At the January 2018 Oxford Farming Conference, then Defra Secretary Michael Gove described soil as the “golden thread” running through the new Environmental Land Management Scheme (ELMS), and confirmed that under it, soil carbon sequestration would be among the public goods for which farmers could claim public funds.

Since then, soil carbon’s climate change mitigation potential has come into sharp focus. At July’s Sustainable Food Trust conference, Farming and Climate Change, Tony Juniper of Natural England and Minette Batters of the National Farmers Union (NFU) both listed carbon sequestration among soil’s critical functions, while Dieter Helm of the Natural Capital Committee and Patrick Holden, the Sustainable Food Trust’s Chief Executive, called for soil carbon to be included in (respectively) the asset baseline and environmental balance sheet of all farm businesses.

International interest in soil carbon goes back to the “4 per 1,000” initiative (to which the UK is a signatory) which aims to increase the amount of carbon in agricultural, grassland and forest soils by 0.4% every year. More recently, August’s IPCC special report, Climate Change and Land, included an increase in soil carbon as among the most significant climate actions in the land use sector, and in September, EU farming ministers met in Finland to debate whether their farming policy plans provide the right framework for improved soil carbon sequestration.

All of this points to an approaching watershed in soil carbon policymaking. However, various obstacles lie in the way, referred to by the IPCC in its special report as the “knowledge, financial, and institutional gaps and challenges” that hinder the upscaling and widespread deployment of soil carbon management.

Addressing these gaps and challenges will be critical for ELMS, if it wants to incentivise carbon sequestration among farmers in England. Specifically, a number of technical, scientific, legal and economic levers need to be carefully interwoven if a fair, coherent, long-term, scalable and effective policy framework is to be developed.

Starting with a scientific rationale, there is broad consensus around the farming practices that can increase soil carbon. These include minimal- or no-tillage, diversifying rotation, cover cropping and grass and legume leys, increased manure application, minimising heavy machinery and the addition of biochar.

The challenge comes when one tries to factor in the varying degrees of effectiveness of these practices in rebuilding carbon, and the underlying factors that influence it, which include crop, soil type, seasonal and climatic conditions across the country. Arable soils have greater scope for increase than grasslands, while scope for sequestration on sandy soils might be as low as 2-3%, versus 30-40% for regions with peaty soil.

Any incentivisation would have to reflect realistic capacity for sequestration – and be location, rather than just crop or practice specific.

Which raises the next challenge – that of measurement. Routine, accurate measurement of soil carbon in agriculture is possible but it can be laborious and expensive, especially when taken at depth. Satellite and drone-based data collection will, in due course, provide ways of measuring agricultural soil carbon accurately and at reasonable cost. In the meantime, the gold standard remains repeated field sampling and laboratory analysis.

Then there is the matter of timeframe. To be genuinely meaningful, sequestration needs to guarantee carbon is kept out of the atmosphere for decades, if not centuries. Soils also have a carrying capacity for carbon, some scientists estimate between 20 and 50 years before global soils are carbon saturated.

Again, this needs to be factored into the design of any incentivisation measures, and the role of legal restrictions (e.g. conservation covenants, clauses in tenancy agreements) should be considered to guarantee longevity. Alongside future sequestration, policy measures should try to ensure current terrestrial carbon stocks remain stable.

Getting to the bottom of all these variables will be key to incentivising soil carbon sequestration – and soil health more generally. Under the existing definition, ‘soil health’ in of itself does not meet the criteria of a public good – something that must be both ‘non-excludable and non-rivalrous’. Since farmers get private, competitive benefit from healthy soil (high-quality, nutritious produce), the ‘public good’ conditions aren’t met, and public funds are therefore not available.

Instead, soil health can only be incentivised as an asset through which other public goods (biodiversity, flood risk mitigation, climate change mitigation) can flow. Since soil carbon is critical to all of these benefits, it might be seen as the golden thread running through the golden thread!

One final significant development will likely stimulate the search for solutions to the challenges laid out above. Until now, the complexity of soil carbon measurement has excluded it from the carbon markets, but this could be about to change. Earlier this year, Indigo Agriculture, a Boston-based agritech start-up, announced that it will start paying farmers up to $15/tonne to store carbon in their soil – an initiative that builds on similar ventures elsewhere in the US and in Australia, and which they hope will lead to the generation of a genuine international carbon market.

The possibility of such a market (including carbon taxes, carbon offsets and carbon credits) would significantly elevate the global economic importance of carbon measurement and throw significant impetus behind finding solutions.

In time, these schemes may provide a lucrative financial injection and share with the public sector the financial burden of delivering healthy soils. However, until they are scalable, only public funding, accompanied by the required educational and regulatory framework can accelerate the widespread behaviour change needed to properly harness soil carbon’s potential.