I've seen this happen may times in the past with hot piles and it's my understanding that the material has heated to a degree that it in fact turns to ash and all that's left in that layer is the mineral material. In other words, the carbon compounds are oxidized and ash is formed as though you've burned the material in a furnace. You effectively lose the organic matter value of the material that turns to ash and it's resultant positive effects on soil structure and microbial health. Therein lies the need to closely monitor temperatures and turning of the pile to maximize the amount of finished compost versus unmonitored progression of the process and overheating and resultant ash.

In order to have ash you would also have charred vegetable matter in with it. I have never seen this, but I have seen my compost full of gray green fungal spores such that I have to turn it standing up wind or with a mask on because of the cloud it forms.

the material has heated to a degree that it in fact turns to ash and all that's left in that layer is the mineral material.

For this you would need rapid oxidation. Daaaaaaang - that was one hot pile the OP made there!

Dan

PS: fungal

I still believe that actual ash can result from high-temperature composting under conditions of low moisture and a pile that is large enough to insulate itself. Spontaneous combustion can be the ultimate result of the over-heating of organic material in a heap. Please enter compost and spontaneous combustion into your search engine to learn about it. In a lecture by Elaine Ingham two years ago that I attended she spoke about the need for constant temperature monitoring as a guide to compost turning in rapid compost production in order to avoid ash production and the subsequent loss of the beneficial effects of the finished product.

Material in a compost pile can get hot enough to produce something that looks like ash, and that is called flash. The material looks as though it was burned in a flash and while it is that ashey grey color still looks like what it was originally. Most often this happens, in my experience, just at the outside of the middle of the pile where digestion is most active.
I have seen that many times and still do not know just why it happens, because even though I try to make each pile of compost the same there are still, and always will be, variations and sometimes it was there and others it was not.

Since it is in the top of the pile it probably is not due to heating. That would occur more centrally. I suspect that it is actinobacteria.

Here is a link that might be useful: Actinobacteria

Let's go back to what the OP actually wrote.

First, the temperature at which most compost constituents oxidize is certainly over 200ºC. I challenge anyone to find credible examples of normal residential compost piles heating up to these temps and combusting spontaneously. Come now folks.

So from the info given the pile was cooling down and the critters were doing their thing after the heat subsided. I'm willing to listen to how bacteria caused the ash-like layer. Or which fungus caused the ash-like layer. And it is a critter doing its thing. When I come across stuff like this I think to myself: 'decomposition process'.

Actual ash? Sheesh. Think about it.

Dan

Thanks to all for the different opinions. Always something new to learn on this forum. I'm thinking since this pile was very well mixed (I use the cement mixer) and heavy with wood fibers it's likely a result of fungal or bacterial activity. I'm also hoping it was not a result of high heat as that could be a disaster in our forest. I'll have to figure out how to manage compost piles when we are not here to tend them.

Just turned over a two week old compost pile made of fresh grass clippings and sawdust. There was an ashy looking layer around six inches down from the surface and the pile was hot and steaming (but not hot enough to burn the hand). So-o-o-o I wouldn't think it really is ash but a byproduct of bacterial action. It was dry and powdery although the surface of the pile was moist and the center was moist and quite warm. Mixed it all up and I'm sure it'll continue to heat and break down. Another pile of loose grass clippings about a week old was almost entirely white all the way through even though it rained all last night. Gotta be a mold or bacteria IMO.

Thanks jbann23, that sounds the same as what I saw in mine.

In my search on the net I did find a report of a man who died after mixing his compost and inhaled dusty stuff. Mine didn't seem to become airborne altho I didn't mix it much but it's probably a good thing to avoid breathing in any fungal spores if possible.

I get those grey dusty spores whenever I compost my grass clippings. Definitely looks like ash and it will give off a cloud of dust when the pile is stirred.

As a boy living in rural Virgina I used to ride my motorcycle down a remote trail where someone had been dumping sawdust. They had a very large pile, perhaps 15 feet tall and 40 feet wide, It had been sitting there composting for a number of years.

