Soil Carbon Credits Explained: How Projects Measure Carbon in the Ground

Soil carbon credits pay farmers and land managers for storing more carbon in the soil. Soil holds a large share of the carbon in land ecosystems, and practices such as cover cropping, reduced tillage, better grazing and the addition of organic matter can increase the amount it holds. The carbon claim is hard to make, though, because soil is variable, slow to change, hard to sample and able to release its carbon again. This article explains how soil carbon projects set their baselines, measure change and deal with the risk that stored carbon is lost.

The starting point. A soil project begins by recording the carbon already in the ground. Projects take samples at set depths across the project area, usually in a stratified design that groups fields by soil type, climate and management history. The baseline then describes what would have happened without the project, often a continuation of current practice. A project on land that was already managed well has a weak claim, because the change it delivers is small.

Measuring change in the ground. Soil carbon changes slowly, and the amount in any single sample varies from one spot to the next. For this reason, projects usually compare samples taken at the same locations over time rather than relying on one measurement. Some use models that estimate soil carbon from management, climate and soil inputs, and calibrate those models against field samples. Methodologies set out how many samples are needed, how deep they go and how often they are repeated. A model that is not checked against measurements can drift, so buyers should ask how it was calibrated.

Why uncertainty matters. The change a project measures is often small compared with the total stock and with the noise in the samples. The uncertainty around a soil credit can therefore be large relative to the credits themselves. Projects handle this by reporting a confidence interval and, in many methodologies, by discounting the credits issued to reflect it. A credit reported without any uncertainty figure should be read with caution.

Permanence and reversal. Soil carbon can be lost quickly if fields return to tillage, if drought reduces plant growth or if land is burned or cleared. This reversal risk is what sets soil credits apart from many other reduction credits. Methodologies usually handle it with a buffer pool, in which a share of credits is held back in case stored carbon is lost, and with commitments that bind the land manager for a set period. The length of the commitment and the size of the buffer deserve close reading.

Leakage and additionality. A farmer who adopts a practice on one field may shift production to another, which can move emissions rather than cut them. Projects are expected to assess this risk. Additionality matters too. If a practice was already common in the region, or was already being paid for by another programme, the project cannot claim the change as new. Verification bodies test both points when they check the project design.

Soil carbon and the wider footprint. Soil carbon projects can bring other benefits, including better water retention, improved yields and less erosion. These benefits are real but separate from the carbon result, and they should be reported on their own terms. Adding them to the tonnes would make the carbon claim look stronger than the evidence supports.

What to check before buying. Ask which methodology was used and which version applies. Check how many sampling points were taken, at what depths and how often they were repeated. If a model was used, look for its calibration details and the uncertainty reported alongside the result. Confirm the buffer pool size, and check that the registry record shows issuance and retirement and that the vintage matches the period of measured change.

Soil carbon credits can support a claim when the measurement is repeated, the uncertainty is shown and the risk of loss is held back in a buffer. Without those elements, a soil credit describes an intention rather than a measured change in the ground.

Frequently asked questions.

Is soil carbon a removal? It can be, when a practice increases the carbon stock compared with the baseline. But the storage is less secure than geological storage, because it can be lost again. Buyers should read the permanence arrangements alongside the claim.

Can a farmer be paid for soil carbon they already store? Generally no. Payment is linked to a measured increase above the baseline, and a project that rewards existing practice has weak additionality.

Do soil samples alone prove a credit? No. Samples show the stock at one point in time. A credit depends on the change between measurements, the method used to estimate it and the uncertainty around that estimate.