Boiling water to sterilize & re-use potting mix?

turquoisegardenia(6a Toronto)February 15, 2011

Hello everyone,

Although I learned a lot by experimenting last year, I'm still very much an amateur, so I would appreciate a bit of advice!

I have quite a few large containers filled with last year's potting mix, which was roughly 5% perlite, 5% vermiculite, 20% coco coir, 10% composted manure, 30% Miracle Gro potting mix, and 30% compost. I'm very much on a budget, and aside from the compost, which is given out free by the city, I shelled out quite a pretty penny for this stuff. The thing is, last year my tomatoes had blight, and something similar seemed to happen to my strawberries (though this could have been due to my Topsy Turvy strawberry planter-- please never get one of these. Don't even get me started!).

So my question is-- if I boil a considerable amount of water and pour it through the soil, will this kill the blight? Obviously it would also kill any beneficial soil microbes too, though because the containers have been outside in the cold all winter anyway (it sometimes gets as low as -40 celsius here in Montreal), wouldn't they be dead anyway? How could I re-populate these soil microbes, by re-introducing new soil? How much would I have to use? Finally, what do you think is the most likely culprit for introducing the blight? I'm thinking likely the compost, or perhaps the manure? I'd hate to simply re-introduce it again by re-using whatever the source was, yet being a frugal gardener, I find it hard to throw out compost or manure!

OK, that was way more than one question, sorry guys :p

Thanks in advance!

Thank you for reporting this comment. Undo

I don't know the answer but I have the exact same conundrum! Anybody?

    Bookmark   February 15, 2011 at 3:08PM
Thank you for reporting this comment. Undo

I don't know that it's even worth saving spent medium of such a mixture... from my own perspective, I'd dump it in the garden and begin with fresh medium built in a more inorganic manner, which will last longer and provide a healthy environment for root growth.

The link below is loaded with excellent information on soil building and how mediums behave in a container environment.

Here is a link that might be useful: Container Soils - Water Movement & Retention

    Bookmark   February 15, 2011 at 3:38PM
Thank you for reporting this comment. Undo
turquoisegardenia(6a Toronto)

Thanks for the advice jodik, but even though I understand the concept and agree that it's likely a better medium, that mix is not an option for me as cost is a barrier. The only bark fines I could find in my area were the kind meant for reptiles, and they want $40 for a 40-litre package (I would need several)! At that price I might as well buy my veggies at the grocery store. I also do not use sphagnum moss as it is not harvested sustainably and, being Canadian (most sphagnum is harvested from our northern peat bogs), I feel it is my duty to protect the habitat of my country's native wildlife. So I'm afraid I'll just have to work with what I have, although again, I appreciate your trying to help. Does anyone else have any ideas?

    Bookmark   February 15, 2011 at 4:02PM
Thank you for reporting this comment. Undo
rhizo_1 (North AL) zone 7

I'm afraid that the addition of manure and compost into container gardening can be asking for trouble, for lots of reasons. If you wish to use something along the lines of what you've been using, you'd be better off adding a great deal more perlite with your MiracleGro.

The problem with the compost and the manure has less to do with the introduction of diseases but with the poor drainage associated with materials that tend to get mucky...especially as time goes by. You may have learned that plants growing in mucky soils are more prone to diseases, and that is certainly true for your tomatoes.

The general rule of thumb for all tomato growers is to never use the same potting mix for more tomatoes and to avoid planting tomatoes in the same location in your garden every year.

To address your question about sterilization,, pouring hot water over your containers won't do the trick, not even close. The entire soil volume has to reach pretty high temperatures in order for soil born pathogens to be affected; soil pasteurization occurs at about 180 degrees. At home, this can be done by taking small amounts of the potting mix and baking in the oven or zapping in the microwave....NEITHER of which I recommend for you if you intend to use the same soil (manure). I don't recommend it for anyone, actually.

You will need to sterilize the containers and your tools, as well, which can be done by a 10% bleach solution.

Most of these plant pathogens are ubiquitous. Keeping this in mind at all times, it's our job to choose plants that may be resistant to those diseases, to do all that we can to grow them under the best conditions we can, and to avoid negative cultural practices such as over or under watering, excess or under fertilization, poor drainage, etc.

