Why Today’s Soil Carbon Market is Built on Half the Ledger

Around the world, billions of dollars in carbon credits hinge on a single metric: soil organic carbon (SOC). The logic is simple. If farmers adopt practices such as cover crops, reduced tillage, rotational grazing, that increase organic matter in their soils, they can sell the resulting carbon gains as credits. But there’s a missing half to this story, and it’s big enough to undermine the integrity of the market itself: The carbon we’re not counting.

That missing half is soil inorganic carbon (SIC): the mineral carbonates (chiefly calcium and magnesium carbonates) that make up a vast, dynamic carbon pool. New science shows SIC is comparable in size to the planet’s SOC reservoir and far more vulnerable to modern agriculture than once assumed. Yet most carbon credit protocols, and almost all transactions, ignore it. The result: a market that may credit gains in one soil carbon pool while ignoring equal or larger losses in the other.

This article lays out the evidence for moving from SOC-only accounting to a Total Soil Carbon (TSC) framework, one that measures both SOC and SIC, to meaningful depths, with defensible measurement and verification methods. To ensure we count the carbon we’re not counting.

1.       A blind spot baked into the rules

For the last decade, leading standards and national programs have operationalized soil credits almost exclusively around SOC. In Australia, for example, the Clean Energy Regulator’s 2021 Estimating soil organic carbon sequestration using measurement and models method explicitly centers on SOC change to 30 cm, paired with modelled emissions for on-farm activities. That focus persists in subsequent guidance and reviews. (2)

There’s nothing nefarious about that history; research, field tools, and agronomic narratives all pointed to SOC. But the assumption underneath, that SIC is geologically inert on policy timescales, has not survived contact with the evidence. (5) The result, we end up with a large source of carbon we’re not counting.

2.       The scale of what we’re missing

A landmark 2024 Science paper compiled 223,593 field measurements and used machine-learning models to map SIC globally to 2 meters depth. The verdict: soils store 2,305 ± 636 billion tonnes of carbon as SIC. This is more than five times the carbon in all the world’s vegetation and roughly on par with global SOC stocks at similar depths. The same analysis warned that soil acidification tied to nitrogen additions could reduce topsoil SIC by up to 23 billion tonnes over the next 30 years, and that at least 1.13 ± 0.33 billion tonnes of inorganic carbon are flushed from soils to inland waters every year. (4)

These are not edge cases limited to deserts. While SIC is concentrated in drylands and alkaline soils, it occurs on every continent and is intimately linked to pH, hydrology, nitrogen inputs, irrigation, and land use, all variables that modern agriculture aggressively modifies. (4)

Despite the scale, the literature shows a historic SIC knowledge gap. A 2024 bibliometric analysis in Geoderma surveyed >47,000 soil-carbon publications going back a century and found that >96% focused on SOC. SIC accounted for <4% of publications and citations, even though the two pools are similar in magnitude down to two meters. (7) Hence, how we end up with a large chunk of carbon we’re not counting.

Translation for markets: We’ve spent decades building methods, models, and finance around half the subterranean balance sheet. That might be tolerable if the ignored half were truly inert. It isn’t.

3.      The trade‑offs hiding in plain sight

3.1.             Nitrogen, acidification, and carbonate loss

Nitrogen fertilizers, ubiquitous in intensive agriculture, acidify soils. Acid dissolves carbonates (SIC), releasing CO₂. The 2024 Science synthesis quantifies the vulnerability; complementary syntheses and reviews detail the mechanisms and global consequences, including the downstream export of inorganic carbon to rivers and oceans. (4)

In practice, many farms counter soil acidification by applying lime, itself a carbonate. Under IPCC Tier 1 methods used in national inventories, liming is counted as a CO₂ source (mass-balance of carbonate applied). The US EPA and UNFCCC guidance both treat liming as a reportable emissions category. Yet current soil carbon crediting rarely nets those emissions against SOC gains in the same fields. (8)

