Building Soil Carbon

 

The following revised article came from information provided by Jon Stika. Jon Stika is a retired Natural Resources Conservation Service (NRCS) soil health instructor. He is also the author of A Soil Owner’s Manual: How to Restore and Maintain Soil Health. Farmers are looking at carbon markets, just as they look at crop markets, to improve their bottom line. The goal is to transfer carbon from the atmosphere back into the soil. The real question is what conservation practices increase soil carbon? If a farmer is looking to sign a carbon sequestration contract, it makes sense they understand carbon cycling and soil carbon storage.

Carbon market contracts use the term “sequestered carbon”. Sequestered carbon includes oil, natural gas, and coal; carbon that is tied up. Do farmers want to sequester carbon or recycle soil carbon more efficiently? Soil carbon is one of the most limiting nutrients for improved crop production. With an estimated world population of 8 billion people today and many hungry people, perhaps efficiently recycling soil carbon is more important than permanently trying to sequester carbon long-term.

The majority (about 80 percent) of soil carbon comes from the remains of soil microbes, soil microbial produced carbon compounds, or living soil microbes themselves. Of all soil carbon, roughly five times as much comes from soil microbes fed sugars or other living plant roots substances compared to the carbon from the decomposition of above-ground plant residues. This is why it is important to have living roots feeding soil organisms as much of the time as possible in an effective carbon-capturing system.

Sugars exuded by plant roots and the outer layer of sloughed off plant cell from plant roots cells feed soil organisms. About 50% of plant absorbed atmospheric carbon goes below ground to grow more roots and feed soil organisms. The soil organisms then feed the plant many soil nutrients those plants need to grow. During this cooperation between plants and soil organisms, the soil organisms don’t respire all the carbon the plants give them as food, some of it becomes more stable soil carbon compounds that we eventually include in a measurement of “soil organic matter.”

Farmers who grow plants to move atmospheric carbon into soil carbon to generate carbon credits for additional revenue need to understand this cycle. No practice(s) will get you additional soil carbon without an understanding of how the soil functions as a biological system. Captured atmospheric carbon transferred into soil carbon is a biological process. The goal must be to make life as good as possible for soil organisms that will do that work.

This can be accomplished by observing four basic principles: disturbing the soil less, growing a diversity of plants, maintain living roots in the soil as much of the time as possible, and keeping the soil covered at all times. There is no one practice that will address all four principles. Every time the soil is tilled, carbon goes out of the soil and into the atmosphere. Soil organisms need a diverse diet from a diversity of plants to thrive. Living roots should be maintained in the soil both during, and beyond, the traditional growing season. Soil cover is the roof of the house where soil organisms live. All four principles must be practiced in concert with each other to achieve the goal of increasing soil carbon.

It is difficult, if not impossible, to accumulate soil carbon just growing a cover crop if there is significant tillage, growing only one or two different crops, or a system that does not keep the soil covered. However, a system that minimizes tillage, includes a diversity of plants species of plants, uses crop rotation and plants cover crop mixtures that maintains living roots in the soil year-round, will likely store soil carbon. Do not focus on practices, focus on principles if you wish to be successful in storing soil carbon!

So, if you are interested in getting paid to store soil carbon, it is important to understand how your management system might do that. Without understanding how soil carbon accumulates, you may not accumulate any carbon after applying a practice or two that did not necessarily create the favorable soil environment for soil organisms to make it happen. While plant residues on the soil surface are a necessary part of making life good for soil organisms, the residue on the soil surface does not contribute most of the carbon that stays in the soil. Only a complete understanding of how soil and plants interact as a biological system will result in successful soil carbon storage and profitable crop production.