I have one of my local privately owned garden centers special order my potting mix for me. I've stopped making my own medium and went back to using, exclusively, a product that is extremely excellent! If you are interested, please drop me an email via my member page. I don't want to advertise it here for fear of appearing like I'm selling for the company. Which I'm not, lol!

    Bookmark   February 15, 2011 at 4:14PM
Thank you for reporting this comment. Undo

I agree with the others that your growing medium may not be the best for attempted sterilization. At any rate, you would definitely have to either oven sterilize or purchase a soil sterilizer to do it right. Pouring boiling water over the soil will not do it.

One thing is you need to separate in your head is that field conditions are not the same as container conditions. You don't need to and probably shouldn't try to duplicate field crop conditions in containers, at least not when it comes to soil. For example, in would be perfectly fine to have worms in field crop soil but not something we want in container plants.

Most likely, your prolonged freezing weather has killed off some nasty stuff but maybe not everything. If you do sterilize, don't worry about good organisms because they may be introduced naturally plus, again, you are not dealing with field conditions (does hydroponics culture require beneficial organisms?). Sterilize your containers if you sterilize soil. If you can't sterilize, at least rotate crops in containers (after amending soil to meet this year's needs for the new plants).

Solar sterilization is also another possibility for you in the future. There is information about this on the internet. I've done it in an idle hoop greenhouse with good results.

    Bookmark   February 15, 2011 at 7:31PM
Thank you for reporting this comment. Undo
Ohiofem(6a Ohio)

I feel for you. I have to have more than 42 cubic feet of potting mix for my container vegetables. The first year I established my garden, I ended up spending almost $200 buying premixed potting soil that I mixed with my own compost. I can't afford to do that every year. I agree with other posters that the ideal potting mix would be Al's mix, which really does cost way less than packaged potting soil. But you have your reasons for not wanting to do that, so how about a compromise?

You didn't say whether you had early blight or late blight. Both are air borne fungi. They are not in your soil, so sterilizing your soil wouldn't even protect you from them. Search on this forum for blight and you will find suggestions about how to deal with it and prevent it. Late blight is probably hopeless, but you can hold early blight in check with fungicides.

The plant that really needs fresh soil every year is tomatoes, and that's because of soil borne problems like verticillium wilt. Build some fresh potting mix just for your tomatoes this year. Maybe you could modify Al's mix by using coir instead of peat. See if you can find a landscaping company that sells pine bark fines as mulch. I found one that sells it for $2 a cubic foot. They also sell cheap builder's sand. In a pinch you could reuse your old soil enriched by some compost and maybe controlled release fertilizer for the other vegetables. They probably won't grow as well as they might in a fresher mix. If you like how your tomatoes do in a faster draining mix, then maybe next year you will think it's worth growing other veggies in that kind of mix. Take it one step at a time. It's supposed to be fun, not a burden.

    Bookmark   February 15, 2011 at 8:58PM
Thank you for reporting this comment. Undo

In my opinion, the most important step you can take is learning and understanding the differences between growing within the confined space of containers versus growing in the ground, and learning how the factors that combine in a container relate to one another... as in what plants require and how they grow, and how and why mediums and moisture behave as they do.

Once you grasp the basic science, the rest is fairly easy.

Since I only need enough medium for my indoor plants and a few pots outdoors, the reptile bark is not a large expense for me, nor are any of the other ingredients in the Gritty Mix. And I really like the fact that I don't have to re-pot yearly, or worry about too much moisture retention, which tends to suffocate and rot roots.

Growing plants should be enjoyable... not a chore... and having a basic understanding of what's happening below the soil surface can really help, I think. Take it slow and easy... learn at your own pace... and use what you learn to make it all enjoyable. You don't need to spend a fortune or change everything all at once.

