How Do You Calculate Soil Carbon Stock?

This article was adapted from “Sampling Design for Quantifying Soil Organic Carbon Stock in Production Ag Fields” first published on Dec 3, 2021, in Crops & Soils magazine. 

The short answer: To get an estimate of the amount of carbon in a given area, like an agricultural field, you need to measure soil organic carbon concentration of the soil, bulk density, and coarse fragment content. The resulting measures are used to calculate your soil organic carbon (SOC) stock.

Break it down: SOC stock refers to the amount of carbon, measured in units of mass, that is present in a given area. Units are typically reported as megagrams of carbon (Mg C) per unit area (often per hectare). Measuring soil organic carbon is useful for both soil health monitoring and carbon accounting for carbon markets.

• For soil health monitoring, measuring SOC concentration from 0-15 cm depth is adequate for understanding how management is impacting carbon sequestration. SOC concentration includes the measure of total soil carbon concentration by dry combustion with the amount of inorganic carbon concentration removed. You can read more about soil carbon measurements here.
• But most voluntary carbon markets are moving toward SOC stock estimates from 0-30 cm depths. Taking SOC stock allows us to directly compare the carbon emissions and carbon mass of the greenhouse gas CO2 in the atmosphere.

Calculating soil organic carbon stock requires measurements of three different things:

1. Soil organic carbon (SOC) concentration, measured in much the same way as soil fertility testing using a composite sample taken to a fixed depth. For more information, check out this article on how to collect soil samples for carbon calculations.
2. Bulk density, a measure of the mass of soil per volume.
   • Bulk density is measured by pounding a soil-coring device into the ground using a slide hammer to collect a known mass of soil, then calculating the oven-dry soil mass.
   • Reported values typically range from about 0.85 g cm^-3 to 1.60 g cm^-3, with the upper end corresponding to compacted soils.
   • Lower bulk density values mean less compact soils.
3. Coarse fragment volume is the third and final measure needed in soils with CF volumes greater than 2% of the total. These are the mineral components of soil—anything bigger than a particle of sand counts as a coarse fragment (so greater than 2 mm).

Whew! And given all that information, here’s the equation you can use to calculate SOC stock:

Where the following is true:

• OC refers to carbon concentration.
• The conversion factor 0.1 changes lab reported values (mg C cm^-2) to the preferred Mg C ha^-1.
• Bulk density refers to bulk density of the fine soil component
• And CF is the volumetric coarse fragment content.

Bonus! Here’s the equation to calculate bulk density:

Curious about the techniques and design you should use for sampling? Read on! You can find details about soil sampling here.

Photo by Edwin Remsberg and courtesy of USDA-SARE.