Soil sampling detects changes in carbon stock

Before any farm management practices are changed, soil samples can help carbon programs get an accurate benchmark of carbon levels in a field. While many carbon programs utilize modeling technology to estimate carbon sequestration, soil samples quantify changes. 

“There’s a requirement around soil samples being taken so that we can make sure that programs are grounded in a representation of the reality of today,” says A.J. Kumar, vice president of sustainability sciences at Indigo Ag.

Unlike typical soil testing for organic matter, soil being tested for carbon sequestration undergoes a higher temperature ‘dry combustion’ carbon analysis and bulk density test.

“That is typically expressed as a percent of organic carbon in the soil,” says Jim Friedericks, director of outreach and education for AgSource Laboratories. “That percentage is the concentration of carbon in the soil. Then, to quantify the tons of stock carbon, you have to determine the density within the soil. Based on the percentage and the bulk density of soil, you can calculate how many tons of carbon there are present in that depth of soil that you’ve measured.”

Soil carbon levels tend to be quite variable, so measuring the carbon stock and capturing changes over time on one field through soil sampling alone is difficult, says Jim Jordahl, a project analyst with the Bioeconomy Institute at Iowa State University.

“Sampling at exactly the same locations is one tool that carbon programs may use to increase the likelihood of detecting changes,” says Jordahl.

Changes to the soil makeup are often not seen for five to 10 years, and carbon program contracts often outline how often soil tests are to be conducted. To ensure consistency across samples, and provide farmers time and cost savings, many carbon companies will do soil testing themselves. 

“Programs follow a very specific and tightly defined protocol in order to provide the trail of documentation and rigor that they require,” Jordahl says. “Most of the carbon offset voluntary market programs include some type of soil sampling that is done by the program or a third party. The farmer is typically heavily involved in terms of providing information about the site and current and past practices on the farm.”

Data from sampling isn’t the only piece to understanding how much carbon is being held in the soil. Other factors, such as soil texture and pH, also have an impact. A full-scale understanding is key to quantifying accurate offsets. 

“The soil sample measurement is not the same thing as the carbon credit,” Kumar says. “If I go to a field and take soil samples and measure the carbon this year and next year, what I’m measuring is the change over time. That’s not necessarily the same as what would’ve happened if I had not done a practice change. Other things influence that, like how much precipitation and erosion a field saw. The soil samples help make sure that we can do the estimations of the impact accurately, but they are not sufficient alone to get that estimate of a carbon credit.”

Editor’s Note: The Context Network is a global agribusiness consulting firm that helps organizations achieve results through strategic management insights and a network of ag industry professionals, creating business solutions that deliver actionable outcomes. Learn more at contextnet.com.