    Bookmark   February 16, 2011 at 4:53AM
Thank you for reporting this comment. Undo
turquoisegardenia(6a Toronto)

Thanks guys! You've really given me a lot of food for thought. I think that is exactly what I'm going to do: buy a few of the components for the mix, try it out with my toms this year (which failed miserably last year anyway, so I'm open to change) and see if it's worth the investment. To be honest, I am still a little skeptical about it. I guess the problem for me is that even though I certainly agree that this mix would drain better and become less compacted, it is not at all what we find in nature. And although everyone says "don't try to replicate nature for your container soil", I still can't get rid of that tiny voice in my head that says "why not?" Provided that I make sure to provide a large number of drainage holes, why should my container become any more compacted or waterlogged than the ground would? I'm not trying to be argumentative, don't get me wrong. I just don't quite "get" it. I mean, we replicate lighting, watering, and feeding conditions found in nature, right? Why is soil the one thing we change?

On another note, I followed a link another member posted (sorry, can't remember who offhand) about the Global Boxes they designed, and was intrigued by their recommendation to use simple reusable cloth grocery bags to plant in. It seems that because the bags provide significantly more aeration and drainage than, say, the Rubbermaid bins I use right now, you can get away with a more conventional potting soil medium. What's everyone's consensus on this? I was thinking of maybe trying that out too as a sort of compromise, to be able to use up the mix I have while still getting the benefits of better aeration and drainage.

    Bookmark   February 16, 2011 at 4:46PM
Thank you for reporting this comment. Undo
Ohiofem(6a Ohio)

I don't know what Global Boxes are, but I have been very happy with Smart Pots. They are cheap, durable and made of a fabric that breathes and drains. Used on bare ground, they are more like mini-raised beds than typical garden pots. I've used them for the past three years. I have no financial interest in this company.

Here is a link that might be useful: Find out more about Smart Pots

    Bookmark   February 16, 2011 at 5:47PM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

The 'why not' of trying to replicate a garden environment in containers is that water behaves much differently in containers than it does in the earth. Heavy (water-retentive) soils, like the one you described in your original post, hold onto water so tightly it will not drain from the container w/o assistance. In order for roots to function properly, they need a substantial volume of air in the soil at all times. Because your soil supports a large volume of perched water, root function/metabolism/growth can almost be counted on to suffer. The link Jodi provided on 2/15 explains it all in detail.

On a scale of 1-10, with 1 being growing in gardens and beds and 10 being full hydroponic growing, container culture is about a 7 or 8 - much closer to hydroponics than gardening. It's natural for gardeners that want to keep close to the earth to want to bring the garden to container culture, but they are so distinctly different that much of what we learned about our gardens should be left in the gardens because it can become so counterproductive, applied to container culture.

IF you use your grocery bags and make sure they have full contact with the ground, it sort of turns them into raised beds, hydrologically speaking. You can also partially bury your containers, which employs the earth as a giant wick, helping to remove the perched water I mentioned.

We can talk about different strategies that might help you use some of the soil you have now, but my advice would be to forgo its use in containers unless you can turn your containers into mini raised beds.


    Bookmark   February 16, 2011 at 5:49PM
Thank you for reporting this comment. Undo
turquoisegardenia(6a Toronto)

Well, I'm a container gardener out of necessity, not choice-- if I had any "ground" to speak of I wouldn't be messing around with rubbermaid bins and the like! Nope, I'm on the third floor, so any options involving the ground are also out. I think, however, I may have found some affordable bark fines after all (please see link below), though it does concern me that the type of wood is not specified. What do you think? Is it the right consistency? Now all I have to do is hope they have it in my local store...

Would it be terribly bad if, for the peat moss portion of the basic mix, I simply re-used my current potting mix? It seems to me like the peat moss component is for water retention, which my my potting mix evidently does quite well!

Finally, what about this "wicking" idea you mentioned in the post linked to above, Al? You said it would be possible to increase drainage with a sort of backwards-running wick watering system? I'm trying to visualize it but a bit confused.

Here is a link that might be useful: Hillview Bark Nuggets

    Bookmark   February 16, 2011 at 9:31PM
Thank you for reporting this comment. Undo

Here's a shot of my many containers last summer. Tomatoes were fabulous. Only two pots and got tons of tomatoes. Planted green beans, 3 varieties of peppers. Planted all my herbs in a large pot on the deck. All the flowers were in containers.

