Certain fungi can digest cellulose and lignin, as can the symbiotic bacteria inhabiting a cow’s rumen.  In this respect the cow is a very clever animal running a cellulose digestion factory in the first and largest of its several stomachs.  There, it cultures bacteria that eat cellulose; then the cow digests the bacteria as they pass out of one stomach and into another.

Plants also construct proteins, the vital stuff of life itself.  Proteins are mainly found in those parts of the plant involved with reproduction and photosynthesis.  Protein molecules differ from starches and sugars in that they are larger and amazingly more 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 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.