The main difference between composting in heaps and natural
decomposition on the earth's surface is temperature. On the forest
floor, leaves leisurely decay and the primary agents of
decomposition are soil animals. Bacteria and other microorganisms
are secondary. In a compost pile the opposite occurs: we substitute
a violent fermentation by microorganisms such as bacteria and fungi.
Soil animals are secondary and come into play only after the
microbes have had their hour.
Under decent conditions, with a relatively unlimited food supply,
bacteria, yeasts, and fungi can double their numbers every twenty to
thirty minutes, increasing geometrically: 1, 2, 4, 8, 16, 32, 64,
128, 256, 512, 1,024, 2,048, 4,096, etc. In only four hours one cell
multiplies to over four thousand. In three more hours there will be
two million.
For food, they consume the compost heap. Almost all oxygen-breathing
organisms make energy by "burning" some form of organic matter as
fuel much like gasoline powers an automobile. This cellular burning
does not happen violently with flame and light. Living things use
enzymes to break complex organic molecules down into simpler ones
like sugar (and others) and then enzymatically unite these with
oxygen. But as gentle as enzymatic combustion may seem, it still is
burning. Microbes can "burn" starches, cellulose, lignin, proteins,
and fats, as well as sugars.
No engine is one hundred percent efficient. All motors give off
waste heat as they run. Similarly, no plant or animal is capable of
using every bit of energy released from their food, and consequently
radiate heat. When working hard, living things give off more heat;
when resting, less. The ebb and flow of heat production matches
their oxygen consumption, and matches their physical and metabolic
activities, and growth rates. Even single-celled animals like
bacteria and fungi breathe oxygen and give off heat.
Soil animals and microorganisms working over the thin layer of leaf
litter on the forest floor also generate heat but it dissipates
without making any perceptible increase in temperature. However,
compostable materials do not transfer heat readily. In the language
of architecture and home building they might be said to have a high
"R" value or to be good insulators When a large quantity of
decomposing materials are heaped up, biological heat is trapped
within the pile and temperature increases, further accelerating the
rate of decomposition.
Temperature controls how rapidly living things carry out their
activities. Only birds and mammals are warm blooded-capable of
holding the rate of their metabolic chemistry constant by holding
their body temperature steady. Most animals and all microorganisms
have no ability to regulate their internal temperature; when they
are cold they are sluggish, when warm, active. Driven by
cold-blooded soil animals and microorganisms, the hotter the compost
pile gets the faster it is consumed.
This relationship between temperature and the speed of biological
activity also holds true for organic chemical reactions in a
test-tube, the shelf-life of garden seed, the time it takes seed to
germinate and the storage of food in the refrigerator. At the
temperature of frozen water most living chemical processes come to a
halt or close to it. That is why freezing prevents food from going
through those normal enzymatic decomposition stages we call
spoiling.
By the time that temperature has increased to about 50 degree F, the
chemistry of most living things is beginning to operate efficiently.
From that temperature the speed of organic chemical reactions then
approximately doubles with each 20 degree increase of temperature.
So, at 70 degree F decomposition is running at twice the rate it
does at 50 degree, while at 90 degree four times as rapidly as at 50
degree and so on. However, when temperatures get to about 150 degree
organic chemistry is not necessarily racing 32 times as fast as
compared to 50 degree because many reactions engendered by living
things decline in efficiency at temperatures much over 110 degree.
This explanation is oversimplified and the numbers I have used to
illustrate the process are slightly inaccurate, however the idea
itself is substantially correct. You should understand that while
inorganic chemical reactions accelerate with increases in
temperature almost without limit, those processes conducted by
living things usually have a much lower terminal temperature. Above
some point, life stops. Even the most heat tolerant soil animals
will die or exit a compost pile by the time the temperature exceeds
120 degree, leaving the material in the sole possession of
microorganisms.
