Before we ask how to compost, since nature is maximally efficient
perhaps it would benefit us to first examine how nature goes about
returning organic matter to the soil from whence it came. If we do
nearly as well, we can be proud.
Where nature is allowed to operate without human intervention, each
place develops a stable level of biomass that is inevitably the
highest amount of organic life that site could support. Whether
deciduous forest, coniferous forest, prairie, even desert, nature
makes the most of the available resources and raises the living
drama to its most intense and complex peak possible. There will be
as many mammals as there can be, as many insects, as many worms, as
many plants growing as large as they can get, as much organic matter
in all stages of decomposition and the maximum amount of relatively
stable humus in the soil. All these forms of living and decomposing
organisms are linked in one complex system; each part so closely
connected to all the others that should one be lessened or
increased, all the others change as well.
The efficient decomposition of leaves on a forest floor is a fine
example of what we might hope to achieve in a compost pile. Under
the shade of the trees and mulched thickly by leaves, the forest
floor usually stays moist. Although the leaves tend to mat where
they contact the soil, the wet, somewhat compacted layer is thin
enough to permit air to be in contact with all of the materials and
to enter the soil.
Living in this very top layer of fluffy, crumbly, moist soil mixed
with leaf material and humus, are the animals that begin the process
of humification. Many of these primary decomposers are larger,
insect-like animals commonly known to gardeners, including the wood
lice that we call pill bugs because they roll up defensively into
hard armadillo-like shells, and the highly intrusive earwigs my
daughter calls pinch bugs. There are also numerous types of insect
larvae busily at work.
A person could spend their entire life trying to understand the
ecology of a single handful of humus-rich topsoil. For a century
now, numerous soil biologists have been doing just that and still
the job is not finished. Since gardeners, much less ordinary people,
are rarely interested in observing and naming the tiny animals of
the soil, especially are we disinterested in those who do no damage
to our crops, soil animals are usually delineated only by Latin
scientific names. The variations with which soil animals live, eat,
digest, reproduce, attack, and defend themselves fills whole
sections of academic science libraries.
During the writing of this book I became quite immersed in this
subject and read far more deeply into soil biology and microbiology
than I thought I ever would. Even though this area of knowledge has
amused me, I doubt it will entertain most of you. If it does, I
recommend that you first consult specialist source materials listed
in the bibliography for an introduction to a huge universe of
literature.
I will not make you yawn by mentioning long, unfamiliar Latin names.
I will not astonish you with descriptions of complex reproductive
methods and beautiful survival strategies. Gardeners do not really
need this information. But managing the earth so that soil animals
are helped and not destroyed is essential to good gardening. And
there are a few qualities of soil animals that are found in almost
all of them. If we are aware of the general characteristics of soil
animals we can evaluate our composting and gardening practices by
their effect on these minuscule creatures.
Compared to the atmosphere, soil is a place where temperature
fluctuations are small and slow. Consequently, soil animals are
generally intolerant to sudden temperature changes and may not
function well over a very wide range. That's why leaving bare earth
exposed to the hot summer sun often retards plant growth and why
many thoughtful gardeners either put down a thin mulch in summer or
try to rapidly establish a cooling leaf canopy to shade raised beds.
Except for a few microorganisms, soil animals breathe oxygen just
like other living things and so are dependent on an adequate air
supply. Where soil is airless due to compaction, poor drainage, or
large proportions of very fine clay, soil animals are few in number.
The soil environment is generally quite moist; even when the soil
seems a little dryish the relative humidity of the soil air usually
approaches 100 percent. Soil animals consequently have not developed
the ability to conserve their body moisture and are speedily killed
by dry conditions. When faced with desiccation they retreat deeper
into the soil if there is oxygen and pore spaces large enough to
move about. So we see another reason why a thin mulch that preserves
surface moisture can greatly increase the beneficial population of
soil animals. Some single-cell animals and roundworms are capable of
surviving stress by encysting themselves, forming a little "seed"
that preserves their genetic material and enough food to reactivate
it, coming back to life when conditions improve. These cysts may
endure long periods of severe freezing and sometimes temperatures of
over 150 degree F.
