Very unlikely, at my compost facility we had to watch for fire constantly but we had windrows of organic matter 10-12 feet high. You have a anaerobic process and some way for the heat to build up without a way to escape which will happen if there is a heat sink like a piece of metal in the pile. Having this type of situation in a small container will happen about 1 in 500,000 or more. A pile of leaves in a bag or up against a concrete wall or basement floor might create the proper conditions where there is enough moisture in the middle for the composting to keep working and the outside dries out, very hot in the center and dry and very combustible outside.

I wouldn't worry period, but I don't worry about much.

Anna, we've heard about this happening in NS twice in the past two weeks. The first was a report about a bag of potting soil combusting in somebody's basement, then another lady called in saying the same had happened to her. It's very hard to believe but who am I to contradict them or the fire departments that responded. Maybe it's a problem with particular batches of potting soil? That should be investigated.

I had never heard of this happening, but I googled spontaneous combustion potting soil, and apparently it can happen with potting soil stored indoors under certain conditions. A related danger is that people don't realize that potting soil is flammable. They think that since it's dirt, it'll make a good ashtray, and a smoldering ember will ignite the organic matter in the potting soil.

Coco Coir, rather than Core

Typical ingredients of potting mix we find here, like Pro-Mix, are: Sphagnum peat moss, perlite, vermiculite, possibly nutrients, possibly limestone. The surprising thing (to me anyway) about the fires is the relatively small amounts of potting mix involved.

For potting soil to spontaneously combust certain conditions need to be present, the right material and just enough moisture. Generally potting soils do not have moisture in the bags, so the homeowner must have added some to that potting soil. So with just enough moisture in a closed bag anaerobic digestion starts and generates enough heat to cause the product to combust. The same thing can happen with slightly damp wool, cotton, flax (linen), or hemp clothing, or any other natural fiber piled up some.

I question the "spontaneously combusted" part of the event. Any spark from any source could lead to this "spontaneous" combustion. I find it more likely that some outside source of ignition entered the dry bag of potting soil. I live in Florida, so I have no idea what types of appliences people have in their basement, but any thing with a plug or pilot light is suspect.
When ever I weld in the garage, I refuse to leave the house for like a day, just to assure myself that a stray spark didn't make it into a flammable nook or cranny, and is smoldering slowly, waiting for the right time to flare up. A imagine fire could start hours after suck a spark under the right conditions.

Agree with Vance. I also think it's less likely that it was heat from composting in just a bag or pot of soil. A more likely scenario is that the potting mix evolved a flamable gas (methane etc.) which was ignited at a distance in a still basement (heater ignitor, etc.).

Still air in a basement is a good place for a vapor to creep across the floor, and a heater sucks air in to support fuel combustion.

I personally know of such a case of "spontaneous" combustion - in fact, I was recently called in to help design their new landscape after the fire that destroyed their home and much of the existing landscape. The culprit was a bag of alfalfa meal, opened and sitting on a covered porch. Moisture entered the bag, the process of decomposition began and sufficient heat was generated to create combustion. Between the OM itself and the paper-based covering of the product, this was enough to ignite wood siding and the rest is history. Confirmed by the fire inspector.

So yes, it can and does happen - may not be the most common phenomenon, but enough to warrant some concerns. Anything that contains unfully decomposed organic matter and suspect moisture is at risk, specially in a restricted environment.

The only way I could see it combusting spontaneously is a very dry soil mix and an electrostatic spark. That happens in large grain holders, I thought

From what I read, it does occur from sparks and sources of ignition (especially when people think it would make a good ashtray).

One source I found was in Colorado, and they pointed out that because of the moisture needed for the potting soil to build up enough heat to spontaneuosly combust, it happens less often in Colorado than in more humid climates. If most of the fires were due to sparks landing in dry material, I would expect Colorado to have more problems, not fewer.

Potting mixes are often pre-moistened.
Here's one of the articles turned up by a google search of potting+soil+fire

Here is a link that might be useful: pei fire

PaulNS - that is exactly the story I had seen on the news - and what prompted my question, evoking many interesting responses. Thanks everyone.

Ann

If a potting mix does not have moisture naturally then moisture is added unless you are dealing with some specialized mix.

Maggie, if that was directed at my most recent post, I wasn't clear in what I was saying. The reason the potting soil is dry in Colorado is that, although it started out moist, it dried out after the bag was open because of the dry air. I should have been more clear.

Spontaneous Combustion occurs without any outside source of ignition. Any organic material could, potentially during the digestions process, develop sufficent heat to combust spontaneously.

Here is a link that might be useful: Spontaneous conbustion

bpgreen, I produced and bagged potting mix and other soils and mulch in the millions of bags. I only say what we did which was similar to many other baggers of organic matter. It is very difficult to bag dry material, if it was dry we added water.

i dont see how compost piles, which reach what, 170F, can lead to the temp of combustion (which is what, 400-600F?)

Hooray, search the web there are plenty of detailed explainations.

I apologize in advance of anyone is offended by my tone.