Most microorganisms cannot withstand temperatures much over 130
degree. When the core of a pile heats beyond this point they either
form spores while waiting for things to cool off, or die off. Plenty
of living organisms will still be waiting in the cooler outer layers
of the heap to reoccupy the core once things cool down. However,
there are unique bacteria and fungi that only work effectively at
temperatures exceeding 110 degree. Soil scientists and other
academics that sometimes seem to measure their stature on how well
they can baffle the average person by using unfamiliar words for
ordinary notions call these types of organisms _thermophiles,_ a
Latin word that simply means "heat lovers."
Compost piles can get remarkably hot. Since thermophilic
microorganisms and fungi generate the very heat they require to
accelerate their activities and as the ambient temperature increases
generate even more heat, the ultimate temperature is reached when
the pile gets so hot that even thermophilic organisms begin to die
off. Compost piles have exceeded 160 degree. You should expect the
heaps you build to exceed 140 degree and shouldn't be surprised if
they approach 150 degree
Other types of decomposing organic matter can get even hotter. For
example, haystacks commonly catch on fire because dry hay is such an
excellent insulator. If the bales in the center of a large hay stack
are just moist enough to encourage rapid bacterial decomposition,
the heat generated may increase until dryer bales on the outside
begin to smoke and then burn. Wise farmers make sure their hay is
thoroughly dry before baling and stacking it.
How hot the pile can get depends on how well the composter controls
a number of factors. These are so important that they need to be
considered in detail.
_Particle size. _Microorganisms are not capable of chewing or
mechanically attacking food. Their primary method of eating is to
secrete digestive enzymes that break down and then dissolve organic
matter. Some larger single-cell creatures can surround or envelop
and then "swallow" tiny food particles. Once inside the cell this
material is then attacked by similar digestive enzymes.
Since digestive enzymes attack only outside surfaces, the greater
the surface area the composting materials present the more rapidly
microorganisms multiply to consume the food supply. And the more
heat is created. As particle size decreases, the amount of surface
area goes up just about as rapidly as the number series used a few
paragraphs back to illustrate the multiplication of microorganisms.
The surfaces presented in different types of soil similarly affect
plant growth so scientists have carefully calculated the amount of
surface areas of soil materials. Although compost heaps are made of
much larger particles than soil, the relationship between particle
size and surface area is the same. Clearly, when a small difference
in particle size can change the amount of surface area by hundreds
of times, reducing the size of the stuff in the compost pile will:
- expose more material to digestive enzymes;
- greatly accelerate decomposition;
- build much higher temperatures.
_Oxygen supply. _All desirable organisms of decomposition are oxygen
breathers or "aerobes. There must be an adequate movement of air
through the pile to supply their needs. If air supply is choked off,
aerobic microorganisms die off and are replaced by anaerobic
organisms. These do not run by burning carbohydrates, but derive
energy from other kinds of chemical reactions not requiring oxygen.
Anaerobic chemistry is slow and does not generate much heat, so a
pile that suddenly cools off is giving a strong indication that the
core may lack air. The primary waste products of aerobes are water
and carbon dioxide gas--inoffensive substances. When most people
think of putrefaction they are actually picturing decomposition by
anaerobic bacteria. With insufficient oxygen, foul-smelling
materials are created. Instead of humus being formed, black, tarlike
substances develop that are much less useful in soil. Under airless
conditions much nitrate is permanently lost. The odiferous wastes of
anaerobes also includes hydrogen sulfide (smells like rotten eggs),
as well as other toxic substances with very unpleasant qualities.
Heaps built with significant amounts of coarse, strong, irregular
materials tend to retain large pore spaces, encourage airflow and
remain aerobic. Heat generated in the pile causes hot air in the
pile's center to rise and exit the pile by convection. This
automatically draws in a supply of fresh, cool air. But heaps made
exclusively of large particles not only present little surface area
to microorganisms, they permit so much airflow that they are rapidly
cooled. This is one reason that a wet firewood rick or a pile of
damp wood chips does not heat up. At the opposite extreme, piles
made of finely ground or soft, wet materials tend to compact, ending
convective air exchanges and bringing aerobic decomposition to a
halt. In the center of an airless heap, anaerobic organisms
immediately take over.