I'd also be interested in the answer. It could be due to a similar reason where our human digestive system extracts the nutrients from the food we eat but there's always something leftover at the end ;-). When I turn over a cooled down shrunken pile it will re-heat but never get as hot (160F) as the original pile. If I never used the finished compost but rather kept on perpetually turning it over and over and over again then it may disappear eventually though it may take years.

Yes, but of course what comes out of a digestive system will sure heat up a compost pile! That's well understood.

You may want to add the Soil, Compost and Mulch forum. In the mean time, see if this helps.,,

https://compost.css.cornell.edu/microorg.html

tj

I will admit that I used to post on the Soil, Compost, and Mulch forum, but five or so years ago I was gently skeptical of some things I was being told, and I got insulted and harangued by one or two people there who I think were veterans of that forum. I've never gone back.

But thank you for the Cornell link. They do make the point that humus is "relatively stable". And yes, I understand that humus is resistant to microbial action. But it does degrade. Wondering what makes it "relatively stable". But nobody seems to want to explain why. In general, all hydrocarbons degrade in the environment, because of bacteria and fungi.

It should be pointed out that, to some extent, even coal and charcoal biodegrade.

Hard to digest things that remain (like lignins) are why it is deemed stable.

tj

True that lignin is not easily biodegradable, but lignin is a ~20% component of vegetable matter. Much more in woody vegetation. Much less in vegetable foliage. If that's all that was left behind, more than 80% of your compost would have volatilized and departed.

Lignin is only one of many hard to digest components and different plants and plant parts contain varying amounts of lignin. That is why pine bark is used in container soils. From NC State…

“All barks are composed of cellulose and lignin. Pine bark is approximately 95% lignin and 5% cellulose. Hardwood barks contain less lignin (55-75%) and more cellulose (25- 45%). Pine bark's high lignin content causes this material to decompose very slowly.”

The whole article…

https://hortscans.ces.ncsu.edu/uploads/f/r/from_the_51e5b0ae566a1.pdf

tj

As I said, much more lignin in woody vegetation. That's pine bark. I don't have any bark or hardwood in my compost. My compost doesn't contain a lot of lignin. But I didn't know that pine bark was used in container soils for that reason. That's smart, because you don't want your container soils volatilizing and disappearing. I use peat moss and regular compost in my container soil, and it does shrink significantly over time. I have to keep adding to top it up.

But the question still stands. What prevents regular compost from largely just disappearing? To the extent that humus degrades very slowly, why exactly is that? If humus actually contains soil nutrients (and everyone says it does), how are those nutrients delivered if the humus doesn't degrade?

Believe it or not, Wikipedia has a decently detailed synopsis on humus - how and when it is formed, how stable it may actually be and what purpose it serves in the soil and to the plants growing in it. Reads like it might have been written by a trained soil scientist, as it matches the data I learned in hort college.

Humus - Wikipedia

Thank you, gardengal48. Yes I did finally see that in Wikipedia. It says that humus is "protected from decomposition by microbial or enzyme action because it is hidden (occluded) inside small aggregates of soil particles, or tightly sorbed or complexed to clays." I've seen that in other places as well. I just don't know what that means. Bacteria don't like getting grit in their teeth? Wikipedia gives a research article reference to that statement, and I've managed to access the paper, but ha ha, there are NO references to humus in it. Lots of references to "SOM" or soil organic matter, though. It looks kind of as if the soil aggregates actually prevent the microbes from accessing the material in that the pathway to the digestable material is "long and tortuous". That's a little hard to understand. Shouldn't be that hard for bacteria and enzymes to get to practically anything. But again, as I said, to the extent humus is indigestable, you aren't going to extract any nutrients from it. Not that fully composted vegetative material has a lot of nutrients, but still ...

Dan, my own curiosity about composting and humus took me into the field of chemistry to find the answer you're looking for. The decomposition breakdown from fresh organic matter to humus, (and called humification) is as you described, but humus is a combination of humic acids, fluvic acids and humin. Each of these 3 names are generic in nature and each are made up from many different elements, compounds and acids and and a very complex combination to say the least. The formation of these combinations in humus are based on four theories so uncertainty still exists, but analysis has identified most of the basic elements, compounds and acids.