IMPOSSIBLE.....that would be a fairy tale since combustion can only take place when the material itself lends itself to take fire. A drying liquid such as linseed oil is usually a good example when it is soaking in a cloth that is allowed to sit in a crumpled state which the air is excluded. Soaking the cloth in water and laying it out flat then defeats any hint of combustion.

There is hardly enough heat in a compost pile to warn off a mouse from entering....how is that to ever start combustion.

Compost piles have been known to generate enough heat to combust. That would be very unlikely to happen in a small pile that somebody would have at home. There was a thread here a year or two ago about a large commercial pile that started burning, and if I remember correctly, there were pictures along with it.

The original topic, though, wasn't about compost piles catching fire, but potting soil. When I first read the post here, I thought it was likely an urban legend, but I googled it and found some credible articles about it.

I had 500 semi loads of mulch stored in an underground quarry. I had an arson fire underground which the fire dept would not fight. It burned for 6 months.

Why wouldn't the fire department fight it? Was it too dangerous because it was in a confined space?

It's awful that somebody would do something like that. I hope you had insurance coverage for it.

This was in an underground quarry large enough to run train tracks in and we drove semi's in to load and unload. They were afraid of the fumes in closed quarters, the owner of the quarry told the fire dept that we had all types of dangerous chemicals stored in the quarry.
To the insurance question, 500 semi loads was worth about 1/2 million. I learned a new insurance term called co-insurance, which you should know and understand if you are in business. Bottom line is I got $8,000 and the quarry sued me and got a million.

Many years ago when I was in High School, the cotton rags soaked in linsead oil that the wood shop students used had to be put in a closed container and put outside the school at night because of the potential for spontaneous combustion. Many woodworking shops have burned down because that simple precaution was not taken. People that do not believe that this occurs have not taken the time to learn material they should be familiar with, this happens far more often than you hear about and is really so common that it is not news unless you have a very large pile of manure or someone dies in the fire.
Give any form of organic matter the right conditions and it will ignite without an outside ignition source.

Maggiemae, an underground fire is one of those happenings that one cannot be sure to ever extinguish...so they usually, if the area is not in any direct danger, they let it burn.

It is a very common occurrence that community dumps....often will have the trash build up heat....and develop methane from the decomposition of such a large source. Usually they try to chimney away such gas by opening a hole and placing a form of chimney there.
Sometimes, the chimney is not sufficient....and fires do result. To ever hope to put sufficient water down there would involve such amounts that possibly the community wouldn't be able to generate. It may also involve drawing such amounts from nearby wells---which also can affect community wells from which water is drawn for community use.

So....like many fires that involve potential problems afterwards if the fire was put out...they let them burn.

And six months burning....that's a small fire. We had one up here in a Toronto suburb...a former dump location...it was allowed to burn for 3 years.
This is a very good example of why people should not ever consider burning out a rodent....skunk, racoon...etc...
or burn out a stump that has its roots going deep underground. Such fire can last for a very long time....and how would anyone really know how it might show up someplace you wouldn't want it to.

"And six months burning....that's a small fire."

Guess that is true by comparison but it was my life's work including my house which was on the loan.

I apologize in advance if anyone is offended by my tone.

A small pot of soil set on fire? Somebody put their cigarette out in it, or some other spark set it alight. That is not spontaneous combustion.

Spontaneous combustion of compost is relatively well-documented but not widely appreciated. It is caused by a combination of high temperatures generated by the composting and a flammable mixture of organic vapours, also caused by the composting, which can ignite at those temperatures, then setting fire to the solid organic material. It is very likely that the majority of compost fires are actually started by an external spark or flame but the proportion is hard to establish.

Shrubs, compost fire is also caused by the heat of composting which then cannot escape and gets hotter and hotter finally reaching the point where combustion occurs.

I had a neighbor lose his farm after the hay in his loft caught fire...destroyed the barn, his entire milking herd, much of his young stock. The cause was listed as spontaneous combustion, both by the fire marshall and insurance. Somehow, there was just enough moisture in his hay to smolder and smolder and smolder and smolder and eventually ignite.

I believe it was in Missouri last year or so a large pile of manure burned for months and months. Yummy. I'll leave it to others to locate this one on the internet.

I've yet to have a soil mix catch a flame, nor is it one I have heard through the grapvine, although I certainly believe it could happen. All it takes is the right set of circumstances.

I recently encountered a freakish fire incident the other morning, 3 a.m. to be exact, when my smoke alarms went off and woke me up. My son looked around to see where it was coming from and as he opened the garage, flames and smoke were billowing out. He reached in and pulled out a closed plastic container, one you buy from walmart or kmart and it had flames shooting out of the lid and side. We opened up the lid and threw water on it, lots to extinguish the flames and dumped out the contents. Inside were a few baby outfits, two coiled ropes-old, old electrical wiring attached to electrical boxes, pair of crocs, garbage bag-empty or at least I think it might have had the baby clothes in it, ceramic pot that had potting soil-dried out. The fact that the most burnt item in the container turned out to be the ceramic pot and another item, old white electric cord with exposed wire led me to believe it was the source of this fire. The container had melted where the pot was and the bottom of the pot was gone. Nobody was smoking, everyone was asleep yet my husband seems to think otherwise. I like forensics and believe that given a source such as what was in the potting soil material and moisture causing it to heat up, it is likely that's what happened. Not sure about the old electric wire if it had anything to do with it. Anyone want to take a guess?

