Sterile Soil – Does it Really Exist?

Robert Pavlis

There is lots of talk about using sterile soil. Some people even sterilize their soil before they use it. Others buy bags of sterile soil. All of this is done in an effort to reduce pests, diseases and sprouting seeds. This all seems to make a lot of sense but there are a couple of basic questions that one should ask;

  • Does sterile soil really exist?
  • Does sterile soil make a difference to your plants?

 

I’ll deal with the second question in my next post. Today I want to discuss the existence of sterile soil.

Sterile soil
Sterile soil – does it exist?

Sterile Soil – What is It?

It is always good to start by defining the terms you will use. What is sterile soil?

One definition I found on line is, “Sterile soil is garden or potting soil that has undergone heat or chemical processing to kill any pathogens and seeds that are in it. ” I think most people would accept this definition, but it is only part of the story. It is also important that the product, once sterilized is kept sterilized until it is packaged, and the package must be such that the product remains sterile until you open it.

A better definition of sterile soil would be “soil that contains no life forms, including seeds, which has been packaged in sterile conditions.”

Compost Science for Gardeners by Robert Pavlis

Sterilizing Soil

You can buy potting soil that is labeled sterile. Most of these products have been treated with chemicals to kill pathogens and seeds. In some cases they have been treated with high temperature steam.

According to Colorado State University (ref 1) “temperatures routinely used to heat soil will not result in completely sterile soil / potting media. The goal is to heat the potting mix to a point that kills the plant pathogens of concern.”

It seems as if the sterilization treatments are not likely to be 100% effective.

Packaging Sterile Soil

If a company does sterilize the soil, they then need to package it in sterile conditions. This would be similar to packaging processed food, or medical supplies. Sterile packaging requires a very high level of cleanliness in order to keep microbes from contaminating the product. It just does not seem like the kind of facility a manufacturer of soil would have?

I contacted some suppliers of so called sterile soil. When I asked them about their sterile packaging facilities they had no idea what I was talking about.

I doubt that any company supplying soil will have a sterile facility to package it. If you know of one, let me know in the comments below – I’d like to talk to them.

Homemade Sterile Soil

You can’t buy the stuff, but you can sterile it yourself. There are many described processes on the internet. Colorado State University ( ref 1) provides information and temperatures for sterilizing soil. You will need a temperature of 212F (100C) for 30 minutes in order to sterilize it (ref 2).

But ……”Excessive soil heating may also increase chance of phytotoxicity due to soluble salts, manganese toxicity, and toxic organic compounds. Soil mixtures high in readily decompostable organic matter (manure, leaf mold, compost) are most likely to give injury when exposed to excessively high temperatures” (ref 2).

Once your soil is sterile you will need to keep it sterile until you use it. This means storing it in sterile containers, and filling pots in sterile conditions – something home owners can’t easily do.

If you are not going to store it in sterile conditions – what is the point of sterilization?

Killing the seeds would be one benefit, but this is not sterile soil.

Is Peat Moss Sterile?

Sphagnum peat moss gets special credit for a variety of features including sterility. Even Pro-Mix, one the the largest producers of sphagnum peat moss in North America clearly states that peat moss is not sterile.

The Sterile Soil Myth

For the home gardener sterile soil is a myth – it does not exist.

At best you can buy products that have been treated to kill most pathogens and most seeds. If it was not contaminated during packaging, it will start getting contaminated as soon as you open the bag.

Don’t be concerned about this. I’ll explain why next week.

References

(1) Start Seed and Transplants in Sterilized Soil; http://www.colostate.edu/Dept/CoopExt/4dmg/Soil/sterile.htm

(2) Using Heat to Eradicate Soil-borne Plant Pathogens; http://phytosphere.com/soilphytophthora/soilsterilization.htm

(3) Photo source; Doran

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

22 thoughts on “Sterile Soil – Does it Really Exist?”

  1. This is an old post, but thought I would add my two cents. I am retired now, but my past history included working in the OR, Sterile Processing and Infection Control.
    “Sterile” is a term that is greatly confused, especially in the public setting. I totally recognize that any soil I buy labeled “sterile” will not be so and even if I treat it myself, by the time I am ready to use it; it will be contaminated.
    Even so, I would like to start out the growing season with the least amount of strikes against me. In the past I have dealt with fungus issues in the soil and root rot when I have been extremely careful not to overwater. Many times my plants get attacked in the spring before they’ve even had a chance to get going. I’m going to try to clean and disinfect all of my pots and equipment to the best of my ability this season and I will buy treated soil as well – knowing that it’s not sterile, but might give my plants a fighting chance.

    Reply
  2. Hi, I once used it to kill weeds and I was happy with the results. So I need to use it again to some parts of my place where there’s too much weeds and is uncontrollable

    Reply
  3. I feel like no one in the gardening world is expecting “sterile soil” to be medical or food-grade sterile. I’m not sure that would even be beneficial.

    We mean that the soil should be free from live pests, weed seeds, and plant diseases. As long as none of those things are re-introduced after sterilization, most would call this sterile.

    For example, if I cough on a surgeon’s scalpel and a bucket of soil, both will no longer be technically sterile. But while everyone would care about the scalpel, no one would care about the soil because that can’t possibly hurt what it’s intended for (plants).

