Do you know what really happens when things rot? Have other garden
books confused you with vague meanings for words like "stabilized
humus?" This book won't. Are you afraid that compost making is a
nasty, unpleasant, or difficult process? It isn't.
A compost pile is actually a fast-track method of changing crude
organic materials into something resembling soil, called humus. But
the word "humus" is often misunderstood, along with the words
"compost," and "organic matter." And when fundamental ideas like
these are not really defined in a person's mind, the whole subject
they are a part of may be confused. So this chapter will clarify
these basics.
Compost making is a simple process. Done properly it becomes a
natural part of your gardening or yard maintenance activities, as
much so as mowing the lawn. And making compost does not have to take
any more effort than bagging up yard waste.
Handling well-made compost is always a pleasant experience. It is
easy to disregard compost's vulgar origins because there is no
similarity between the good-smelling brown or black crumbly
substance dug out of a compost pile and the manure, garbage, leaves,
grass clippings and other waste products from which it began.
Precisely defined, composting means 'enhancing the consumption of
crude organic matter by a complex ecology of biological
decomposition organisms.' As raw organic materials are eaten and
re-eaten by many, many tiny organisms from bacteria (the smallest)
to earthworms (the largest), their components are gradually altered
and recombined. Gardeners often use the terms organic matter,
compost, and humus as interchangeable identities. But there are
important differences in meaning that need to be explained.
This stuff, this organic matter we food gardeners are vitally
concerned about, is formed by growing plants that manufacture the
substances of life. Most organic molecules are very large, complex
assemblies while inorganic materials are much simpler. Animals can
break down, reassemble and destroy organic matter but they cannot
create it. Only plants can make organic materials like cellulose,
proteins, and sugars from inorganic minerals derived from soil, air
or water. The elements plants build with include calcium, magnesium,
potassium, phosphorus, sodium, sulfur, iron, zinc, cobalt, boron,
manganese, molybdenum, carbon, nitrogen, oxygen, and hydrogen.
So organic matter from both land and sea plants fuels the entire
chain of life from worms to whales. Humans are most familiar with
large animals; they rarely consider that the soil is also filled
with animal life busily consuming organic matter or each other. Rich
earth abounds with single cell organisms like bacteria,
actinomycetes, fungi, protozoa, and rotifers. Soil life forms
increase in complexity to microscopic round worms called nematodes,
various kinds of mollusks like snails and slugs (many so tiny the
gardener has no idea they are populating the soil), thousands of
almost microscopic soil-dwelling members of the spider family that
zoologists call arthropods, the insects in all their profusion and
complexity, and, of course, certain larger soil animals most of us
are familiar with such as moles. The entire sum of all this organic
matter: living plants, decomposing plant materials, and all the
animals, living or dead, large and small is sometimes called
_biomass._ One realistic way to gauge the fertility of any
particular soil body is to weigh the amount of biomass it sustains.
_Humus_ is a special and very important type of decomposed organic
matter. Although scientists have been intently studying humus for a
century or more, they still do not know its chemical formula. It is
certain that humus does not have a single chemical structure, but is
a very complex mixture of similar substances that vary according to
the types of organic matter that decayed, and the environmental
conditions and specific organisms that made the humus.
Whatever its varied chemistry, all humus is brown or black, has a
fine, crumbly texture, is very light-weight when dry, and smells
like fresh earth. It is sponge-like, holding several times its
weight in water. Like clay, humus attracts plant nutrients like a
magnet so they aren't so easily washed away by rain or irrigation.
Then humus feeds nutrients back to plants. In the words of soil
science, this functioning like a storage battery for minerals is
called cation exchange capacity. More about that later.
Most important, humus is the last stage in the decomposition of
organic matter. Once organic matter has become humus it resists
further decomposition. Humus rots slowly. When humus does get broken
down by soil microbes it stops being organic matter and changes back
to simple inorganic substances. This ultimate destruction of organic
matter is often called nitrification because one of the main
substances released is nitrate--that vital fertilizer that makes
plants grow green and fast.
Probably without realizing it, many non-gardeners have already
scuffed up that thin layer of nearly pure humus forming naturally on
the forest floor where leaves and needles contact the soil. Most
Americans would be repelled by many of the substances that decompose
into humus. But, fastidious as we tend to be, most would not be
offended to barehandedly cradle a scoop of humus, raise it to the
nose, and take an enjoyable sniff. There seems to be something built
into the most primary nature of humans that likes humus.
In nature, the formation of humus is a slow and constant process
that does not occur in a single step. Plants grow, die and finally
fall to earth where soil-dwelling organisms consume them and each
other until eventually there remains no recognizable trace of the
original plant. Only a small amount of humus is left, located close
to the soil's surface or carried to the depths by burrowing
earthworms. Alternately, the growing plants are eaten by animals
that do not live in the soil, whose manure falls to the ground where
it comes into contact with soil-dwelling organisms that eat it and
each other until there remains no recognizable trace of the original
material. A small amount of humus is left. Or the animal itself
eventually dies and falls to the earth where ....
Composting artificially accelerates the decomposition of crude
organic matter and its recombination into humus. What in nature
might take years we can make happen in weeks or months. But compost
that seems ready to work into soil may not have quite yet become
humus. Though brown and crumbly and good-smelling and well
decomposed, it may only have partially rotted.
When tilled into soil at that point, compost doesn't act at once
like powerful fertilizer and won't immediately contribute to plant
growth until it has decomposed further. But if composting is allowed
to proceed until virtually all of the organic matter has changed
into humus, a great deal of biomass will be reduced to a relatively
tiny remainder of a very valuable substance far more useful than
chemical fertilizer.
For thousands of years gardeners and farmers had few fertilizers
other than animal manure and compost. These were always considered
very valuable substances and a great deal of lore existed about
using them. During the early part of this century, our focus changed
to using chemicals; organic wastes were often considered nuisances
with little value. These days we are rediscovering compost as an
agent of soil improvement and also finding out that we must compost
organic waste materials to recycle them in an ecologically sound
manner.