The upshot is a double loss for genuine climate accounting: acidification dissolves in‑situ SIC and liming adds a reportable emission, while the market books only the SOC gain. Hence, a project can earn credits for SOC increases while its nitrogen regime and liming quietly erode the SIC ledger and emit CO₂; none of which appears in the offset calculation. Indeed, there’s a chunck of carbon we’re not counting. (2)

3.2.             Depth matters: gains up top, losses below

Shallow sampling (0–30 cm) is common in protocols, but carbon dynamics don’t stop at the spade’s edge. Re-sampling studies and long-term trials increasingly find SOC gains near the surface paired with losses deeper in the profile, yielding smaller or even negative net changes to 1 meter. A U.S. Upper Midwest 30‑year study that measured density-corrected SOC to 90 cm found row‑crop systems lost SOC, while prairie and well-managed pasture maintained it, illustrating how shallow or incomplete methods can overestimate benefits. At larger scales, recent China-wide work shows upper‑soil SOC increases offset by deeper losses over the last four decades, again reinforcing that whole-profile accounting is necessary for credible claims. (6)

Markets built on 0–30 cm and soil organic carbon (SOC) only snapshots risk paying for partial stories due to the carbon we’re not counting.

3.3.             The SOC–SIC “see‑saw”

A 2025 meta‑analysis in Global Change Biology modelled management effects on both pools. It found that while many practices boost SOC, trade‑offs (notably from mineral fertilizer use and certain restoration pathways) can reduce SIC, partly offsetting overall soil carbon sequestration. The study estimates a global soil carbon sequestration potential of ~1.5 Gt C yr⁻¹, but that figure assumes SOC+SIC are accounted together. (5) In other words, the climate math only adds up when Total Soil Carbon is the unit of account, and we include the carbon we’re not counting today.

4.      Measurement: from lab benches to field designs

4.1.             The lab problem no one priced in

Even within SOC-only frameworks, measurement error is now a headline issue. A January 2025 study in SOIL ran a blind lab comparison across eight service labs and tested common processing steps (sieving, grinding, drying) across 12 soils. Findings:

• Mechanical grinders failed to remove coarse material consistently, causing higher variance.
• Not oven-drying samples underestimated total C and SOC (3.5% and 5% on average).
• Crucially, in calcareous soils, failing to remove SIC before dry combustion risks overstating SOC, because the instrument measures total carbon unless an acid pretreatment or separate SIC measurement is performed.

The paper explicitly calls for routine SIC testing and standardized pretreatment in any calcareous context. For markets, that’s not an academic footnote, it’s the difference between paying for SOC stock versus missing out on the carbon we’re not counting. (4)

4.2.             Direct measurement beats “model-and-hope”

On the field side, a 2025 study led by Yale demonstrates that direct measurement and re‑measurement across hundreds of fields is feasible at scale and can substitute for over‑reliance on predictive models. The authors show that sampling roughly 10% of fields across many farms, over realistic timeframes, yields statistically robust estimates of soil carbon change, exactly the kind of design carbon buyers thought was too expensive to do. (1)

One of those methods is Carbon Asset Solutions’ Mobile Inelastic Neutron Scattering. This method measures up to 225 Hectares or 550 Acres a day over a continuous path of 1.5m wide and 30cm depth, at the atomic level. The result of the more accurate measurement method and by far larger soil sampling that measures Total Soil Carbon ánd other key soil characteristics at the same time: 80% increased accuracy in soil carbon measurement compared to traditional lab testing, and contextual information to act upon it.

Hence, we can and we must measure Total Soil Carbon at large-scale because the cost of not measuring leads to incredible losses financially and operationally. (1)

5.      What this means for farmers, buyers, and regulators

For farmers

A Total Soil Carbon framework doesn’t “punish” dryland or alkaline farmers, it protects them from incorrect incentives. Under SOC-only rules, a farmer might chase SOC gains with fertilizer regimens that dissolve their SIC bank account. Under TSC, the same farmer is rewarded for managing acidity, optimizing nitrogen, and adopting irrigation practices that stabilize both pools. As such, taking into account the carbon we’re not counting can lead to financial gains and improved practices. A double win!. (5)