I used bagged potting soil mixed with small bark and some perlite. Had no problems with drainage, disease or roots. Had terrific results and will do the same thing this year.

Just use what you have available. Bark chips are easy to find. I used Orchid bark because I had it on hand.

Basil, Parsley, Sage, Thyme

Hot peppers

Feel free to email me if you want more info.


    Bookmark   February 16, 2011 at 11:26PM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

It's pine bark, but it's hard to say by the picture if the size of the bark is appropriate. It looks like it's quite large. If you read the link Jodi left for you upthread, you probably realize that particle size and distribution is very important to how a container soil performs.

Here is something I wrote to illustrate the importance of particle size/distribution:

Obviously, I think the grower's soil choice when establishing a planting for the long term is the most important decision he/she will make. There is no question that the roots are the heart of the plant, and plant vitality is inextricably linked in a hard lock-up with root vitality. In order to get the best from your plants, you absolutely must have happy roots.
If you start with a water-retentive medium, you cannot improve it's aeration or drainage characteristics by adding larger particulates. Sand, perlite, Turface, calcined DE ...... none of them will work. To visualize why sand and perlite can't change drainage/aeration, think of how well a pot full of BBs would drain (perlite), then think of how poorly a pot full of pudding would drain (bagged soil). Even mixing the pudding and perlite/BBs together 1:1 in a third pot yields a mix that retains the drainage characteristics and PWT height of the pudding. It's only after the perlite become the largest fraction of the mix (60-75%) that drainage & PWT height begins to improve. At that point, you're growing in perlite amended with a little potting soil.

You cannot add coarse material to fine material and improve drainage or the ht of the PWT. Use the same example as above & replace the pudding with play sand or peat moss - same results. The benefit in adding perlite to heavy soils doesn't come from the fact that they drain better. The fine peat or pudding particles simply 'fill in' around the perlite, so drainage & the ht of the PWT remains the same. All perlite does in heavy soils is occupy space that would otherwise be full of water. Perlite simply reduces the amount of water a soil is capable of holding because it is not internally porous. IOW - all it does is take up space.

If you want to profit from a soil that offers superior drainage and aeration, you need to build it into the soil from the start, by ensuring that the soil is primarily comprised of particles much larger than those in peat/compost/coir, which is why the recipes I suggest as starting points all direct readers to START with the foremost fraction of the soil being large particles, to ensure excellent aeration. From there, if you choose, you can add an appropriate volume of finer particles to increase water retention. You do not have that option with a soil that is already extremely water-retentive right out of the bag.

I fully understand that many are happy with the results they get when using commercially prepared soils, and I'm not trying to get anyone to change anything. My intent is to make sure that those who are having trouble with issues related to soil, understand why the issues occur, that there are options, and what they are.

Many use their current potting soils (like MG or used soils as a smaller fraction of a new media they put together specifically to improve drainage & aeration, so yes - you CAN use some of the soil you have as a fraction of a new soil, but keep in mind that you aren't going to rid yourself of the drainage/aeration and high PWT issues problems unless your soil is weighted heavily toward larger particles than your current soil is currently comprised of.

When you add a little extra perlite and/or pine bark to a peat-based soil, you don't change the drainage characteristics, the height of the PWT, or aeration. You DO decrease the total volume of water the soil CAN hold, and that IS a plus, but that's about all you really accomplish. Add a LOT of (appropriate size) bark and perlite = job done.

The physics of how water behaves in container soils doesn't change because someone might assure you it does. As surely as the cow is barred from jumping over the moon by the law of gravity, soils are bound by physical laws to exhibit rather specific and predictable characteristics based on their particle size and distribution (enter the pudding analogy into evidence).

If you choose not to go with a soil that has a considerable fraction of larger particles, there are ways to help you deal with excess water retention when it occurs. There is a lot of good information about dealing with water retention if you follow the embedded link. When you get to where you're deciding about how you want to proceed with your soil, we can talk more specifically about tailoring a plan that best supports your methodology.

Take care.


    Bookmark   February 17, 2011 at 2:56PM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

.... thought you might enjoy a few pictures ....


    Bookmark   February 17, 2011 at 3:06PM
Thank you for reporting this comment. Undo

Wow, beautiful pictures you guys.

