Sterile Soil – Does it Really Exist?

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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.”

Microbe Science for Gardeners Book, 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

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