For buyers

If you’re paying for climate outcomes, you want net carbon stored in soils, not just one pool measured with ambiguous lab SOPs. Ask for TSC accounting, including disclosure of SIC protocols. If a project can’t supply them, price the risk accordingly. (3, 8)

For regulators and standards

Integrity reviews, like Australia’s periodic review of the SOC method, should explicitly evaluate whether current sampling depth, lab procedures, and system boundaries (liming, nitrogen) align with the state of the science. Where they don’t, update the method to require TSC, deeper sampling, and lab standardization. (2)

6.      The bottom line: stop paying for half a story

The promise of soil carbon markets is real: pay land stewards to restore and protect the planet’s largest terrestrial carbon stock. But a market that measures only organic carbon at shallow depths, with inconsistent lab practices, and ignores inorganic carbon losses and liming emissions, is not fit for purpose.

The science has moved. The tools exist. The policy windows (like Australia’s method review) are open. The fix is conceptually simple: Count what counts; include the carbon we’re not counting. Make Total Soil Carbon, not just SOC, the basis of credits. Measure it properly. Sample deep enough. Net the emissions you cause. And stop balancing the world’s climate books on half a ledger.

>>> Start your Total Soil Carbon Journey today with Carbon Asset Solutions, book you Total Soil Carbon measurement today <<<

Sources & further reading

1. Potash, E. et al.  (2025) “Measure-and-remeasure as an economically feasible approach to crediting soil organic carbon at scale.” Environmental Research Letters, DOI: 10.1088/1748-9326/ada16c, https://iopscience.iop.org/article/10.1088/1748-9326/ada16c
2. Clean Energy Regulator (Australia). 2021 SOC method and guidance; periodic review (2025). https://www.legislation.gov.au/F2021L01696/asmade/2021-12-02/text/original/pdf,https://cer.gov.au/schemes/australian-carbon-credit-unit-scheme/accu-scheme-methods/estimating-soil-organic-carbon-sequestration-using-measurement-and-models-method, https://consult.dcceew.gov.au/periodic-review-of-the-soil-organic-carbon-method-2021
3. Even, R. J. et al. (2025). “Large errors in soil carbon measurements attributed to inconsistent sample processing.” Soil (vol. 11, no. 1). https://doi.org/10.5194/soil-11-17-2025 , https://soil.copernicus.org/articles/11/17/2025/
4. Huang, Y. et al. (2024). “Size, distribution, and vulnerability of the global soil inorganic carbon.” Science (384). https://doi.org/10.1126/science.adi7918, https://www.science.org/doi/epdf/10.1126/science.adi7918
5. Liao, Y. et al. (2025). “Inorganic Carbon Should Be Considered for Carbon Sequestration in Agricultural Soils.” Global Change Biology (vol. 31, no.4). doi: 10.1111/gcb.70160, https://publications.goettingen-research-online.de/handle/2/149047?cs=mla
6. Zhou, Z., et al. (2025). “Carbon gain in upper but loss in deeper cropland soils across China over the last four decades.” Proceedings of the National Academy of Sciences (PNAS) (vol. 122, no. 1). https://doi.org/10.1073/pnas.24223711221 AND Dietz, CL., et al. (2024). “Soil carbon maintained by perennial grasslands over 30 years but lost in field crop systems in a temperate Mollisol”. Communications earth & environment. (Vol. 5, no. 1). https://doi.org/10.1038/s43247-024-01500-w
7. Raza, S. et al. (2024). “Inorganic carbon is overlooked in global soil carbon research.” Geoderma (vol. 443). https://doi.org/10.1016/j.geoderma.2024.116831
8. US EPA (2026). “Agriculture Sector Emissions”. EPA United States Environmental Protection Agency. https://www.epa.gov/ghgemissions/agriculture-sector-emissions AND UNFCCC (n.d.). “Liming (3G)”. United Nations Framework Convention on Climate Change. https://unfccc.int/resource/tet/ba/ba8-02_L8_CS_NID.pdf