Study Reveals UK Soil Carbon Levels May Be Underestimated

AHDB pilot reveals 30% of farm soil carbon lies below standard measurement depth

British farmland may be storing far more carbon than current measurements suggest. A new pilot study has found that standard soil carbon assessments could be missing a third or more of the carbon actually held in agricultural soils. The issue centres on sampling depth, and the findings have implications for carbon accounting, farm policy, and emerging carbon markets.

The Agriculture and Horticulture Development Board has been running an Environment Baselining Pilot across 178 farms in England, Scotland, and Wales. Early results from 170 of those farms indicate that roughly 30% of soil organic carbon sits below the 30 cm depth that most carbon accounting methodologies currently measure. That means existing baselines may systematically underestimate carbon stocks across a significant proportion of UK farmland.

The pilot covers beef and lamb, cereals and oilseeds, dairy, and pork enterprises. AHDB delivered the work with support from Quality Meat Scotland and Hybu Cig Cymru, the Welsh meat promotion body. Soil samples were taken at multiple depths to establish a more complete picture of carbon storage. The results suggest that around 95% of estimated carbon stocks on participating farms are held in soils, with the remainder stored in trees and hedges.

However, the distribution of that soil carbon appears to be less straightforward than standard sampling assumes. A substantial share lies in deeper soil horizons that are not routinely included in carbon stock assessments. As a result, farms using conventional measurement protocols may be recording significantly lower carbon totals than their soils actually contain.

Shallow sampling misses deeper carbon stocks across varied soil types

Current carbon accounting frameworks typically measure soil organic carbon to a depth of 30 cm. This approach is widely used because it captures the most biologically active layer of soil and aligns with international reporting standards. Nevertheless, the AHDB pilot suggests this convention may not reflect the full reality of carbon storage in agricultural soils.

The pilot recorded an average soil carbon stock of approximately 128 tonnes per hectare across participating farms. That figure already represents a substantial carbon reservoir. Some individual fields, however, recorded totals exceeding 600 tonnes per hectare. The highest values were found in the Cambridgeshire Fens and the Somerset Levels, where deep organic peat soils naturally hold very high carbon stocks.

These findings align with broader soil science research. A 2010 study found that estimated soil carbon within 2 metres of the surface was 141.3 kg C/m², compared to just 36.1 kg C/m² when based only on conventional sampling depths. That fourfold difference illustrates how limited sampling can drastically understate total carbon stocks. Consequently, farms with deep soils or organic horizons below 30 cm may be storing far more carbon than their official baselines suggest.

The pilot also revealed large variation between fields and soil types. Carbon stocks differed markedly depending on soil composition, land use history, and local hydrology. This variation matters because it means no single sampling protocol will suit every farm. Businesses with peat soils, deep loams, or varied topography may need tailored measurement approaches to capture their true carbon inventory.

Moreover, the pilot’s findings raise questions about the comparability of carbon data across farms. If one farm measures to 30 cm and another to 1 metre, their reported totals will not be directly comparable. That inconsistency could distort carbon market transactions, policy evaluations, and supply chain reporting. For carbon accounting to be reliable, measurement protocols need to be both accurate and standardised.

Implications for farm carbon baselines and emerging market schemes

These findings have practical consequences for UK farmers, particularly those entering carbon markets or responding to supply chain carbon reporting requirements. Many private and public carbon schemes rely on soil carbon baselines to calculate sequestration or storage credits. If those baselines are systematically understated, the additionality claims underpinning carbon credits may be compromised.

For example, a farm that establishes a baseline using 30 cm sampling may appear to sequester additional carbon through changed management practices. In reality, much of that apparent increase could simply be carbon that was already present at depth but not initially measured. That creates a risk of double-counting or overstating genuine carbon gains. Therefore, measurement protocols need to reflect the full soil profile to avoid inflating the benefits of interventions.

Similarly, businesses seeking to demonstrate carbon storage for sustainability reporting or tender compliance may find their claims questioned if they rely on shallow sampling. Public sector procurement guidance, including PPN 06/21, requires suppliers to report their carbon footprint and demonstrate progress towards net zero. Inaccurate soil carbon data could undermine those submissions, particularly for land-based businesses where soil represents the largest carbon pool.

The financial implications are also significant. Carbon credits derived from inaccurate baselines may face challenges in verification or sale. If auditors or buyers require deeper sampling to validate claims, farms may need to re-measure their soils, potentially at considerable cost. That uncertainty could deter participation in carbon markets or delay the development of credible farm-level carbon schemes.

