How To Measure Soil Carbon

Measuring carbon in the atmosphere for the Northern Hemisphere is tracked by sensors (some developed by Lincoln Nebraska’s LI-COR Biosciences) at the Mauna Loa, Hawaii monitoring location. Atmospheric carbon data is updated on a regular basis and available for use by anyone. Unfortunately there is not an online network that continuously monitors soil carbon. In 2006 when carbon was becoming a “hot” topic, USDA-NRCS developed a Soil Carbon map of the world using various data sources:

Why monitor soil carbon?

Even before we brought cows to our pastures in 2011, folks were claiming cattle were “more damaging to the planet than CO2 from cars” (2006 article)! We actually felt a need to collect soil carbon data to defend our operation in the future.

The idea that raising beef in the image of nature as a contributor to greenhouse gases does not make sense when you think of the huge herds of bison that created the deep carbon rich soils of North American. The claim that cattle raised on pasture is an atmospheric carbon source has since been debunked by the 2019 Quantis-General Mills analysis of the White Oak Pastures beef operation in Georgia. Note: DS Family Farm utilizes the same pasture production principles that White Oak Pastures uses to raise beef except we do not slaughter on the farm.

The image below is slide 17 from the PDF Version of the Quantis Life Cycle report showing that “Net total emissions” of producing beef in natures image results in -3.5 Kg CO2-equivalent emitted per Kg of fresh meat produced. This means CO2 is sequestered when raising beef in natures image, again this just makes sense. So enjoy Pasture Grazed Beef for the flavor and for what it does to heal our planet!

Measuring soil carbon – methods

Back to the past. In 2011 we came across Peter Donovan’s “Soil Carbon Challenge”. His document “Measuring soil carbon change” was about the only step-by-step information we could find at the time to monitor soil carbon. Overall we were impressed with the protocol!

More recently others have entered the soil carbon measuring scene including:

• Point Blue Conservation Science
  • Scroll down to Rangeland Monitoring Network – Protocols.
• Regenerative Organic-Certified (ROC)
  • Current link to ROC Soil Sampling Guidelines (PDF)
• Savory – Ecological Outcome Verification (EOV)
  • Look for link to current version (Ver. 2.0 PDF)
  • Review section “LONG TERM ECOLOGICAL MONITORING”.
• To stay up with current Soil Health Monitoring protocols and policy keep tabs on the Soil Health Institute.

Suggestions to start soil carbon monitoring

After trying to monitor soil carbon over the past 8 years, we are happy to list some suggestions and steps below. Consider measuring the soil carbon on your own farm, garden or lawn. Following these steps with about $50 of new equipment, you will be setup to monitor soil carbon.

Before you begin, read the protocols:

• Select Your Soil Carbon Monitoring Site(s):
  • Review Peter Donovan’s document, Point Blue and Savory’s suggestions to identify site locations. Settle on a system that works for your operation, simple and repeatable.
• Sampling Depths:
  • Depends on how much work you want to put into it. We like the Savory depths of 0-4 in., 4-8 in., 8-12 in. and 40 inches (converted from metric system). This allows you to combine the top three samples for an average “top foot” analysis.
  • Want to start easy and give it a try? Just run through the entire process at the 0-4 inch depth.

Steps to determine soil carbon

#1 Download this modified Soil Bulk Density guide

This modified bulk density guide was used at the 2019 Nebraska Sustainable Ag conference. Teacher questions were removed and edits were made to clarify the calculations.

#2 In Field Bulk Density – each depth

Besides Savory EOV, most other soil carbon methods are based on determining Bulk Density at each sampling depth. Refer to pages 4 & 5 in the modified guide for the step by step procedure and watch the video below.

This is a fairly invasive sampling procedure. Imagine for each sampling depth you will need to excavate a soil pit to insert the 3″ diameter cylinder to collect a bulk density sample. Tip: at deeper depths, drive the cylinder in horizontally rather than the vertical method shown at the soil surface.

Key Equipment You May Need To Obtain:

• 3 inch diameter steel tube cut to 5 inches in length.
  • Any local auto parts store will have a straight 3″ diameter exhaust pipe that you can purchase for around $10. If you have a chop saw (or know someone that does) simply cut the pipe to 5″ length. Use a file to smooth rough edges.
• Digital Scale that will weigh 600+ grams to +- 0.1 gram.
  • Find on Amazon for around $15.

#3 Calculate Bulk Density

After completing the field work and office lab work of drying down the sub sample, use Table 2 page 6 in the modified guide to determine Soil Bulk Density in grams/cm3 for each depth sampled. Pages 7 & 8 helps you calculate Soil Water Content, Soil Porosity and Water-Filled pore space for each bulk density ring collected.

Note, at DS Family Farm we are going to further investigate the Savory EOV “Soil Equivalent Fixed Mass” method in place of Bulk Density in the future. Appears to be less invasive with similar results.

#4 Representative soil sample – each depth

Collect a “representative soil sample” by collecting 8 soil cores using a soil probe for each sampling depth. When you have 8 soil cores for each depth, mix the 8 cores together. Label the mixed soil cores by depth and send the soil samples to a lab of your choice that will give % Soil Organic Carbon using the Dry Combustion Method. We use Ward Labs in Kearney Nebraska and request “Organic Carbon”. This will cost around $10 per sample.

#5 Download this modified Organic Matter guide

This modified organic matter guide was used at the 2019 Nebraska Sustainable Ag conference. Teacher guided questions were removed, edits were made to clarify the calculation steps and Table 4 was added for % Organic Carbon samples.

This guide is really pretty cool. It allows one to make a number of estimates concerning Soil Carbon, Organic N, Organic P and Organic S utilizing bulk density (calculated in step 3 above) and any past soil sample that would have % Soil Organic Matter. Just complete Table 3 if you have % OM available to get the estimates.

#6 Calculate Metric Tons of Carbon and CO2 equivalent

Table 4 of the modified Organic Matter guide was added to utilize % Carbon from lab tests using the Dry Combustion Method. This is the same calculations used in the Peter Donovan document referenced above.

#7 Repeat in the future

Recommendations at this time is to repeat soil carbon monitoring every 3 years. A tip we liked in the Peter Donovan document was to keep a soil sample for future reference. For example, if you collect soil in 2020, collect enough soil to send some to the lab and store the other portion of the sample in a cool dry place. In the future, this stored sample could be used if new testing methods become available or as a check against lab results.

There you go. A longer than normal post but hopefully this information will give you the confidence needed to try soil carbon monitoring on your own property. We will share some of our results in a future post.