Here is a good paper to read if your you want to dig deeper into the subject and appease your curiosity. "ORGANIC MATTER, HUMUS, HUMATE, HUMIC ACID, FULVIC ACID AND HUMIN: THEIR IMPORTANCE IN SOIL FERTILITY AND PLANT HEALTH. " ENJOY!!

OK, the issue isn't about how humus forms. It's about why it doesn't decay quickly. And, as I've said repeatedly, to the extent humus or humic substances don't degrade readily, it's hard to understand how they can provide usable nutrients in the soil, which Dr. Pettit thinks they do.

Did you bother to read the Wiki article? They explain it very well.

No they don't. I read it carefully and quoted what they said. Did you not bother to read my response? They explained it in a way that I simply don't understand. How exactly does "hidden (occluded) inside small aggregates of soil particles" happen? Soil mineral particles are largely roundish (rather than flatish) crystals. You stack roundish things together and there are gaps all over the place. If I try to hide in a pile of beach balls, it's not going to work that well.

" Soil mineral particles are largely roundish (rather than flatish) crystals. You stack roundish things together and there are gaps all over the place. If I try to hide in a pile of beach balls, it's not going to work that well. "

Not true! Clay - which is present in most soils to varying degrees - is formed in flat planes or flakes. And the article did explain about flocculation and aggregation and how both mineralization and humification regardless of soil type encourage aggregation.

Oh, does clay form boxes around hydrocarbons? Who knew? But again, some real explanation of the geometry of small aggregates of soil, and how they shield hydrocarbons would be appreciated. Clay particles are of the order of a micron in size. So are the bacteria that do hydrocarbon degradation.

It occurs to me that another way of looking at it is that if soil mineral aggregates effectively shield hydrocarbons from microbes, there has to be LOTS more mineral soil than hydrocarbons. Are hydrocarbons really a minority constituent of humus?

It's a very complex process and I'm still having trouble seeing the big picture, so when you figure it out please post the answer. I few years ago I bought a couple bags of humic acid and mixed it into a plot of sandy ground. I couldn't tell any difference but maybe I didn't need it, or I was ripped off with nearly no humic acid in it. There are some selling humic acid with zero or little in the bag so pick your source carefully if interested.

Some people say that "humic acid" is, in fact, just humus. Lots of good things about humus, but not necessarily as a nutrient. I am reading that humus actually has a somewhat indirect role in soil fertility. That is, since humus itself is almost inert, and doesn't decompose much (for reasons at least I don't quite understand), it's largely not a direct source of nutrients. But it is porous, and holds onto nutrients and prevents them from leaching. It also allows air and water to permeate the soil better. It acts to buffer soil acidity and alkalinity, and that can contribute to nutrient availability. So humus is good! It's kind of funny that several references pretend that humus itself is a good source of nutrients like phosphorus, potassium, magnesium, sulfur, calcium, and nitrogen. But that doesn't really seem to be the case. I guess to the extent that humus does eventually break down, it could be considered a minor source of nutrients, but that's exactly the deal with compost. Compost in soil breaks down slowly, and while it has minimal NPK, that NPK is largely not released immediately. In that sense, compost is a weak fertilizer, but by no means an immediate one. Some references suggest that humus is important for the all-important mycorrhizal fungi, but others say that it's the fungi that help make the humus. Kind of striking how much semi-contradictory material there is out there about humus. I saw a marvelous website talking all about how composting leads to "hummus". Maybe if you dig chickpeas and tahini into your garden, your plants will benefit?

As I read further, I've learned that, to some extent, humus actually chemically bonds with some clays, and that bonding is partly responsible for it's resistance to degradation. So one has to assume that pure vegetative material won't turn into humus. You need an admixture of mineral soil. What's a little surprising is that humus produced as the end result of composting is only 1-5% of the soil you start out with. You don't get much. That would suggest that, indeed, a large fraction of the hydrocarbons in a compost pile volatilize and disappear. Of course, a lot of that is just evaporating water. That's my experience. I have a kitchen compost pile that I dump a couple of pounds a week into, and the pile size never really increases that much. All the more reason to harvest compost from your pile before degradation has completely stopped.