Recently, I’ve come across several discussions and articles on the need to sterilize potting soil or garden soil. One person went so far as to say that your plants will grow better in sterile soil. Interesting, given the fact that truly fertile soil is anything but sterile – it’s teeming with all manner of good bacteria and beneficial microorganisms, which are all murdered if you follow most of these directions. I can understand some situations that one would want to sterilize garden soil, but not to sterilize potting soil.
Trying to sterilize the soil in an outdoor growing bed in the oven or microwave is mystifying. But people want to know how to do this, even though it will a) smell awful, b) waste energy, and c) consume a lot of their time. Most of them are worried about weed seeds that they wish to cook, rather than pull when if they had simply pulled the weeds before the undesired plants started flowering, there would be very few weed seeds to deal with. Destroying the life in your soil means you will only have to invest more time and money rebuilding it so your plants will be awesome. It’s a lot less costly, and less time-consuming to simply pull a few weeds once a month. You can also mulch with shredded leaves after planting to stop a lot of those seeds from sprouting.
Now, there are times that it becomes necessary to sterilize soil due to disease. Not everyone has enough room to move the garden for a couple of years until the problems naturally correct themselves, and it’s certainly better to sterilize than a chemical solution. But don’t use your oven, or microwave – harness the power of the sun. It is possible to sterilize soil with the sun and not kill the all-important beneficial microbes. No – it won’t be an instant fix. But if you’re going to grow stuff, you can’t expect it to be done well without some planning and effort. It’s in your, and your plants’ best interests to use a method known as “soil sterilization”. This will destroy the weeds, weed seeds, diseases, and other harmful pathogens that might be in your soil. It can be done on a small plot or large scale like on a farm, and it gives you even more benefits while the cleansing magic is taking place. The sun can speed up the process of compost being generated into nutrients that plants can access… so yes, properly sterilized soil IS the key to growing healthier plants.
But what about potting soil?
There’s no reason to sterilize potting mix right out of the bag. Potting mix and seed starting mixes are always made from sterile ingredients. So, you don’t have to worry about making it more sterile, just fill your containers or germination tray cells, and plant. If you’re trying to make sure it has no bug eggs in it, cooking them will work, but why kill the benefits it contains when you can spray harmful or nuisance insects should they appear? Use something approved for organic growing.
If the bag says “potting soil,” and contains actual dirt, not a mix of peat moss, bark, coir, etc – you cannot container grow plants in that anyway – they will suffocate or rot, whichever comes first. You can use it with other ingredients to make your own potting mix, or mix a bit in with a prepared potting mix, but… it should have been sterilized before being bagged to prevent diseases and alien seeds from spreading hither and yon. And, if you’ve used some out of the bag and it sprouted weeds, take it back to the store for a refund, or let the sun do its job.
Recycled potting mix?
There’s nothing wrong with recycling your old container media, in fact it’s very wise. Outdoor pots should have been emptied in the fall after the first frost, the roots of the old plants removed and added to the compost pile, with the remaining loose mix set aside to rest in the sun over the winter. Indoor container soil should also be cleaned of roots and debris, before being allowed to rest or composted. If you’ve grown tomatoes in some of the pots, it’s best to make sure you don’t use it for tomatoes again for a couple of years, but it’s fine for anything else you grow. Except starting seeds.
Potting mix blends are not the same seed starting blends, because a finished seedling and larger plants have different root needs than sprouting or infant plants. Seed starting in partially decomposed used potting media could lead to problems with damping-off and lowered germination rates. Best to start these with fresh media designed for seed growing, and also when potting up in recycled potting mix, use partially new media – about 25%. As the peat moss and bark disintegrate naturally over time, it will turn into straight muck, which is not going to give you healthy plants. In three years’ time, Miracle-Gro Potting Mix will reduce to 1/3 the volume in the same pot with the same plant growing in it. The poor roots will be residing in nothing but goo when wet, and almost a brick when dry. (Yes, this is the voice of experience speaking. I couldn’t find the right pots to divide and transplant!)
If you’re worried about weed seeds in your recycled potting mix, the sun will definitely kill them in solar sterilization, but it will also speed up the decomposition process of all-natural materials it contains. You might decide that to maintain good media properties as long as possible it is smarter to bake it in the oven, but if you do this, be sure to put the lost microbes back with compost tea.
Learn more: How To Do Soil Solarization (UC Davis)
Store-bought potting mix is not always sterile. I bought a bag of potting mix at my reputable local garden center, the brand they recommended. I wanted to pot up my previously purchased house plants, about 10 of them. In a couple weeks I started seeing “fruit flies” in the house, but I had no fruit out to draw them in. After several more weeks of unsuccessfully battling the small flies with various remedies, I realized all my houseplants were infested with little gnats crawling through the soil. A bit of researching on different forums, I realized that the potting soil was the culprit. Next time I used the same bag of soil, I exposed it to boiling water. No fungus gnats!
They are usually sterile when packaged. But that can be months before you bought it. In the meanwhile gnats and other little critters make a home of that bag and there you go. Or they just do a poor job, they exist too. Thanks for your comment.
Store Potting soil is never sterilized.
All the big box store potting soil contains weed seed. So, try this experiment buy any bag you want from any big box store pour it into flat, water it, put it under lights and a clear dome and see what sprouts in about 14 days. You will be horrified what grows. The let it grow and ID all the weeds. You have got be too naive to think you can buy soil that cheap and it has been sterilized of weed seed. Try different brands and see.
I have in past purchased Miracle Grow potting soil and it had bugs. So how do I make sure it has no eggs and bugs in the soil when I am about to use it?
What about to get rid of / kill squash vine borer larvae or pupae (or whatever it is that overwinters in the soil) ?
