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?
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 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.
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.
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.