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.