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