Garden Myth: You Can Use Sterilized Garden Soil to Start Seeds

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Sterilizing potting soil is generally a waste of time. Photo: sublimesucculents.com

Question: On Facebook, I saw that you can sterilize the soil from the vegetable garden in the oven and use it to start seedlings indoors in the spring. What do you think?

Danielle Dumoulin

Answer: You could try that, but I certainly wouldn’t recommend it. 

It really isn’t possible to sterilize soil in the way you imagine. Of course, years ago, everybody sterilized their own soil to kill off bug and worms and such, but at that time, packaged seed sowing mixes didn’t yet exist, nor you could even buy potting soil. So, gardeners tried their best to turn garden soil into soil appropriate for growing seeds, with often disappointing results. Which is why pretty much no one does this anymore. 

Of course, the other reason for not sterilizing soil is that doing so stinks. Expect to have to abandon your house for a few hours while your kitchen airs out after you’ve finished cooking up the soil! 

If you want to try anyway, here is a recipe from back in the time, taken from an old book:

“Simply place a quantity of moist—not sodden—soil in a baking tin, cover it, and bake it for exactly one hour in a 180˚F (82˚C) oven. This will destroy weed seeds, soil pests, and disease organisms without simultaneously killing useful bacteria.”

(Oddly, the book failed to mention the smell.)

Note that the soil must be moist, because it is mainly water vapor that kills the microbes, weed seeds (at least, most weed seeds), insects, earthworms, slugs and other inhabitants of garden soil. You don’t want to bake dry soil. That can cause chemical changes, making it water-repellent (you certainly don’t want that!) and possibly creating toxic organic compounds.

Sterilization Doesn’t Sterilize

The purpose of this sterilization is to kill the harmful microbes, etc. found in the soil, but the resulting soil is not really sterilized. Some bacteria and fungi survive the treatment. At any rate, once the soils cools, microbes quickly return, carried by the air, human hands, contaminated tools, etc. Unfortunately, the very fungi you least want to find in soil you’ll be sowing seeds in, those that cause damping-off disease, are among the first to return. 

Outside of a labaratory, you really can’t keep soil sterile. Photo: felixioncool, pixabay.com

Outside of a laboratory, it simply isn’t possible to keep soil sterile. Germs always find their way back in!

So, you didn’t really sterilize the soil, but you will have killed off some microbes and especially larger pests, like slugs and fungus gnats.

Turning Good Soil Bad

The unpleasant smell and the microbes that move back in are not the only problems. 

Garden soil is simply not good soil for indoor use. Photo: theprairiehomestead.com

In general, soil harvested from the garden is simply not suitable for indoor gardening. It tends to be heavy, dense and poorly aerated (whereas ideally, the opposite would be true: for seedlings—or for that matter, pretty much anything you’d want to grow indoors—you’d want light and airy soil). It drains poorly and reduces oxygen flow to the roots. Outdoors in the ground, there are all sorts of little creatures at work loosening it up … but you just killed them off, remember? You’ll also have to add mycorrhizal fungi, beneficial fungi that heating the soil killed off. In addition, garden soil contains stones and rocks and other detritus that can hinder rooting. 

Also, heating the soil increases the risk of phytotoxicity due to the increased concentration of soluble salts and the formation of toxic organic compounds. It can even affect the soil’s pH (weird things happen when you heat soil!). In other words, heating soil profoundly changes its nature and makes it less suitable for growing plants.

Damping-Off Disease

When seedlings keel over and die, you know you’ve made a major booboo! Photo: therustedgarden.blogspot.com

The main problem with using sterilized garden soil to grow seedlings, however, is damping-off. This disease, actually caused by several different unrelated fungi whose spores are present in the air, develops rapidly in garden soil once it is remoistened. This is the disease that causes young seedlings to flop over and die, their base brown and seemingly pinched. And it can totally ruin your sowing efforts.

When you plant seedlings in this kind of soil, therefore, it becomes a race against time: the seedlings have to grow quickly before the fungi attack them. Sometimes they succeed, sometimes they fail.

Modern commercial soils—based on peat, coir, bark, perlite, vermiculite, etc.—are not perfectly sterile (no potting soil producer will claim that their product is sterile), but it turns out that none of the ingredients are very conducive to the fungi that cause damping-off. I’m not saying damping-off can’t happen when you use commercial seed sowing mix, but it’s much rarer. Even if you just add a handful or so of garden soil to your sowing mix, you considerably increase the risk of damping-off taking over.

💡 Helpful Hint: When you’re sowing seeds, open a fresh bag of potting soil. Harmful fungal spores are much less likely to be already present than in soil taken from a bag you opened 4 or 5 months ago. Keep the previously opened potting soil for other purposes, such as repotting houseplants, filling container gardens or even potting up seedlings (once they’re well established, damping off is no longer an issue). Just don’t use it to start seeds. 

When I was a young gardener and used home-sterilized garden soil for my seedlings, we regularly lost them—often all of our seedlings!—to damping-off. It was considered necessary to treat the soil with not-terribly-organic antifungal products, such as No-Damp (oxine benzoate), in order to succeed. With modern potting mix, damping-off is much rarer and, in most cases, no special treatment is needed. Many experienced gardeners who’ve grown their own seeds for years have never even experienced damping-off. 

___________

In other words … the place for garden soil is in the garden, not indoors. I’m all for recycling and reusing and saving money, but … never skimp on soil and certainly not on the soil you sow your seeds in. The type of mix sold for starting seedlings (you could also use all-purpose potting mix or houseplant potting mix, which are much the same thing) is designed for indoor growing and a wise gardener would stick to them.

9 thoughts on “Garden Myth: You Can Use Sterilized Garden Soil to Start Seeds

  1. Glen Spurrell

    Thanks Larry for yet another interesting and useful post. You made me realize how many years I’ve been sowing seeds and how, in that time, commercial potting soil must have changed. Thank heavens damping off is mostly now a thing of the past. And you reminded us all of “No-Damp” and its odd smell.

  2. JOLJ

    I Sterilization soil with clear plastic 24′ &75′ for 4 months. It was to kill weeds & yes it was not a true Sterilization. It did kill perrennial weeds without using round up.

  3. Benjamin Go

    good day sir I just want to ask question or advice about termite in our area there plenty of them I plan to spray ISOPTEX 5SC would you recommend it is there any harmful effect to our surrounding plant? what is the best option for the elimination of thier colonies. Thanks you sir.

  4. Oh, too much work. I know that sanitation is important for some seed, for which I would not mind purchasing the appropriate medium. For me, it just never came up. Regardless, I would not want garden soil, sterilized or otherwise.
    However, I have indirect experience with seed that are weirdly finicky! Some of the native species ‘expect’ their native unsterililized soil. That is not the weird part of it. A few types want to experience fire prior to germination. They will not be fooled by baking in an oven. They want the real thing. They get placed on the surface of the soil in flats, and then covered with dry pine needles, which are then ignited! It is not as easy as it sounds. Without the heat of a forest fire, dry pine needles are not very combustible. They need to be fanned, which is messy and . . . risky.

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