how much organic matter is lost in the composting process?

The short answer: all of it.  You can compost it, and then the breakdown continues, and with rich microbes on top of a rich soil, in time, 100% of the organic matter will be gone.  All that will be left is the "ash" that is not the organic matter.  

Most of what was there was:

  - water
  - nitrogen
  - carbon

Water re-joins the water cycle by going down to the water table, or up into the atmosphere.

Nitrogen has a few hundred paths of denitrification.  Simply off gassing into the atmosphere (remember, the air we breathe is 78% nitrogen) is just one way.   Being chomped on and digested by critters is another.

Carbon gets chomped and digested by oodles of organisms too.  And it also gets to find it's way back into the atmosphere (CO2 and CO being the most popular).

20 years ago I made a LOT of compost.   I remember making compost piles that fit into my 4x4x4 bins.   They started off heaping WAY out of the bins.  About 90 cubic feet.  HUGE!   And I would wet them, and turn them about four times.   They got so hot and steamy.  And I ended up with a small pile that had fully cooled no matter how much I turned it.   It fit into the 4x4 footprint of the bin easily.   Running a lot of guesses and math, I would estimate that the final product was about 10 cubic feet.   I had a small screen to fish out the bigger rocks and sticks.  I think I ended up with about 8 cubic feet.  

I also think that as I was gathering materials for my big compost pile, I think my raw materials may have shrunk 10% to 20% before my compost pile was even built.  

So I think I started with about 100 cubic feet of raw material, and I ended up with about 8 cubic feet.

But if it took a few months until I used it, it would shrink even more.  

And then when I place it where it was going to do it's magic, it would, of course, shrink further into oblivion.  

So, when talking about all of this, I've always been really quick to say "95% loss".    Which is what I wrote in my new book.  Which went in front of a lot of reviewers.   Many didn't seem to object to this number, but a few did.   So I backed off to 90% and went about my day.  

Newer reviewers are now objecting to "90%".  Saywhat?  No way!

So shawn went on a research expidition to find some white papers that would carefully measure volume and maybe even mass.  Not much out there.  There was one document that mentioned volume and mass, but their composting seemed .... weird.   7 days of composting.   Composting just one material.  There were some that did 100 days of composting (should be plenty) but with just one or two materials.  No mention if the pile was turned - or what they considered "done".  

There were some other anectodal reports that measured loss by height.   You can lose 95% of your mass and just 65% of your height.  Imagine a six foot tall pile and a 2 foot tall pile.   The six foot tall pile has 20 times the volume, but only 3 times the height.

My plan at this point is to leave "90%" and reference this thread.   Maybe over the next few days, people can share their recollections.   And over the next few years, people can share actual pics and measurements.

Thats hard to quantify. When i filled my bin with yucca leaves it easily fell by your number or more. Think about all the air with 500 yucca plants in a pile.

Shrinkage is shrinkage. It's gonna happpen whether a person agrees thats its happening or not. Is there a part 2? Do THIS (bury it?) and the shrinkage is less?

wayne fajkus wrote:Do THIS (bury it?) and the shrinkage is less?

Move to the desert, where organic matter tends to mummify rather than compost.  Then you can keep it "forever."  It won't grow anything in that state, of course.....

I have to agree with fearless leader here, 90% to 95% loss of mass volume seems spot on.
I compost mostly manures mixed with lots of straw, twigs and leaves.
A 4 foot cube will reduce in 90 days to a 4x4x2 foot area, the soil under this mass will be totally awesome soil, very soft, full of worms and with a microbiome that is hard to measure because it is so stacked with bacteria, fungi and all the other good critters that it is very hard to count them.
Because of this I usually build heaps where I want to plant once the compost is ready, most of my compost goes around the orchard trees, except for the small amount (2 inches) that I leave for mulch on the new garden plot.
Over the years I've tried lots of methods and finally settled on this one for best fit all around.

If you use a scale and weigh your starting heap and your finished heap, the mass retains about 70% by weight dry, but volume shrinks tremendously no matter if you turn it or not.

