"Compost" and "composting" is a fairly specific kind of process, it's just not correct to call what happens in a landfill "composting" (or contrast with a midden which is just a pile of kitchen refuse, and also not "composting".)
The key difference in composting (as contrasted with just piling up waste material) is the specific formation of a kind of ecosystem within the pile. A healthy compost pile produces it's own heat. It's a concentrated and accelerated form of the natural processes that happen in healthy soil, and in many ways it really is a kind of organism.
Anyway, in a landfill you get your pockets of mostly organic biomass, and decomposition happens, but it tends to be anaerobic (which generally is less healthy from the POV of human metabolism) and the results don't get integrated back into ecosystems rapidly (isolation is not complete, so they ooze back into the water table but they are almost always accompanied by toxic chemical wastes, so it doesn't really help.)
https://en.wikipedia.org/wiki/Aerobic_organism
https://en.wikipedia.org/wiki/Anaerobic_organism
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"Compost" and "composting" is a fairly specific kind of process, it's just not correct to call what happens in a landfill "composting" (or contrast with a midden which is just a pile of kitchen refuse, and also not "composting".)
The key difference in composting (as contrasted with just piling up waste material) is the specific formation of a kind of ecosystem within the pile. A healthy compost pile produces it's own heat. It's a concentrated and accelerated form of the natural processes that happen in healthy soil, and in many ways it really is a kind of organism.
(The dividing line between organism and ecosystem is not sharp. E.g.: https://en.wikipedia.org/wiki/Microbial_mat https://en.wikipedia.org/wiki/Trichoplax https://en.wikipedia.org/wiki/Portuguese_man_o%27_war#Coloni... and so on... )
Anyway, in a landfill you get your pockets of mostly organic biomass, and decomposition happens, but it tends to be anaerobic (which generally is less healthy from the POV of human metabolism) and the results don't get integrated back into ecosystems rapidly (isolation is not complete, so they ooze back into the water table but they are almost always accompanied by toxic chemical wastes, so it doesn't really help.)