_Size of the pile._ It is much harder to keep a small object hot
than a large one. That's because the ratio of surface area to volume
goes down as volume goes up. No matter how well other factors
encourage thermophiles, it is still difficult to make a pile heat up
that is less than three feet high and three feet in diameter. And a
tiny pile like that one tends to heat only for a short time and then
cool off rapidly. Larger piles tend to heat much faster and remain
hot long enough to allow significant decomposition to occur. Most
composters consider a four foot cube to be a minimum practical size.
Industrial or municipal composters build windrows up to ten feet at
the base, seven feet high, and as long as they want.
However, even if you have unlimited material there is still a limit
to the heap's size and that limiting factor is air supply. The
bigger the compost pile the harder it becomes to get oxygen into the
center. Industrial composters may have power equipment that
simultaneously turns and sprays water, mechanically oxygenating and
remoistening a massive windrow every few days. Even poorly-financed
municipal composting systems have tractors with scoop loaders to
turn their piles frequently. At home the practical limit is probably
a heap six or seven feet wide at the base, initially about five feet
high (it will rapidly slump a foot or so once heating begins), and
as long as one has material for.
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