Hello and thank you for this information.
Last year I had gnats infesting all of my plants. This year, and probably as an overkill, I sterilized in the microwave soil mix. Did I make a huge mistake and what do I need to add to bring back the nutrients and I possibly depleted by microwaving. Thank you and look forward to your answer.
I have Japanese beetle grubs in one outside patio pot. I have put dawn detergent and Listerine in the pot. I have also poured very hot water in the pot. Little things are still surviving but not as many. On Saturdays when I need a break from housecleaning, I go outside and use a cute little shovel to turn the soil over and pop the grubs. I don’t want to use harsh chemicals (yes, I know dawn and listerine are chemicals but not as harmful) I don’t plan to put vegetables in this pot but the darn grubs killed one of my beautiful evergreens.
What should be my next plan of action? It’s a really big pot.
Thank you so much for this information. I knew it was folly to put soil in an oven to bake for the sake of my oven first and the sterilization, also the stink. But the solar article in the link will be my next endeavor. Thanks again for this informationm.
Good overall advice, even though I will not necessarily follow it. For a starting mix I use regular soil with a little sand and compost. To rule out disease-causing pathogens I microwave the soil for about three minutes. I’ve never had issues with germination; actually, I have pretty good germination rates. I also recycle my starting mixes, after giving them a good soaking, letting them to dry a bit and microwaving.
As for compost tea, I don’t think it should be recommended. Robert Pavlis of gardenmyths[dot]com, who does a good job debunking many gardening myths, has a post on the NPK value of compost tea. His conclusion is that it contains few nutrients.
Barney
Your last statement is ignorant. I wrote the book “Compost Tea Making”. One of my major reference books, “Teeming With Microbes” by Jeff Lowenfels is excellent. Educate please.
The old NPK think is neanderthal. That is one of the reasons–promoted by big Ag–that agricultural soils world wide are suffering, and the quality of the produce is poor.
Compost Tea, done well produces amazing results.
I came across your article while looking for a solution to root aphids, not fungus gnats, which, like B. French says, have come with every $!&#% bag of soil I bought for the last 2 years, no matter the brand. *Of course* I don’t want to kill everything in the soil, but it’s impossible to get rid of root aphids once they are present. What plants aren’t killed by RA outright barely survive. I’m planning on sterilizing my store bought soil in the oven, and then will add compost to get the beneficials back, an important point that’s being missed in other DIY sterilization methods. I hear what you’re saying about using the sun, but i can’t risk it.
happy gardening 🙂
“It is possible to sterilize soil with the sun and not kill the all important beneficial microbes. ”
You seem to be really confused. First, sterilizing kills all life. Second, the sun doesn’t get the soil hot enough to sterilize it. It gets it hot enough near the surface to kill some things, but it’s not as if all the bad things die above some temperature and the good things don’t. You’ll kill a range of things, including plants, insects, and microorganisms – some good and some bad.
Hi Jacob,
Glad you stopped by. While that statement might sound confused, one must consider that insects and microbes, being ambulatory, will head for more comfortable conditions beyond a thin layer of potting mix frying in the sun. Eggs and fungi created for dark, cool, and moist conditions, however, cannot escape and will be eradicated either by heat or UV light. And if spread out on a dark surface, the midsummer sun can cook a sunnyside-up egg, so it can also cook insect eggs.
Yet another comment about fungus gnat infested soil. I still have a 2 cu ft bag and my only option is to sterilize it. Only this one brand had fungus gnats; my other pots in different soil had zero gnats. I don’t trust that producers are sterilizing their products.
Every single bag of potting soil, of every brand, that I have purchased in the last four years has contained eggs and/or larvae of fungus gnats. If not eradicated they cause tremendous problems and spread throughout the house. I’ve contacted the various suppliers and been told the gnats are coming in after I open the bags. Nope. I’ve opened a sealed bag fresh from the store and seen the tiny larvae crawling around. I’ve put some fresh soil in a sealed jar and soon the little creeps are flying about in the jar. The only thing to do is heat treat the soil, then follow up with Bti (Bacillus thuringiensis s. israelensis) and use sticky traps to monitor for increased populations.
Well, of course! Why accept the responsibility for those insect eggs. Kind of hard to pinpoint where the eggs came from.
If the bag has air holes in it though, and it was ever stored outside and got wet, the gnats could have laid eggs in it after it left the packing facility. They won’t lay eggs in dry media. Still, it’s a big problem.
As mentioned in the post, sterilizing to control insect eggs is common, but when I wrote this post more people were worried about weed seeds and diseases than bugs. Still, it will kill off beneficial microorganisms before reaching temps that destroy eggs. 30 minutes after the potting mix/soil reaches 160 degrees. Only weed seeds and heat-resistant plant viruses survive that kind of heat.
Hope you’re adding microbes back after sterilizing.
I’ve had the same problem with fungus gnat infested potting soil as B.French described, and finally decided to sterilize the soil in the oven. How does one go about adding microbes back? I’m an amateur indoor plant hobbyist. Thanks
Hi Geoff,
You can add compost, manure, and fertilizers to your soil. Here is an easy home soil test that will help you figure out what it’s missing. https://gcmag.co/Soil-tests
Happy gardening!
I lost some valuable indoor plants to a fungus that was in the “sterile” potting soil mix.
Hi Lisa,
That’s really not surprising. Retailers store bagged potting mix outdoors. The bags have air holes in them, but it’s not enough air flow for the contents to dry out – especially when it a fine mix. This wet anaerobic environment provides the perfect environment for fungi to grow. And it also causes faster deterioration of the chunky natural ingredients in the mix.
If you sterilize it, you can always add microbes with a high quality compost tea. Most progressive gardening shops will sell it ready to use by the gallon.