I just made up some container mix using compost, perlite and lots of pine bark. Skipped the peat because of two reasons, cost cutting, and the compost I'd managed to get really held onto water when it got rained on. So far, what I've planted in it seems to be doing well (knock wood). Previously I've used sand, composted manure and bags of top soil to make container mixes, and it worked fairly well.

I have a question. Could you substitute Melaleuca mulch instead of pine bark? It's cheap. I also like the idea of using a horribly invasive species here in Florida in a useful way.

    Bookmark   February 17, 2011 at 5:06PM
Thank you for reporting this comment. Undo

Never heard of it. What is it?


    Bookmark   February 17, 2011 at 11:53PM
Thank you for reporting this comment. Undo

It appears that we're talking more about short term plantings, here... and not so much about plants that will have to remain in the same medium for more than a year, say.

Once we go beyond a single growing season, it becomes even more imperative to use a medium that isn't decomposing... one that is durable and more inorganic.

So, keep in mind the length of time your planting will be in the same medium when choosing ingredients.

    Bookmark   February 18, 2011 at 11:21AM
Thank you for reporting this comment. Undo


Melaleuca is a paper bark type of tree originally from Australia. Back at the beginning of the last century the geniuses who ran Florida sprayed the seeds all over the everglades with the idea that the trees would dry them up and make the land usable, a very Wylie Coyote type of plan. Only after wards did they realize how important the Everglades are to the Florida ecosystem.

It has now displaces hundreds of thousands of acres of native species, and is the first to pop back up after the natural fires we have here. It spreads by fire. Preventing native species from sprouting. About the only way to get rid of the thing is to turn 'em into mulch. The much is also the only natural mulch that discourages termites and other insects so it's the only one safe to use up against your house.

    Bookmark   February 18, 2011 at 11:32AM
Thank you for reporting this comment. Undo
tapla (mid-MI z5b-6a)

Melaleuca (punk tree) bark is sometimes used as a substitute in container media for SOME of the pine bark, but it has been found that the % of melaleuca bark should be well under 50% of the o/a volume. There are a number of organic products and byproducts that are used in some container media, but at low %s. E.g., coir is generally kept to under 10-15% in commercial container media for several reasons - high pH, possible salt content, extremely high K content among them.


    Bookmark   February 18, 2011 at 3:16PM
Thank you for reporting this comment. Undo
turquoisegardenia(6a Toronto)

Jane- HOLY MOLY! Is that some sort of mutant basil strain? I'm lucky if mine gets to a quarter that size. Gorgeous garden you've got there!

    Bookmark   February 18, 2011 at 5:21PM
Thank you for reporting this comment. Undo
greenman28 NorCal 7b/8a

Maybe a Mammoth Basil?


    Bookmark   February 18, 2011 at 9:27PM
Thank you for reporting this comment. Undo

Funny, actually I have no idea what kind of basil.
I hardly fertilize, so I was surprised too. Mammoth sounds good to me.

Thanks for the complements. First time growing veggies in containers and they all turned out great.

Swamp, I have to agree you need to be careful using too much compost. I just noticed your zone and with heat and humidity, you can run into problems using compost. I can't advise about your use of Melaleuca.

I use small bark and perlite mixed with bagged soil so it is quite loose. In the containers with the tomatoes, I added some sphagnam moss to hold more moisture.

You need to make a mix which drains quickly and doesn't hold too much moisture if you water often, or get frequent rain.

I do not water often and like a mix which doesn't dry out too fast. Your choices depend on your environment and watering habits.

Good luck,

    Bookmark   February 19, 2011 at 1:54AM
Sign Up to comment
More Discussions
Culinary Herbs?
I need to buy some culinary herbs. Where is the best...
Al's Gritty Mix -- A Learning Experinece
I came to this forum a few weeks ago in an attempt...
Summer Squash
Last spring/summer, I planted a summer squash that...
Can a peat based mix be used for 2 seasons?
Hello - Considering compaction, etc can a peat based...
"tapla" root pruning question
Hi Al, I bought another Brush Cherry, and wanted your...
© 2015 Houzz Inc. Houzz® The new way to design your home™