I did a side by side way back in 1983 with two identical heaps that were 4' cubes, one turned and the other left alone.
Starting weights were 115.76 lbs. and 115.78 lbs. end weights were both 80 lbs.
Both heaps were layered with each layer being compressed with a 40 lb. flat tamper, then allowed to spring back for 1 hour before the next layer went on, this was repeated until the cubes were level full.
Layers were 14 inches thick before compressing with the tamper.
The components were; dried cow chips, fresh from a square bale straw, fresh cut grass clippings.
Each layer was misted via watering head and hose after compression.
The trial went on for 90 days with the turned subject cube being turned once every two weeks throughout the trial period.
At the end of the 90 days neither cube was sieved.
cubes were 1 inch thick pine lumber and they were weighed at start and finish with a certified truck scale.

I was actually surprised at the end results, I had anticipated the unturned cube to be less completely decomposed and to weigh more than the turned cube, I was wrong in that anticipation.

Error, I said mass when it should have been volume (the strike through)

Good point on the air, Wayne.  Volume is much more accurate than height, but mass is probably the best way to quantify it, leaving out biodiversity.  I've never quantified it, but I would guess that a compost pile is 50% air and compost may be around 10-20% air; just a WAG.  If we weigh it before and after we can get a more accurate picture.  Strictly speaking, in a lab environment, I'd probably want to look at the mass from a dry matter standpoint.  How much dry matter is in the initial pile and how much is in the finished pile.

Hmm, interesting takes. My experience has been somewhere between 50 and 75% decrease in volume from the starting pile to the finished compost. I also tend to compost with lots of woody chips that I keep in the finished product so that probably helps. I'm not really clear on what the point that Paul is making though. I know that he is not a fan of compost for various reasons but the purpose of compost seems to me to be to take a wide array of waste products and decompose them in a controlled manner so that you can put the biology where you want it without having to deal with as much volume as mulch (which can also be hard to spread in certain areas at certain times of year depending on what's growing there). I don't think that compost is intended to supply organic matter (that would be the job of mulch, green manure, cover crops, companion plantings, etc) so the fact that much of the organic matter is lost to the air isn't a terrible loss in my view.

Mass reduction and C loss was significantly higher in the turned windrows than in the unturned windrows

I can't access the full study. Study
But I've seen similar observations made elsewhere.

Other studies have found N loss is correlated with how many times its turned.

A quick google and people are claiming volume losses of 10-50%
People composting using the Berkeley method claim no volume loss.

It seems to me, if you look at it that way, "all of it" is the answer to how much organic matter is lost whether you compost it or not.  It's just the time frame that is different.  If you put wood in a hugel bed, eventually, it will all be lost, but it will take longer because less of the "good stuff" is available immediately.  With compost, it is lost sooner but more "good stuff" is available immediately.  Both are a trade-off, with something like wood chips being in-between.

 

By instinct I would agree with the "90% by volume" thing, but then what you are left with is much more dense than the starting material. I'm not sure I would be able to put any kind of reasonable figure on percentage of mass lost. And then you have that some materials are inherently wetter, or less dense to start with.

I think the only meaningful measurement would dry-mass before to dry mass after comparison. But then that figure would have little meaning to most people, because it is not a relevant figure for people actually making compost in practice.

Personally, I would just ignore any figures and crack on with doing it. We are getting our chicken next month, and our compost heaps will be moving into their run. We will be moving from nice orderly bins to heaps that the chickens will kick and turn for us. Any sense of "how much is lost" becomes meaningless in that kind of situation.

I remember one of Geoff Lawton's videos where he say no loss of mass for hi 28-day compost piles. I admire Geoff but my composting was never as fast as his and somebody said it was probably because Geoff has lots of interns to do the turning for him:

I do believe David the Good is here on Permies.

I stick to the advise of Mike McGrath and chop-up and process my carbon (endless falling leaves) as finely as possible before adding to my bins so my compost probably finishes faster than David the Good's piles. Hard part is I use garden shears since I don't have a leaf blower/shredder mentioned in the ted talk:

I don't think the amount the volume decreases is really in question. What I think is relevant is where it is all going.

