Should You Sterilize Potting Soil?

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December 11, 2015

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 – its 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 little 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 years until the problems naturally corrects itself, 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 your going to grow stuff, you can’t expect it to be done well without some planning and effort. It’s in yours, 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 it’s 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 years, but it’s fine for anything else you grow. Except starting seeds.

Potting mix blends are not the same a 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 disintegrates 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)

Callie

Callie

Contributing Writer at Garden Culture Magazine
Only strangers knock on the door at Callie's house. People who know her don't bother if the sun is shining - they know to look in the garden.
Callie

10 Comments

  • 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.

  • 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

  • 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.

  • “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.

  • Michele von Baltimore April 15, 2018

    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 🙂

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