    Perhaps there’s a better word to use than “sterile” when talking about potting mix, but that’s more a question of semantics. I think we all understand what we mean when we call soil sterile…

    Reply
    • So you know that everyone who uses the term sterile soil knows what it means? In my experience, when the wrong word is used it leads to confusion.

      And the so-called sterile soil is not necessarily free of plant pathogens. It needs to be really sterile for that condition to be met.

      Reply
  4. Unless one using “sterile” soil is only planting in clean containers and keeping them sealed up indoors, pathogens will be brought by any bug, critter walking or flying by, leaf falling or wind blowing across your soil.

    Reply
  5. I call BS. I have been sterilizing my soil since day 1. It is heated to 212 degrees F at 15 PSI for 45 minutes. It kills everything. Then I use a hydrogen peroxide spray on the inside of the containers after washing them with soap and water. The soil goes straight from the pressure cooker into the container. I have not had soil pests or fungi since. In fact, even the gnats and flies are gone. I would say that without a doubt soil can absolutely be sterilized and in fact, yields much better results.

    Reply
    • 1) The article is not about what you do to sterilize soil – it is about commercial products that claim they are sterile.
      2) You missed a big part of the article. As soon as you put it into a pot, or plant something in it – it is no longer sterile. In fact just moving it from your hot pot to the container will contaminate the soil unless you do this in a sterile laminar hood.

      Re: “I have not had soil pests or fungi since” – how did you test for these? Humans can’t see these things.

      Reply
  6. Thanks for another great post. I checked the sources and their sources in turn, and I am now sure that my method of de-pathogenizing used potting soil is harmless and effective – I use a slow-cooker over night, temperature reaching 99 ⁰C.

    I was worried this temperature could produce harmful by-products, but now I m not, anymore. Not for typical peat/sand-based commercial potting soil mix.

    Reply
  7. when I search to buy sterile soil on internet, ( Home Depot, Lowe’s and amazon)
    It led me to organic soil products. Are these 2 name interchange? Is organic soil the stores sell the same as sterile soil?

    Reply
  8. The problem for me last year using Home Depot organic garden soil was gnats.

    Had grow lights, seedling heating mats in garage setting. Ruined good organic seedlings. Don’t want to use chemicals.

    Tried heating soil in oven in the house – just ridiculous. There had to be a better way?

    Reply
  9. It is neither a ploy or a marketing trick. Soil sterilization has been practiced for centuries either solar or fire and now steam and chemicals. It is incredibly important in propagation beds (indoors and outdoors). Greenhouses usually fumigated or bombed (compressed canisters) before starting new seed cycles.

    In order to understand the need for soil sterilization a quick soil and its life refresher. Soil content can be divided basically into 4 areas, the soil texture (clay, sand and silt content) and structure which states the aggregation. Further to this you may say there is organic matter although not necessary (plant debris, animal manure, etc) and finally life (nematodes, bacteria, virus, fungi).

    The purpose of sterilization is to remove potential seeds and possible pathogens (bacteria, fungi, etc) which could damage plant parts. More so on a delicate process such as plant propagation (seedlings, cuttings, grafts). Not so much on mature plants. The link below illustrates some temperatures required to kill several pathogens.

    http://phytosphere.com/soilphytophthora/soilsterilization.htm

    Finally, in response to Emmanuel Roux comments. Yes microbial life is important for plant life. There is symbiotic relationship between fungi (mycorrhizae) and roots, nematodes aerate the soil and provide valuable manure, bacteria involved in gas exchange, etc. However, none of those as important as keeping potential pathogens away while the new seed starts to grow. Most nutrients are already available for germination in the endosperm.

    Reply
    • You missed the whole point of the post. Just because people go through steps that accomplishes some sterilization, does not mean the resulting soil is sterile. Once sterile – soil would nothing nothing living in it – that does not happen.

      Reply
  10. While I agree with everything you say, I believe (and correct me if i’m wrong) sterilizing soil and/or compost (in an oven or micro-wave) before using it indoors, serves a purpose in terms of ensuring that one is not bringing outdoor pests inside where no natural predators exist.

    Reply
    • Sterilizing your own soil certainly has some value. If by pest you mean animals and insects, then the sterilization will take care of them. From the references in the post you can see that killing pathogens is a bit trickier but can be done.

      Other than animals and insects, is it important to kill other life forms? That is a question for another post. Many people, including myself, never clean our pots that get reused over and over again for seedlings. It has never been a problem.

      Reply
  11. I’ve always wondered about “sterile soil”… thanks for exploring this… anecdotally I doubt home gardeners ever have sterile soil but hopefully for seeding purposes, they’ve eliminated the pests (including weeds) when propagating.. looking forward to your next installment…
    CHEERS

    Reply
  12. It thought that microbial life was important to good soil ???
    I have been using good compost mixed with coconut coir for seedlings and i have very rarely had problems
    I have grown quite a few in a semi commercial setting. Could it be that “sterilized soil” is just a convenient marketing ploy to build in a perceived value added trick to sell overpriced dirt.
    Just wondering?
    Thks for the post , looking forward to next week.

    Reply
    • Microbial life is critical in the garden. Since people fertilize in pots it is less important in pots.

      The general public puts a lot of value on cleanliness – hence sterile soil sells better. People using bagged soil for seedlings are also concerned about disease like damping off – sterile soil sells better for this app as well.

      It is marketing.

      Reply

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