Furthermore, government policy increasingly ties incentives to environmental outcomes, including carbon storage. Schemes such as the Sustainable Farming Incentive and future Environmental Land Management payments may incorporate soil carbon metrics. If those metrics are based on incomplete data, payments may not accurately reflect actual environmental benefit. Consequently, policy design needs to account for the limitations of current measurement standards.

The issue extends to supply chain expectations as well. Food and retail businesses are setting net zero targets that depend on reducing Scope 3 emissions from agriculture. Many are asking suppliers to measure and report soil carbon as part of that effort. If farmers use standard 30 cm sampling, their reported totals may not align with the deeper carbon stocks their soils actually contain. That mismatch could complicate supply chain carbon accounting and make it harder for businesses to verify their emissions reductions.

What the AHDB pilot found about farm carbon stocks

• Around 30% of soil organic carbon on participating farms sits below the commonly measured 30 cm depth.
• Early analysis from 170 farms shows that approximately 95% of estimated carbon stocks are held in soils, with the rest in trees and hedges.
• Average soil carbon stock across the pilot was around 128 tonnes per hectare, with some fields exceeding 600 tonnes per hectare.
• The highest carbon stocks were recorded in the Cambridgeshire Fens and Somerset Levels, where deep organic peat soils are common.
• Large variation exists between fields and soil types, indicating that no single measurement approach will suit all farms.
• Current carbon accounting methodologies typically measure only to 30 cm, which may underestimate total soil carbon by a third or more.

Measurement standards may need updating to reflect deeper carbon storage

The pilot’s findings suggest that standard soil carbon measurement protocols may require revision. If a third of soil carbon routinely lies below 30 cm, then continuing to measure only to that depth will persistently understate carbon stocks. That has implications for baseline accuracy, policy evaluation, and market integrity.

One option is to extend sampling to 1 metre or deeper, particularly on farms with organic soils or deep horizons. That would capture a more complete picture of carbon storage and reduce the risk of undercounting. However, deeper sampling is more expensive and time-consuming, which could deter uptake. Practical guidance will need to balance accuracy with cost and feasibility.

Another consideration is soil type. The pilot found that peatland and organic soils in areas such as the Fens and Somerset Levels hold particularly high carbon stocks at depth. For those soils, shallow sampling is especially inadequate. Conversely, shallow or stony soils may contain little carbon below 30 cm. Therefore, measurement protocols may need to be tailored to soil type rather than applied uniformly across all farms.

Standardisation remains important nonetheless. If different farms or schemes adopt different sampling depths, their data will not be comparable. That could fragment carbon markets and complicate policy design. Industry bodies, regulators, and standards organisations will need to coordinate on updated protocols that reflect the full soil profile while remaining practical to implement.

Businesses using carbon reporting services for ESG compliance should review their current soil carbon data in light of these findings. If your farm has deep soils or organic horizons, your existing baseline may understate your carbon stocks. Re-measuring at greater depth could reveal a larger carbon inventory, which may affect your reporting, carbon market participation, or supply chain commitments.

Similarly, farms developing carbon baselines for the first time should consider whether standard 30 cm sampling will capture their full carbon stock. For example, businesses with peat soils, alluvial deposits, or deep loams may benefit from deeper sampling. That initial investment in accurate measurement could provide a more robust baseline for future carbon accounting and reduce the risk of disputes over additionality or sequestration claims.

Training and capacity building will also be important. Soil sampling and carbon analysis require technical skill, and many farm advisers may not be familiar with deeper sampling protocols. Training on Scope 3 emissions and carbon accounting can help businesses and their advisers understand the implications of measurement depth and interpret soil carbon data accurately.

Where to find further information on soil carbon and farm baselines

The AHDB pilot is ongoing, and further results are expected as more farms complete their baselining work. AHDB has published initial findings on its website, and updates will be available as the pilot progresses. You can follow developments through the AHDB website.

For broader guidance on soil carbon measurement and management, the Department for Environment, Food and Rural Affairs provides resources on soil health, carbon sequestration, and environmental land management. The Environment Agency also offers guidance on peatland management and protecting high-carbon soils.

Businesses seeking support with carbon reporting and net-zero program compliance can access structured programs that cover baseline measurement, emissions accounting, and reporting frameworks. That support is particularly relevant for suppliers responding to PPN 06/21 or other public sector procurement requirements.

Finally, industry bodies such as the Institute of Environmental Management and Assessment and the Chartered Institution of Water and Environmental Management provide technical resources on soil carbon science and measurement standards. Those resources can help businesses understand the science behind the AHDB findings and assess whether their current measurement protocols are fit for purpose.