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2.2 Chemical Characteristics
When describing organic matter, we use
what is called the carbon-to-nitrogen ratio,
or the C:N ratio. Carbon is used to build the
bodies of soil organisms, and nitrogen is used
in proteins and enzymes responsible for the
metabolism of soil organisms. If we were to
take a soil bacterium, grind up its tissue, and
then measure its carbon and nitrogen content,
we would get an idea of how much carbon
relative to how much nitrogen we would
need in the soil to grow that bacterium. This
is why we use the C:N ratio to understand the
quality of the organic matter for growth of soil
organisms.
All organisms typically need more carbon
than nitrogen. However, with a high C:N
ratio (greater than 24:1) there is not enough
nitrogen for soil microbes to support their
metabolism. Although there may be enough
carbon to grow their bodies, they will be
unable to carry out the reactions needed for
growth. They will use the nitrogen already
in the soil, which can result in limiting the
amount of nitrogen available to plants. This
will also lead to more organic matter loss.
When the C:N ratio is low, organisms will free
up the excess nitrogen in the organic matter
and release it into the soil in a readily available
form, where it can be used by plants. This
excess nitrogen may also become a problem
if it is leached out of the soil and carried by
runoff into waterways, causing contamination.
Sometimes we also see mention of the C:P
(phosphorus) and the C:N:P:S (phosphorus
and sulfur) ratios. These are conceptually the
same as C:N. Different plants and microbes
have different requirements for these
nutrients, relative to their carbon needs.
Dryland soils tend to be low in both C and
N as plant growth is limited by water, and
the C:N ratio can vary, depending on land
use history and vegetation type present. It is
important to remember that the carbon in
soil comes largely from plant material, either
from growing or from organic mulch, but that
nitrogen available for use by plants mostly
comes from soil microorganisms or human
inputs. All this is to say that both of these
elements are essential for soil health, but
routes for entering the soil ecosystem are very
different.
Note: Looking at nutrient cycling provides
us with a way to understand how soils are
connected in space to other ecosystems and
the atmosphere and in time through nutrient
availability and depletion dynamics.
C:N Ratio