First, I'd have to question the need for sterilization at all. Root rot of container plantings is not a fungal issue....it's a water issue. A potting soil with excessive water holding capacity (anything peat based) and too small a particle size (again, anything peat based) will cause drainage to be slow, aeration poor and the PWT high. Roots die from excessive moisture and lack of oxygen.....not a fungal organism.

I'd suggest that rather than sterilizing, you just need to improve the quality of the potting soil. Since we have no idea of what sort of potting soil you are working with, it's hard to make specific suggestions. But you want something that approaches the 5-1-1 mix at the very least....maybe even the gritty mix.

What were the primary components of the soil? I wonder if the problem is the actual physical texture of the potting soil rather than any particular pathogen.

I ask because I have never heat sterilized my potting medium, I don't reuse it, either. My career as a professional horticulturist often led me down the path of container culture, either in production greenhouses or on the 11 acre tree and shrub nursery I started and operated in coastal South Carolina.

I concentrated on the texture of my potting and propagation mixes, using ingredients that created a high level of porosity and rapid drainage with little opportunity for soil borne root rot pathogens to ever be a problem. Again, I never baked or steamed a batch of potting soil, not professionally and never as a home hobbyist (which I am now).

Thanks for getting back to me!

As far as I've understood it, root rot is caused by roots being waterlogged due to overwatering or insufficient drainage, but what really causes the issue is a water mould. This is why changing the wet medium is normally not enough, or, to quote Wikipedia:

"Many cases of root rot are caused by members of the water mold genus Phytophthora; perhaps the most aggressive is P. cinnamomi. Spores from root rot causing agents do contaminate other plants, but the rot cannot take hold unless there is adequate moisture. Spores are not only airborne, but are also carried by insects and other arthropods in the soil. A plant with root rot will not normally survive, but can often be propagated so it will not be lost completely. Plants with root rot should be removed and destroyed."

Also, as mentioned, the soil used is in a big black bag, so it's draining capacity is not really the issue, I'm more interested in the sterilisation part, seeing as I have not had any issues with root rot since :)

Cheers,

Martin

Did you read the first part of the article? "Usually, this is a result of overwatering. In houseplants, it is a very common problem, and is slightly less common in outdoor plants. In both indoor and outdoor plants, it is usually lethal and there is no effective treatment.

The excess water makes it very difficult for the roots to get the air that they need, causing them to decay. To avoid root rot, it is best to only water plants when the soil becomes dry, and to put the plant in a well-drained pot. Using a heavy soil, such as one dug up from outdoors can also cause root rot."

Root rots do not have to have a pathogenic source. And non-pathogenic root rots - anoxia - are far more common than those from an identifiable pathogen (for plants beyond the seedling stage), at least in a home setting. Greenhouse production is another matter and offers a different set of circumstances that provides a much greater risk of pathological infections.

The draining capacity of your potting soil IS the issue!! It is always the issue with container grown plants :-) btw, it being in a big black bag now is not of concern - it is how well and how fast it drains once in the container. If it is the common, heavily peat-based muck that most commercial potting soils are comprised of then I can pretty much guarantee that it does not drain as well or as rapidly as desired and is excessively water retentive. And that is what is causing the rot, not a fungal pathogen.

Of course you are free to do whatever you like and sterilize your soil if you think that is necessary or will help. But unless you change out the soil to one that offers the fast drainage and proper aeration neccesary for healthy root development, you are just as likely to encounter root rot again, sterilization or not!!

I did, but I do not see how that is relevant seeing as I'm using the same soil (that is, from the same batch) for other plants without any issues. I have, so to speak, learnt my lesson and am quite successfully avoiding the problem these days. However, seeing as the article mentions throwing out the diseased plant and soil I was figuring there's something that goes wrong with it once the spores activate, and as it seems a waste throwing away soil I wanted to recuperate it. The aforementioned toxins is also something I was intrigued to read about, but that was just something that piqued my curiosity ;)

Martin, there are various other factors that could be why the soil did not affect other plants in the same manner....larger drainage hole(s), less developed root system (no or limited contact with the PWT), the affected plant was under other stressors, etc. But if you visit the Houseplants, Cacti and Succulents or even the Citrus forum, you will see plenty of evidence as to how the potting soil can influence whether or not a containerized plant will develop root rot. And these are NOT pathogenic causes at all but rather anoxic causes, with excess moisture drowning plant roots and depriving them of oxygen. That is by far the most common reason for root rots in containerized plants beyond the seed starting/seedling stage, not phytophthora or any other fungal issue.

Great, thanks for the feedback! So, basically the consensus is that there should not be any issue recycling the soil in a normal way (that is, through the compost in my case) as long as I keep paying attention to the water retention of the soil when planting, and if I would still want to sterilise soil I should also be prepared to keep the windows open :)

Or do it outside, on a gas grill for example.

Being somewhat of a curious cat (knowing well where I will end up one day) I decided to sterilise and start to dry out both a bit of drying medium and some worm castings separately in the oven to be able to check the difference in water retention and see if any tweaking is necessary. Running the oven at 120 °C did not create much of a smell actually, when opening the door it was similar to what you would get when repotting a plant (although a much warmer feeling, obviously), so I'm thinking that it may depend on the soil composition. Anyway, thanks again for the feedback and happy gardening!

I grew up with my father sterilizing soil in our oven every spring, so, to me, the smell is pleasantly evocative in the same way that the smell of anisette transports me to childhood Easter memories.

I had learned the hard way. Prior to sifting some soil I had put it in the oven at 175°c to dry out quicker. ,Within 2 days of planting some 20+ succulents, most were bare-root, they started to show extreme stress. When I took a look at the roots, they roots were gone, no signs of ever existing. Anything left from below the surface was black*. The plants only planted 24 hours previous still had some roots. They looked like they had rotted, not root rot, more like being damaged from a chemical.

My conclusion is, YES. High heat may make the soil too TOXIC for your plants to survive.

My soil contained peat, perlite, pumice, lava rock, some coir and from a little bit of reused soil. The old soil still had a small amount of fertizer and the empty blue & green slow release capsules.

I think most likely the old cooked fert. still in the soil. Perhaps, but less likely the pumice or perlite had released some chemical from the high heat. (Remember, even new soil purchased planting mixes include a starter fertilizer)

I washed off the plants. Of the plants I was starting from cuttings, I rubbed off the black. Which appeared to have been the epidermis or a layer of the epidermis. I say layer, because there was still a protective coating below what I removed. After damaging a few plants, I switched to just a through rinse.

Most plants survived. In general, succulents come with amazing survival adaptations. I think many plants grown indoors would have croaked.

Not too toxic, per say, but definitely not for use immediately after sterilization until you amend it to contain the good flora/fauna (fungi, nutrients, etc) AND ensure it is capable of normal water retention. Its possible to heat it so much so that water will run right through the medium (and not in the way you want), basically depriving the roots of ANY moisture and/or the basic needs to keep the plant alive. You can, and likely will, bake the good stuff out of the soil/less medium, as heat at that temp doesnt discriminate and blasts everything to sterildom (not a word, but i like it). Its up to you to make your medium full of plant loving, well, needing goods.

Baking used soil in the oven doesnt smell nearly as much as microwaving it! you can bake more at a time, typically, than nuking it.