more certain pasteurization or sterilizing techniques are called for. If you are not dealing with a disease problem, you don't need to sterilize or pasteurize the soil in which things are happily growing, ever.

Thermophilic bacteria beyond the "compost" range are highly unlikely to be in your potted plants to any notable extent. There's a fair bit of disagreement between the "sterilize everything in a steam autoclave" and "soil bacteria/fungus that make it through a hot compost pile (which is one method of pasteurization) are a good thing" camps when it comes to seed starting.

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If you intend to re-use a pot in which a plant had disease issues, tossing the potting soil and sterilizing the pot itself is the better option. Most authorities recommend sterilizing it with a bleach solution rather than boiling water, but I suppose doing both (separately) will work just fine.

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I have nothing to compare it against, but have sterilized soil and pots using hydrogen peroxide (H2O2) several times and haven't had issues using the soil after.

Generally you dilute about 1/2 cup to 1 cup of peroxide per gallon of water--the specific ratio may depend on the strength of the peroxide you're using. Then you can saturate the soil and scrub/rinse the pots with it. Afterwards you need to allow a fair amount of time for the H2O2 to do its thing, and for it to eventually transform into H2O. I usually spread the soil out on large trays and allow it and the pots to air-dry for 24-48 hours, until fully dried.

After the soil has been prepped I mix in some fresh compost and/or compost tea to ensure there's nutrients and some beneficial bacteria. The amount depends on how depleted I think the soil is--using a soil test would be wise.

H2O2 is a bleach, and has the benefit of rendering itself safe (provided you give it enough time). As I said I've never compared it to other methods, or even controls--my mother in law suggested doing it when I was getting ready for my second season of container gardening and it made sense and was simple, so I've done it regularly.

And beware that since it's a bleach it can stain/lighten colors of whatever it comes in contact with. If you get it on your clothes or other surfaces you can rinse it thoroughly to prevent damage.

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    Be aware that if you don't want stabilizers in your H202, then you'll want to use the food grade kind. May 17, 2022 at 1:31
  • ah, interesting, and good to know. I've always used the general stuff from pharmacy or health-items area of the store and wasn't aware of there being a distinction
    – STW
    May 17, 2022 at 14:32
  • The food grade kind is a lot more potent (and can be dangerous if mishandled); so, be careful; I believe you're supposed to dilute it. I think the reason it's so potent is because it doesn't have stabilizers to keep it from turning into water. May 17, 2022 at 23:37

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