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Why Compost Warms Up--Thermodynamics

Daniel Roe
Poster: Daniel Roe @ Sat Jan 17, 2009 4:04 pm

I was looking over the stats for this site, and apparently it gives me a list of things people searched for to get here. I see here that a surprising number of people turn to us for information about why their compost is heating up, and what that has to do with thermodynamics.

That's really weird, but I happen to know the answer (is that weird too? I don't know). I'm supposed to be studying hundreds of pages of pathology notes at the moment, but I think I can procrastinate just a little bit longer to answer this incredibly important question.

Every chemical reaction can either require energy or give off energy. For instance, making carbon dioxide and water into sugar and oxygen requires energy (when plants do this, they use sunlight and it's called "photosynthesis"). Most other organisms besides plants turn the oxygen and sugar created by plants back into carbon dioxide and water, which actually gives off energy.

Each process is long and convoluted, but the laws of thermodynamics state, among other things, that the conversion of energy from one form to another is never 100%. While none of the energy is ever destroyed, some of the energy gets misdirected into heat. This is why a perpetual motion machine--a machine that moves constantly without stopping or requiring any outside energy for all eternity--is impossible. Some of the energy of the machine will be shunted constantly into undesirable heat, which the thermodynamic laws' authors call "entropy."

So how does this relate back to compost? All living things which eat sugar are going to convert that sugar into energy used to power other chemical reactions. That energy is packed into a little molecule-sized battery known as ATP. That ATP plugs into other molecular machines (enzymes), dumps its energy into them, and then ejects. Some of the energy that goes into forming ATP and in the process of dumping ATP's stored energy into those machines turns into particles which fly out in all directions (infa-red photons AKA "heat"). Some warmth is necessary to keep atoms moving around so life can keep on going, but much of the heat created by the organisms is unnecessary, and simply due to the impossibility of a 100% exchange of energy from one form to another.

The bacteria and/or fungii are unlocking the energy stored in the sugar and other molecules of the compost and using it to build other molecules. Some of the energy is needlessly bled off, creating heat. They don't really want to be doing this, but they can't help it, because it's actually impossible due to avoid, which is what the second law of thermodynamics states.

If you have a problem with your compost heating up, you're kind of SOL. These little bugs have don't like it either, but if millions of years of evolution can't solve the problem, you definitely can't.

Update: Hank has pointed out that the heat created by bacteria in compost actually ends up killing them! This is apparently a necessary part of the composting process. Shows what little I know about gardening.

As a side note, I can actually think of one living thing that creates heat for the sake of creating heat. Infants are born with small patches of special fat cells on their neck and other places. These fat patches are different from other fat cells, and when looking at them you can actually see that they are brown (fat is usually yellow or beige, this is Brown Fat). Like most cells, Brown Fat cells have mitochondria, which are little chemical converting doohickeys that float around in the cell and convert whatever usable crap floats into them. The mitochondria in Brown Fat cells, however, are defective by design. They basically take the complex chemical conversion system and "short it out." Chemical "A" turns to B, B and B is thrown away when normally it would be used to make chemical C. The lack of chemical C tells the mitochondria to turn more A into B, which creates a lot of extraneous heat. In the same way you can short out a battery by connecting the positive and negative poles together, these fat cells short out this chemical system and create heat (also like shorting a battery). It's thought that these fat cells help keep the newborn's brain warm when it is most vulnerable, which is the first few days after birth. By the end of those few days, all the fat in the brown fat cells is burned and they die.

Well I hope this explains all that for you strange Googlers.

Keywords: Science  Compost 
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Hank Sat Jan 17, 2009 4:23 pm
Maple, alder, elm....

This is a very astute post

Note for strange Googlers : you may think it's amazing that it's the bacteria themselves that are causing the heat increase which we desire primarily for the purpose of, you know, killing bacteria. There are two reasons for this :

1) Bacteria are too busy reading 'Guitar World' to communicate with each other -- they're learning the solo to the latest Children of Bodom single when they should be going "hey, Phil -- does it seem to be getting a little warm in here? Maybe we should stop eating for a while.."


And if you have, as Dr R_e says, a 'problem' with your compost pile heating up, you are probably a vegan. Compost needs to peak in the range of 117 ti 140 Farenheit degrees to fully convert to the poison-free humus-like product you want to be putting near your food.
Daniel Roe Sat Jan 17, 2009 5:00 pm
You know, I think it's really awful that bees are exploited to make honey.

In the same vein, I think it's horrific how we use bacteria as our microscopic beasts of burden, only to thank them by fooling them into burning themselves to death! Imagine, you're a tiny bacteria enjoying a fine piece of garbage, when all the while you're heating up until your insides break down and you die!

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Bio: Daniel Roe
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Daniel is Medical Resident from the southwest US. Prior to medicine, he worked in IT as a consultant, programmer, web designer/developer, and technician.

Health, music, economics, libertarianism, computers

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