University of Vermont Extension
Department of Plant and Soil Science
Anytime News
Article
GOOD SOIL, GOOD AIR
Dr. Leonard Perry, Extension
Professor
University of Vermont
Did you know that by promoting a
good soil, one high in organic matter, you are improving air quality? This is done by reducing carbon dioxide, one
of the main culprits implicated in global warming, through a process called “carbon
sequestration”.
Carbon dioxide is added to the
atmosphere in many ways. Prior to the
industrial revolution, the main source was breakdown of organic matter
from
soils. Now it is mainly produced from burning
fossil fuels. Yet carbon dioxide is
taken from the atmosphere by plants.
Although plants respire as we do, adding some carbon dioxide back to
the
atmosphere, the net result is a reduction if the plants end up as
organic
matter. Although the largest amount of
carbon is dissolved in oceans, the second largest amount is stored in
soils. These are often called carbon “sinks”.
The carbon in the carbon dioxide is
tied up into stable forms, or “sequestered”, in soil organic matter which is 57
percent carbon. This process begins by
the plant, through the process of photosynthesis, turning carbon from carbon
dioxide into various compounds for growth.
As plants decompose with the help of soil microorganisms, forming
organic matter, the carbon is held in a short term manner. At this early stage, if organic matter is
disturbed or broken up, the carbon can be oxidized and released back to the
atmosphere. Otherwise, it can end up in
longer term and more stable storage as in “humus”. This is merely the dark brown, spongy, and most
stable
form of organic matter that can last for many years, even centuries. You might think of the initial organic matter
as a bank account for carbon, the humus as a long-term investment account.
Keeping these few facts in mind, it
makes sense that the more plants you have, the more carbon dioxide will be
taken from the air. Dense biomass such
as from cover crops in your garden, and dense landscape plantings, result in
more carbon trapped. Cover crops and
mulches also prevent soil erosion, a main means of soil exposure and so loss of
carbon.
Preventing deforestation, and
planting trees, also are seen recommended to add plant mass and reduce carbon
dioxide. Two cautions are to only plant
trees where they will aid the soil, and where they naturally occur. In wetlands, grasses might be more
appropriate and enrich soils more quickly.
Trees can absorb much water and nutrients from some soils, creating a
poor habitat for soil microorganisms, resulting in less organic matter than if
smaller plants with more leaf litter were grown.
Once carbon is trapped in plants,
there is the need to turn it into organic matter in the soil, either through
compost, mulches, or even leaving grass clippings on lawns or leaf litter in
beds. Otherwise, carbon can be oxidized
and lost back to the air, not to mention the fossil fuels spent blowing leaves,
and carting residue off to a landfill.
These fossil fuels of course add more carbon dioxide back to the
air.
Zero or minimum tillage of soil
results in the least oxidation of the carbon, minimizing the amount lost back
to the air. Minimum tillage, as well as
methods to reduce compaction, results in
less disturbance to the earthworms and soil microorganisms that actually take
the plant debris and make it into organic matter and humus bound into
soil. Minimum or zero tillage has the
added benefit of reducing use of fossil fuels if powered equipment is used.
Another aspect to be aware of is
that these microorganisms need some nitrogen to convert plant
debris to organic matter, roughly in a ratio of ten pounds carbon to one pound
of nitrogen.
So
if insufficient fertility in soils, or even in a compost pile, less if any
organic matter will be made. This means
little of the carbon will be held, but can return to the air as carbon dioxide.
Legume crops such as clover, alfalfa, and soybeans can be used to help provide
this nitrogen. Legumes and their associated bacteria take from the air or “fix”
nearly 17 million tons of nitrogen a year, which is worth about $8 billion.
One scientist estimated that from a
garden in Maine
just under a half acre, with high organic matter (7 percent, which is about
double the usual), that about 19 tons of carbon had been sequestered from the
atmosphere over a ten year period. With
the average American releasing six or more tons a year, this one garden offset
the emissions from an average American for a three year period. On the larger scale, one estimate is that up
to 20 percent of targeted reduction in carbon dioxide emissions could come from
soil sequestration by agriculture.
Grow plenty of crops and plants, and
recycle their debris back to your soils.
By building your soil organic matter and treating it properly, you’ll be rewarded with better plant growth
as well as knowing it is helping reduce carbon dioxide from the atmosphere.
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