DEAR NEW FARM:
research into how organic farming sequesters atmospheric carbon
and nutrients: Can you explain to me how all the carbon
and nitrogen can be sequestered and still be able to supply
plants with the nutrients they need? I am in discussion with
and educating our local soil scientist and vegetable extension
agent. He is quite disbelieving regarding the fact that organic
works but is open to learning.
We put our research agronomist Dave Wilson on the case. Here’s
what Dave had to say:
Not all of the carbon and nitrogen is sequestered long-term,
only a small portion of it is. Some is sequestered and some
is active in nutrient cycling and available for plants.
Organic matter originates from plant residues or animal
manures, which are decomposed by soil organisms. As organic
matter is added to the soil and then starts decomposing,
the carbon and nitrogen from that organic matter begins
cycling in both soil-chemical cycles and soil bio-chemical
cycles. As the organic matter is decomposed, approximately
50 percent of the carbon is retained for cell structure
and reproduction by the soil microbes. Approximately 40
percent of the carbon is lost through microbial respiration
as carbon dioxide, and the remaining carbon becomes part
of the soil humus, which is very resistant to further microbial
Total soil organic matter (SOM) can be separated into
A smaller “active fraction” (AKA labile fraction
because it is unstable) consists of plant microbial and
animal products that are easily decomposed. This portion
of the organic matter gets cycled through the soil microorganisms
and is re-released into the soils as sugars, amino acids
and other small molecules. These are from plant materials
and manures that are easily decomposed and "transformed"
by soil microbes.
That small portion of the decomposing organic matter that
eventually becomes "humus" in the soil is also
commonly called the "resistant" or "stable"
organic matter. The humus is the portion of the soil organic
matter that is recalcitrant, and it improves many of the
soil quality characteristics. Humus is a complex organic
material consisting of a series of relatively high molecular
weight molecules. It is brown to black in color. Humus is
formed by secondary synthesis reactions. It is important
due to its favorable contribution to soil cation exchange
capacity (CEC), water adsorption and soil structure stability.
Humus enhances the formation of soil aggregates, thereby
controlling pore size distribution, and the flow of water
and air into and out of the soil. Humus also increases the
resistance of the soil to erosion.
Soil organic matter is a source as well as a sink of atmospheric
CO2 and contributes to plant growth through its effect on
physical, chemical and biological properties of the soil.
Organic matter has a nutritional function in providing a
source of nitrogen and phosphorus, which are essential to
plant growth. In a biological context, organic matter profoundly
affects the activities of micro-floral and micro-faunal
us with comments, suggestions and questions.