Now, if we are trying to say that a 90%-95% loss in volume is justification for not composting, ever, I think I would have a different opinion.

If it were all offgassing, I might have a problem with that. But I don't think it's doing so, but rather being locked up in soil life, the more mobile living component of which will leave the composting environment for other food sources once there's too much competition or not enough food left, leading to the vastly improved soil surrounding and underneath the composting area.

Also, what of materials that we want to hot-compost for bacteriological reasons, or for reasons of breaking down contaminants?

I definitely agree that it's a more complicated issue than most people realise, and that doing it wrongly can be wasteful. But the Ruth Stout method of ditching scraps under thick mulch isn't always applicable, and has led to rodent issues in the wrong locales, much as how, in areas of high pest populations, even the best-constructed hugelbeet, should it contain edible scraps, will be burrowed into by critters in search of those scraps, or of some place to nest.

I think that, considering what can be gained by composting in the right manner for the situation, this is an important question to look at. I think that a better one might be, "How much organic matter is lost using different processes of biological breakdown?" This might include hot and cold composting, the aforementioned Ruth Stout method, different and staged methods of insect decompositon, including vermiculture, and any other method I have left out.

I would also be interested to know the volume of organic matter "lost" should a pile of woodchips be colonised by fungi and decomposed in open air, which is effectively what happens to damp woodchip mulch over time.

-CK

I've read that at the end of the decomposition process, perhaps 2% of that biomass becomes humus.  The other 98%+ eventually gasses off and returns to the atmosphere where it becomes available to plants to uptake during the photosynthesis process.

What turns hard, lifeless clay soil into black crumbly soil?  Carbon.  Humus is the stable gummy black carbon stuff that remains and does not break down any further.  Humus is relatively stable -- it doesn't break down any further.  Thus, if you are losing 98% or more of your carbon in the decay process, it takes centuries to build top soil.  When you think about it that way, how many zillions of tons of biomass did it take to create a barrel of oil?  The quantities are staggering.

So while compost does dramatically increase soil carbon quickly, within 5 years, most of that has gassed-off.  You've got to repeatedly add more and more every year.  In nature, this happens through the growth cycle of annuals and perennials, pumping carbon into the soil via the root network, and dropping a new layer of leaves and spent plants onto the soil surface.  A year later, 98% or more of it is gone.  Thus, the goal of composting has to be more than increasing soil organic matter.  It's about feeding the biologic life, predominantly the microbial bacteria and fungi.

Perhaps a metaphor would be illustrative.  Think of compost like a tent.  You set it up, people live in it, and good stuff happens.  People sleep in there.  They eat meals, play cards, make music, make love, and teach their children funny camp songs.  The tent makes camping possible, but it's not the end-all-be-all.  No tent lasts forever.  After a year that tent will be spent and used up.  It'll have holes in it, the zippers will be broken, and junior threw a flaming marshmallow into the side of it and burned a big hole into it.  You need a new tent every year so that the good stuff that happens in that tent can continue.  So even while the great majority of compost eventually gasses off, while it remains, it is housing life.  Compost becomes the tent in which microbial life does all its good stuff.

I like your reasoning, Marco.

I also wonder if composting in the presence of biochar, or rather charcoal becoming biochar, results in the absorption/adsorption of materials that are otherwise lost to the atmosphere.

-CK

Chris Kott wrote:

I also wonder if composting in the presence of biochar, or rather charcoal becoming biochar, results in the absorption/adsorption of materials that are otherwise lost to the atmosphere.

Absolutely.  Biochar also becomes a "tent" in which microbial life moves into and lives.  But unlike compost, it doesn't deteriorate.

However, microbial life eats compost, whereas biochar isn't easily digested.  Both are needed.

Perhaps a better metaphor would be that of a reef out in the ocean.  Biochar is a reef that attracts life and offers protection and the opportunity for the multiplication of life.  Compost is the food that floats past the reef, feeding all the life that makes its home there.