Testing Soil Carbon

In this newly emerging world of carbon credits, carbon markets, and carbon banking, the need to quantify and document soil carbon content is becoming more and more important. The challenge with this new market is that the testing methods for soil carbon have not been standardized across the industry. Carbon can be analyzed by several different methods and each method can give you different results.

At A&L Great Lakes Laboratories, soil carbon can currently be analyzed by 3 different methods; loss on ignition, combustion, and Walkley-Black oxidation.

The loss on ignition method is the quickest and cheapest method for estimating soil organic carbon. The process is fairly simple. A portion of the dried and ground soil sample is weighed, then put into an oven to burn off the soil organic matter. The sample is then weighed again, and the change in weight is equal to the organic matter content of the soil. From the organic matter content, the organic carbon content can be calculated by assuming that 58% of the organic matter is carbon.

The combustion method is a way to determine total soil carbon. The simplest explanation is that a weighed portion of the soil sample is heated to the point that both the mineral and organic fractions of the soil combust and the gases are then analyzed to determine the total carbon content. While this is a very accurate method, there is no way to determine whether the carbon originated from the organic matter in the soil, or the mineral fraction. The liming materials that are generally used consist of calcium and magnesium carbonates, and this test will detect this form of carbon also.

The Walkley-Black method determines the organic carbon content of a soil by measuring amount of carbon that is oxidized in a reaction with dichromate through a back titration. While this method is generally considered the most accurate method to determine soil organic carbon, it is also the slowest and most expensive. It also requires the handling of hazardous materials, so it is generally the least preferred method for occupational and environmental safety reasons.

As the carbon farming industry continues to evolve, the methods for analyzing carbon are likely to change as well. When comparing results from separate sampling events, be sure that you know what sampling procedure and method were used so a fair comparison can be made.