nitrogen. But these are abundant only where the earth is rich in
humus and minerals, especially calcium. So in a soil body where
large quantities of fixed nitrogen are naturally present, the soil
will also be well-endowed with a good supply of mineral nutrients.

Most of the world's supply of combined nitrogen is biologically
fixed at normal temperatures and standard atmospheric pressure by
soil microorganisms. We call the ones that live freely in soil
"azobacteria" and the ones that associate themselves with the roots
of legumes "rhizobia." Blue-green algae of the type that thrive in
rice paddies also manufacture nitrate nitrogen. We really don't know
how bacteria accomplish this but the nitrogen they "fix" is the
basis of most proteins on earth.

All microorganisms, including nitrogen-fixing bacteria, build their
bodies from the very same elements that plants use for growth. Where
these mineral elements are abundant in soil, the entire soil body is
more alive and carries much more biomass at all levels from bacteria
through insects, plants, and even mammals.

Should any of these vital nutrient substances be in short supply,
all biomass and plant growth will decrease to the level permitted by
the amount available, even though there is an overabundance of all
the rest. The name for this phenomena is the "Law of Limiting
Factors." The concept of limits was first formulated by a scientist,
Justus von Liebig, in the middle of the last century. Although
Liebig's name is not popular with organic gardeners and farmers
because misconceptions of his ideas have led to the widespread use
of chemical fertilizers, Liebig's theory of limits is still good
science.