Q&A

DEAR NEW FARM:

Regarding your 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.

Atina Diffley
Minnesota

 

DEAR ATINA:

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 decay.

Total soil organic matter (SOM) can be separated into two components:

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 organisms.

Dave Wilson

 

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