I seriously doubt you can blame the pot or its soil. Potting soil - or garden soil - will not spontaneously combust. That is - infrequently - caused by the high heat generated by the decomposition of organic matter (helped by some moisture and a confined space) and there is just not enough decomposable organic matter in either potting soil or garden soil to create such a situation.

I can't find anything in your list of contents in that bucket that should be a big risk. Potting mix can heat up (i.e. compost) IF it has moisture to get cooking and sufficient volume. If that stuff was sitting there for some time it was likely dried out (or dry when it was put there). I'm not seeing it. But weird things happen.

Peat moss is the first stage of organic matter being made into coal and peat is used as a fuel in many parts of the world. Ireland uses peat moss to fuel its electric generators. Fires in peat bogs are pretty common occurrences.

Potting mixes are made of various organic materials, shredded bark, peat moss, etc., which are combustible materials, they will burn and will spontaneously combust given the right conditions.

http://environxsolutions.com/about-peat-fires/

But fires in peat moss are NOT pretty common occurrences. In fact, I have never heard of ANY spontaneous combustion from any sort of potting soil. You would need a very refined set of circumstances for that to happen and by your description, you didn't begin to come close :-))

Thanks for your comments! Although this happened with the lid on and I’m not going to dismiss the science part of combustion because as an investigator the source of ignition was the pot with soil and it erupted in flames from the garage door going up. It’s crazy and I’m glad I’m alive to tell it!

If that actually happened, it's a one in a billion event.

BTW on the topic of charcoal (this was new to me so I looked it up), freshly made charcoal is most likely to spontaneously combust within a few days (after cooling it down - otherwise it wouldn't be spontaneous). Apparently after awhile the surface oxidizes enough in the air (or something like that I assume) that it isn't as reactive. So don't worry about the bag of briquettes in your garage.

It just happened to me! Went out to my greenhouse at 5:30 am and it smelled like smoke and was hazy. I looked for the source and my bag of ProMix soil was smouldering away. The plastic bag was starting to melt, and the top layer was already ash. Pretty scary.

WHAT?!

I thought maybe the OP had some sort of oiled rope.

I'd argue that the main takeaway here should not be debating whether spontaneous combustion of potting soil is possible or extremely rare or impossible (although fire marshals say it is, and it is pretty well established it exists in other circumstances/other materials, so can't be ruled out).

The main takeaway is: some types of potting soil (esp peat) are extremely flammable and should not be stored in places where they would cause fire damage, even in a garage or greenhouse or shed. Whether combustion is from some unknown spark or biological or chemical reaction or other, it can burn - even if it seems almost impossible.

From some experience with peat - it can be very flammable; it can smolder unnoticed for quite some time (with not much, almost no, smoke showing); heat can build up inside due to it being tightly-packed (structurally); there may be unexpected situations like it being at high temperature internally - smoldering - just waiting for enough air to flame up.

It just shouldn't be stored like this, even if likelihood is low.

parnelli13's incident could have been caused by biological decomposition activity - i.e. the Pro Mix was composting. If it was damp and had some undigested organic matter in it, it's remotely possible. Unlikely, since anything the size of a bag of potting soil (or even a bale of peat) is too small to generate or retain that much heat.

Bizarro stuff.

This very thing happened this morning to a pot full of Miracle Gro potting soil that was sitting on the porch throughout the winter. Slow burning-- but very hot and long lasting. It's below freezing and the embers were still burning as I dug through the pot to break it down. Water put it out.

Scary, glad that was outdoors. Good to keep in mind to not store stuff like this indoors. I presume the pot was open to the air? Or did it get moistened?

I cannot figure out what would ignite the burning. The pot and it's contents (potting soil and the roots of the perennial plant that was in it). It must've slow burned all night, burning through the side of the plastic pot,. Luckily it was sitting in a clay dish. I've had porch gardens for many year and have never had this sort of thing happen. It's frightening. And yes, I would never store potting soil in any bag or container inside a home or garage.

Pure speculation: I wonder if ice/ice particles could have caused a lensing effect/concentration of the sun like a magnifying glass to raise heat and ignite the mix.

Written as I marvel at some of the strange aspects of how strong sun in cold temperatures causes odd melting / water run-off. Real-life examples of 'albedo' (reflection of sun): my yard has dozens and dozens of odd sunken leaves, moss, etc in the snow - the darker leaves warming up enough in the sun to melt straight down in the snow, in some spots four inches or more.

Just had a very similar experience. Dry soil with fertilizer in a plastic pot on the balcony in direct sunlight. caugfght fire 40cm big flames took my fence could have taken a lot more if my neigbou didnt see it and told me in time.