What is Soil Carbon and How Can It Be Measured?

Conversations about soil carbon are becoming rather common at grower meetings, but what exactly is soil carbon? Soil organic carbon is the carbon fraction in the soil that originated from plant material, the source of the carbon within the parent plant material was CO₂ from the atmosphere. The basic goal of the carbon market is to convert CO₂ from the atmosphere into plant biomass, and then keep it sequestered in the stable humus and/or resistant organic matter to avoid the release of carbon back into the atmosphere . 

Many simply equate soil carbon to soil organic matter. While soil organic matter does contain carbon, soil organic matter is more than carbon. The percent of carbon can vary depending on the soil organic matter parent material, how stable the soil organic matter is, and depth within the soil profile. While the Van Bemmelen factor from 1890, based on the assumption that soil organic matter is 58% carbon, is commonly used to calculate soil organic carbon from soil organic matter values on a soil test. Since 1890 a wide range of studies have shown soil organic matter to be from 52% to 40% carbon. While it is correct that increasing soil organic matter results in more organic carbon in the soil, exact determination of organic carbon volume from soil organic matter data alone is challenging.

Testing for total carbon in soil via combustion is a laboratory method performed at ALGL. However, there is both organic and inorganic carbon in the soil. The primary inorganic carbon in soil are calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃), the two main components of ag lime. Because past management (lime applications) vary by field, estimations of lime-based carbon content is not practical.  ALGL also offers the Walkey Black, a wet chemistry test specifically for organic carbon. The difference between the two can identify the inorganic soil carbon content. Unfortunately, these tests are slower and more costly than basic soil fertility test that most producers are accustomed to.

These tests generate a value that represents the concentration of the total or organic carbon in the soil sample. To determine a carbon ton/acre value a soil bulk density is needed. Soil Bulk densities are most accurate when determined on a non-disturbed soil sample.

Sources:

Methods of Soil Analysis, Part 3 – Chemical Methods, Soil Science Society of America, Madison, WI, 1996.

Hoyle, FC 2013, Managing soil organic matter: A practical guide, Grains Research and Development Corporation, Kingston, viewed 15 October 2018, https://grdc.com.au/resources-and-publications/all-publications/publications/2013/07/grdc-guide-managingsoilorganicmatter.