Underground Accounting: The Messy, Contested Science of Measuring Soil Carbon

There's a number that a lot of people in sustainable agriculture really want you to believe in. It shows up on farm websites, in carbon credit marketing materials, in USDA grant applications, and on the packaging of products sold at premium prices in natural food stores. The number is supposed to tell you how much carbon a given farm is pulling out of the atmosphere and locking into the ground. It's supposed to be proof that the food you're buying is doing something genuinely good for the planet.

The problem? Nobody can fully agree on how to measure it — and the industry has been remarkably quiet about that inconvenient fact.

What Soil Carbon Actually Is (And Why It's Complicated)

Soil organic carbon, or SOC, is the carbon stored in the organic matter that makes up living, healthy soil. It comes from decomposed plant material, microbial activity, root systems, and a hundred other biological processes happening just below the surface. When farmers talk about building soil health — through cover cropping, reduced tillage, composting, rotational grazing — increasing SOC is one of the primary goals.

The logic is straightforward enough: healthy soil stores more carbon, which means less of it floating around in the atmosphere as CO2. That's a real thing. The science behind it is solid at a basic level. Where things get genuinely complicated is in the measurement.

Soil isn't uniform. It varies by depth, by location within a single field, by season, by moisture levels, and by the specific microbial communities living in it at any given moment. A soil sample taken from one corner of a 40-acre farm can look completely different from one taken 200 feet away. And that's before you even get into the question of which testing methodology you're using — because there are several, and they don't always agree with each other.

The Method Problem Nobody Wants to Talk About

The two most commonly used approaches to measuring SOC are loss-on-ignition (LOI) and dry combustion analysis. They sound technical, and they are, but the important thing for consumers to understand is this: they can produce meaningfully different results from the same soil sample.

LOI is cheaper and more widely available, which makes it popular among smaller farms and regional soil labs. Dry combustion is generally considered more accurate, but it's more expensive and less accessible. There are also newer methods — including near-infrared spectroscopy and remote sensing technologies — that are being promoted by tech companies as scalable alternatives. Each of these has its own margin of error, its own set of assumptions, and its own boosters with financial interests in its adoption.

When a farm or a carbon credit program tells you they've measured their soil carbon, they are almost certainly not telling you which method they used, how many samples they took, at what depth, or how that data was validated. And without that context, the number is close to meaningless.

Carbon Credits and the Corporate Incentive to Oversimplify

The voluntary carbon market has poured gasoline on this problem. Over the past several years, a wave of ag-tech startups — many of them backed by major food corporations and investment firms — have built entire business models around paying farmers to sequester carbon and then selling those credits to companies looking to offset their emissions.

The incentive structure here is worth examining carefully. The more carbon a farm appears to sequester, the more credits can be sold, the more money flows through the system. That's not a neutral context in which to be developing measurement methodologies. Several independent researchers have raised concerns that some of the most widely used carbon accounting models in the voluntary market are systematically overestimating sequestration — sometimes by a significant margin.

A 2021 investigation by CarbonPlan, a nonprofit that analyzes carbon removal claims, found that a major USDA forest carbon offset program had overestimated credits by more than 30 percent. Similar concerns have been raised about agricultural soil programs, where modeling assumptions about how long carbon stays in the ground can dramatically inflate projected sequestration numbers.

This doesn't mean every carbon credit is fraudulent. It means the verification infrastructure simply isn't mature enough to back up the confidence with which these claims are being made.

The Permanence Problem

Even when carbon is genuinely sequesrated, there's another question that rarely gets asked in marketing materials: how long does it stay there?

Soil carbon is not a vault. It's a dynamic system. A drought, a flood, a change in tillage practices, a shift in cover crop management — any of these can release stored carbon back into the atmosphere relatively quickly. The scientific term for this is "permanence," and it's one of the thorniest issues in the entire field of carbon accounting.

For carbon sequestration to meaningfully address climate change, the carbon needs to stay in the ground for decades, ideally centuries. But most soil carbon monitoring programs only verify storage over periods of three to ten years. What happens after the contract ends, after the farm changes hands, after a major weather event? The honest answer is: nobody's tracking it closely enough to know.

What Farmers and Buyers Should Actually Be Measuring

None of this means regenerative farming practices are a scam. Cover cropping, reduced tillage, and rotational grazing do produce real, documentable improvements in soil health. The issue is that the industry has leaned so hard into carbon sequestration as a marketing claim — and as a financial instrument — that it has outrun the science.

For farmers genuinely interested in soil health, researchers like those at the Rodale Institute and the Land Institute suggest focusing on a broader suite of indicators: soil biology (microbial activity, fungal networks), water infiltration rates, aggregate stability, and organic matter percentage over time. These metrics are harder to commodify, but they paint a more honest picture of what's actually happening underground.

For consumers and buyers, the ask is simpler but more demanding: stop accepting carbon sequestration claims at face value. Ask farms what methodology they used to measure. Ask which third party verified it. Ask how deep the samples went and how many were taken. If they can't answer those questions, the number on the label is more aspiration than fact.

The soil beneath a well-managed farm is one of the most extraordinary things on earth — a living system of staggering complexity that we're only beginning to understand. It deserves better than being reduced to a marketing metric. And so do the people trying to eat their way toward a healthier planet.