A couple of times I rode by and noticed a little smoke coming out of the pile but didnt think much of it. A few weeks later it really ignited and the FD was out there trying to extinguish it.

I dont think spontaneous combustion would be possible in a backyard compost pile since most of our piles are much smaller and cooler.

Here is a reply to the question of ash versus fungal end-product in a compost pile that I received from a professor at Virginia Tech who specializes in compost:

"Compost could (but rarely) get hot enough to smolder
and ash. Fungi as you've described occurs much more often."

It's mold spores and dry fungal mycelium, that's all.

You can't "ash" organic matter in a compost heap unless it's a really HUGE one.

Luckygal related,

"In my search on the net I did find a report of a man who died after mixing his compost and inhaled dusty stuff. Mine didn't seem to become airborne altho I didn't mix it much but it's probably a good thing to avoid breathing in any fungal spores if possible."

Sounds more like a heart attack to me.

I've inhaled plenty of organic aromatics while gardening, and no it wasn't that stuff because the smoke bothers me, with no ill effects. There have been plenty of chest pains, though, from the exercise.

If someone finds me dead, just turn the compost pile over the body.

Terran

I've read about working w/ compost to be quite beneficial, puts in a few bacteria that we don't have any more due to our love of antibiotics.

This may really be something to take seriously. I know I'm not turning overly dry compost if clouds of dust arise.

Here is a link that might be useful: Aspergillosis

Oh man! We aren't going to be able to Gump (Forrest) it on this one? Instead of "One less thing" there's one more thing to worry about. That's not good for the ticker.

Terran

Terran---- I've seen plenty of organic aromatics in my days but I did not inhale----

Bill Clinton

I frequently get this grey stuff (it ain't ashes in my piles). Mix, add some moisture, it's gone, not an issue.

I also see it a lot in the bags of grass clippings that come out from town, maybe as many as 50% of the bags if I don't get them emptied within a few days.

Lloyd

P.S. JB (chuckle)

Makes me wonder which comes first, a sense of humor or the desire to compost. Must be linked. :-D

My uncle died from a fungus from decomposing material. Hay. Some called it a form of "farmers lung". The doctors called it something much more complex. The lungs provide a perfect wet atmosphere and before you know it the fungus takes off. I always try to avoid breathing in the dust and sometimes wear a mask.

I have never had a hot compost pile that didn't get crazy with actinomycetes. I thought it was one sure sign you're doing it right.

We homeschool our kids, so we do science in the backyard. Yesterday, when I turned my week old pile, I also had my 7 year old take down observations of a compost pile (outdoor temps, compost temps, appearence, etc.). Learning how to record observations is part of the 3rd grade requirements. One thing that he noticed was that just under the surface it looked like the pile was infused by powdered sugar. The temperature was 140 degrees. He noted that the "powder" is only present in the hot spots. Yes, it's actinobacteria. It's funny because ash never crossed his mind, but sugar sure did. He said he thinks it is a "fungus or mold". I told him that due to the heat, smell, and the fact that that it's powdery rather than strung together (like fungal hyphae), I think it might be a particular bacteria and told him to look up thermophilic actinomycetes later and we can do a lesson on that separately.

Is the fungus dangerous for plants? is it ok to spread around my cherry tree which is already suffering with fungus?

No, it is not dangerous to plants. The type of fungal organisms found in compost (and mulches and sometimes potting soils) are saprotrophs and feed on/digest only dead organic matter, not living plant tissue.

I know I'm late to this one but dusty material like this may be spores of some kind - ideal way for them to spread further. Which doesn't change any of the conclusions, you don't want to breathe any kind of dust like substance, but harmless for plants and compost.

Got a very dusty residue from some slime molds on a pile recently (after they'd dried out fully).

You all need to know this isn't just a compost issue. I've replaced dozens of grass spots in my yard because of grub damage and after prepping the spots for seed I can dig in some areas of bare dirt and get a cloud of ash and smoke. Literally l think the heat will burn the exposed soil. I hate it, it continues to return and l over water. Taking care of a beautiful green lawn is pure hell, from kids to grubs to dog pee to mushrooms and ashy soul. FML