Inhabitants of leaf litter reside close to the surface and so must
be able to experience exposure to dryer air and light for short
times without damage. The larger litter livers are called primary
decomposers. They spend most of their time chewing on the thick
reserve of moist leaves contacting the forest floor. Primary
decomposers are unable to digest the entire leaf. They extract only
the easily assimilable substances from their food: proteins, sugars
and other simple carbohydrates and fats. Cellulose and lignin are
the two substances that make up the hard, permanent, and woody parts
of plants; these materials cannot be digested by most soil animals.
Interestingly, just like in a cow's rumen, there are a few larvae
whose digestive tract contains cellulose-decomposing bacteria but
these larvae have little overall effect.
After the primary consumers are finished the leaves have been
mechanically disintegrated and thoroughly moistened, worked over,
chewed to tiny pieces and converted into minuscule bits of moist
excrement still containing active digestive enzymes. Many of the
bacteria and fungi that were present on the leaf surfaces have
passed through this initial digestion process alive or as spores
waiting and ready to activate. In this sense, the excrement of the
primary decomposers is not very different than manure from large
vegetarian mammals like cows and sheep although it is in much
smaller pieces.
Digestive wastes of primary decomposers are thoroughly inoculated
with microorganisms that can consume cellulose and lignin. Even
though it looks like humus, it has not yet fully decomposed. It does
have a water-retentive, granular structure that facilitates the
presence of air and moisture throughout the mass creating perfect
conditions for microbial digestion to proceed.
This excrement is also the food for a diverse group of nearly
microscopic soil animals called secondary decomposers. These are
incapable of eating anything that has not already been predigested
by the primary decomposers. The combination of microbes and the
digestive enzymes of the primary and secondary decomposers breaks
down resistant cellulose and to some degree, even lignins. The
result is a considerable amount of secondary decomposition excrement
having a much finer crumb structure than what was left by the
primary decomposers. It is closer to being humus but is still not
quite finished.
Now comes the final stage in humus formation. Numerous species of
earthworms eat their way through the soil, taking in a mixture of
earth, microbes, and the excrement of soil animals. All of these
substances are mixed together, ground-up, and chemically recombined
in the worm's highly active and acidic gut. Organic substances
chemically unite with soil to form clay/humus complexes that are
quite resistant to further decomposition and have an extraordinarily
high ability to hold and release the very nutrients and water that
feed plants. Earthworm casts (excrement) are mechanically very
stable and help create a durable soil structure that remains open
and friable, something gardeners and farmers call good tilth or good
crumb. Earthworms are so vitally important to soil fertility and
additionally useful as agents of compost making that an entire
section of this book will consider them in great detail.
Let's underline a composting lesson to be drawn from the forest
floor. In nature, humus formation goes on in the presence of air and
moisture. The agents of its formation are soil animals ranging in
complexity from microorganisms through insects working together in a
complex ecology. These same organisms work our compost piles and
help us change crude vegetation into humus or something close to
humus. So, when we make compost we need to make sure that there is
sufficient air and moisture.
Decomposition is actually a process of repeated digestions as
organic matter passes and repasses through the intestinal tracts of
soil animals numerous times or is attacked by the digestive enzymes
secreted by microorganisms. At each stage the vegetation and
decomposition products of that vegetation are thoroughly mixed with
animal digestive enzymes. Soil biologists have observed that where
soil conditions are hostile to soil animals, such as in compacted
fine clay soils that exclude air, organic matter is decomposed
exclusively by microorganisms. Under those conditions virtually no
decomposition-resistant humus/clay complexes form; almost everything
is consumed by the bacterial community as fuel. And the
non-productive soil is virtually devoid of organic matter.
Sir Albert Howard has been called the 'father of modern composting.'
His first composting book (1931) _The Waste Products of
Agriculture,_ stressed the vital importance of animal digestive
enzymes from fresh cow manure in making compost. When he
experimented with making compost without manure the results were
less than ideal. Most gardeners cannot obtain fresh manure but
fortunately soil animals will supply similar digestive enzymes.
Later on when we review Howard's Indore composting method we will
see how brilliantly Sir Albert understood natural decomposition and
mimicked it in a composting method that resulted in a very superior
product.
At this point I suggest another definition for humus. Humus is the
excrement of soil animals, primarily earthworms, but including that
of some other species that, like earthworms, are capable of
combining partially decomposed organic matter and the excrement of
other soil animals with clay to create stable soil crumbs resistant
to further decomposition or consumption.