complex. Most significantly, while carbohydrates are mainly carbon
and hydrogen, proteins contain large amounts of nitrogen and
numerous other mineral nutrients.

Proteins are scarce in nature. Plants can make them only in
proportion to the amount of the nutrient, nitrogen, that they take
up from the soil. Most soils are very poorly endowed with nitrogen.
If nitrate-poor, nutrient-poor soil is well-watered there may be
lush vegetation but the plants will contain little protein and can
support few animals. But where there are high levels of nutrients in
the soil there will be large numbers of animals, even if the land is
poorly watered and grows only scrubby grasses--verdant forests
usually feed only a few shy deer while the short grass semi-desert
prairies once supported huge herds of grazing animals.

Ironically, just as it is with carbon, there is no absolute shortage
of nitrogen on Earth. The atmosphere is nearly 80 percent nitrogen.
But in the form of gas, atmospheric nitrogen is completely useless
to plants or animals. It must first be combined chemically into
forms plants can use, such as nitrate (NO3) or ammonia (NH3). These
chemicals are referred to as "fixed nitrogen."

Nitrogen gas strongly resists combining with other elements.
Chemical factories fix nitrogen only at very high temperatures and
pressures and in the presence of exotic catalysts like platinum or
by exposing nitrogen gas to powerful electric sparks. Lightning
flashes can similarly fix small amounts of nitrogen that fall to
earth dissolved in rain.

And certain soil-dwelling microorganisms are able to fix atmospheric