The protein content of tree leaves is very similar to their ratio of
carbon (C) compared to nitrogen (N)
Sometimes plants store food in the form of oil, the most
concentrated biological energy source. Oil is also constructed from
sugar and is usually found in seeds. Plants also build structural
materials like stem, cell walls, and other woody parts from sugars
converted into cellulose, a substance similar to starch. Very strong
structures are constructed with lignins, a material like cellulose
but much more durable. Cellulose and lignins are permanent. They
cannot be converted back into sugar by plant enzymes. Nor can most
animals or bacteria digest them.
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.
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.
Liebig suggested imagining a barrel being filled with water as a
metaphor for plant growth: the amount of water held in the barrel
being the amount of growth. Each stave represents one of the factors
or requirements plants need in order to grow such as light, water,
oxygen, nitrogen, phosphorus, copper, boron, etc. Lowering any one
stave of the barrel, no matter which one, lessens the amount of
water that can be held and thus growth is reduced to the level of
the most limited growth factor.
For example, one essential plant protein is called chlorophyll, the
green pigment found in leaves that makes sugar through
photosynthesis. Chlorophyll is a protein containing significant
amounts of magnesium. Obviously, the plant's ability to grow is
limited by its ability to find enough fixed nitrogen and also
magnesium to make this protein.
Animals of all sizes from elephants to single cell microorganisms
are primarily composed of protein. But the greatest portion of plant
material is not protein, it is carbohydrates in one form or another.
Eating enough carbohydrates to supply their energy requirements is
rarely the survival problem faced by animals; finding enough protein
(and other vital nutrients) in their food supply to grow and
reproduce is what limits their population. The numbers and health of
grazing animals is limited by the protein and other nutrient content
of the grasses they are eating, similarly the numbers and health of
primary decomposers living on the forest floor is limited by the
nutrient content of their food. And so is the rate of decomposition.
And so too is this true in the compost pile.
The protein content of vegetation is very similar to its ratio of
carbon (C) compared to nitrogen (N). Quick laboratory analysis of
protein content is not done by measuring actual protein itself but
by measuring the amount of combined nitrogen the protein gives off
while decomposing. Acacia, alder, and leaves of other proteinaceous
legumes such as locust, mesquite, scotch broom, vetch, alfalfa,
beans, and peas have low C/N ratios because legume roots uniquely
can shelter clusters of nitrogen-fixing rhizobia. These
microorganisms can supply all the nitrate nitrogen fast-growing
legumes can use if the soil is also well endowed with other mineral
nutrients rhizobia need, especially calcium and phosphorus. Most
other plant families are entirely dependent on nitrate supplies
presented to them by the soil. Consequently, those regions or
locations with soils deficient in mineral nutrients tend to grow
coniferous forests while richer soils support forests with more
protein in their leaves. There may also be climatic conditions that
favor conifers over deciduous trees, regardless of soil fertility.
It is generally true that organic matter with a high ratio of carbon
to nitrogen also will have a high ratio of carbon to other minerals.
And low C/N materials will contain much larger amounts of other
vital mineral nutrients. When we make compost from a wide variety of
materials there are probably enough quantity and variety of
nutrients in the plant residues to form large populations of
humus-forming soil animals and microorganisms. However, when making
compost primarily with high C/N stuff we need to blend in other
substances containing sufficient fixed nitrogen and other vital
nutrient minerals. Otherwise, the decomposition process will take a
very long time because large numbers of decomposing organisms will
not be able to develop.