One important element for spontaneous combustion is that the conditions not permit heat to escape well. Often, here on GardenWeb, folks say that 4x4x4 is really the smallest you'd want a pile to be, if getting a good hot pile is your goal. And I think folks who say that are thinking of temps well below combustion. I agree with josko that spontaneous combustion in a 4x4x4 pile isn't a significant danger; with the materials most of us compost, I think radiation and convection would be sufficient to move enough heat out of the pile to keep the temps well below the danger zone. Now, if you're composting linseed-soaked cotton rags, all bets are off (my sense on those is that you can get spontaneous combustion with a very small volume of linseed-soaked cotton rag!).

Lloyd, you sometimes make very large piles, right? Not just the fantastic cinderblock-bin piles you show us, but also long windrows? (Or am I mixed up and that's someone else?) I'm curious about the highest temps you've seen in your largest piles, if you've ever measured them.

Feel free, anyone, to correct me if I'm wrong =:)

My compost never gets hot enough to worry.

Curt

First, to answer the OP, yes organics can spontaneously ignite.

Jon in Oregon is the dude with the impressive cinderblock bins.

If you can ignore my sniping in this thread, there are two links I posted towards the end that help me to understand a bit about compost fires. Certainly, there are many more threads and links available but those two links are good 'nuff for moi.

I closely monitor the larger piles/windrows for temp and moisture during the thermophilic phase, never met the conditions for spontaneous combustion. 165 is fairly common although I prefer it to be not quite that hot. Even the wire bin got to 162ish but it had cardboard around it and a paper bag cover so convection wasn't as free as it is in the windrows.

Lloyd

It's often seen in hay barns and piles of wood chips.

My take on that composting "flower pot" fire is that someone dropped a cigarette or smoldering match into the pot, and the plastic of the pot was what did most of the damage, because there's no way a flowerpot that small is going to get hot enough to catch fire.

My uncle lost a barn to hay spontaneously combusting, so it does happen.

Coal isn't compost.
Its a well known fact that when large areas of what was once garbage dumps heat up, they can, in fact, begin to burn. Such burning can last for days, weeks, months...and local fire departments have had to pour thousands of gallons of water into such grounds. Sometimes, chimneys can be put down...and the gas from the composting ignited on purpose. That is a way to prevent such gas from erupting in a more unmanageable way and possibly exploding.
But that is a rare occurrence....very rare.

Certain materials do heat up and given certain circumstances can self-ignite. But a compost pile.....not a chance!

Lloyd

Yeah Babyyyyy ;-)

That's what I'm talking about ! !

February 10 filled Cinderblock cavities with concrete.

March 20 Found some leaves now I need some greens ;-)

April 6 I found some greens... YeeHaw ;-)

April 7 more greens ;-)

April 14

April 22nd

April 30th

I realize the above is just an opinion however it CAN happen - enough so that precautions are published on how to avoid such an occurrence. And there have been numerous scientific studies on just this topic, so I'm not sure if it is necessary to encourage too many more via government grants :-)) To be realistic, it doesn't happen very often in a backyard compost pile but only because the conditions necessary for spontaneous combustion have not been satisfied:

Recipe for spontaneous combustion in compost --
Key conditions:
�biological activity
�relatively dry materials
�dry pocket
�large, well-insulated piles
�limited air flow
�time for temperature to build up (150-200F optimum for conbustion)
Other factors:
�non-uniform mix of materials
�poor moisture distribution
�difficult to know temperature throughout a pile
�lapse or oversight in monitoring

It was not a compost pile, but I am personally aware of organic matter causing a house fire. It happened to clients of mine who stored an opened bag of alfalfa meal on their front porch. The top of the bag was folded down to limit the amount of air but sufficiently loose to allow moisture to penetrate. The combination of moisture encouraging the biological activity in the decomposing meal to heat up sufficiently to combust combined with a small but adequate source of oxygen to fuel the combustion resulted in a total structural fire.

And dotty, if you do a Google Image search you should be able to turn up a couple of decent photos of a combusting compost pile. In fact, the first one that turns up under "compost fires" is of my own local commercial compost facility burning last summer. True, most of the photos are of large commercial operations undergoing combustion, but not sure too many home composters have the presence of mind to photo theirs going up in flames.......too busy attempting to put them out!! :-)

This is my friend Elmer, we are going to fly down the Rogue River Yee Haw ;-)

KissinTazzy: There have been some very large Compost burns, but what most people have never seen was a "backyard" compost pile burn.

Lloyd

Spontaneous combustion is a strange thing. I suspect it begins on a microscopic level. It only takes a tiny volume of material to start feeding on itself to get it started. An itty bitty spot in the maelstrom where the conditions are just right for a few minutes. You couldn't stick a thermometer in and measure temperatures capable of combustion though.