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.