Pillar guide
What is soil organic carbon (SOC)?
Soil organic carbon is the carbon stored in soil organic matter — the decomposing plant, microbial, and animal material in soil. It is one of the largest and most manageable pools in the land carbon cycle, and building it can both improve soil health and remove CO₂ from the atmosphere.
Definition: soil organic carbon
Soil organic carbon (SOC) is the measurable carbon component of soil organic matter. It accumulates as plants add carbon to soil through roots and residues, and it is lost as microbes decompose that material and respire CO₂. The balance between these flows is what land management can shift.
Where it comes from
Plant roots, residues, and exudates add carbon; soil life transforms it into stable and active organic matter.
Where it goes
Microbial decomposition respires carbon back to CO₂. Net change depends on inputs versus losses.
Why it's manageable
Practices like reduced tillage, cover cropping, and residue retention can tip the balance toward accumulation.
SOC vs SIC: organic vs inorganic soil carbon
Soil holds two carbon pools. SOC (organic) sits in organic matter and responds to management over years to decades. SIC (inorganic) sits in carbonate minerals and changes over much longer timescales. Agricultural soil carbon projects focus on SOC because it is the pool land management can realistically influence.
Stocks vs flux: what is actually credited
A SOC stock is how much carbon is in the soil at a point in time (for example, tonnes of carbon per hectare to a set depth). Flux is the rate of change in that stock over time. Carbon crediting is about the change in stock — the additional carbon accumulated because of the project — not the total stock already present.
Stock
The quantity of carbon stored now, measured to a defined depth and corrected for bulk density.
Flux (stock change)
The gain or loss over a monitoring period. Credits reflect additional, verified stock change — not the pre-existing stock.
Why soil organic carbon matters
Building SOC delivers two linked benefits: healthier, more productive, more resilient soils, and net removal of CO₂ from the atmosphere. That dual value is why soil carbon sits at the intersection of agronomy and climate.
Soil health
More organic matter improves structure, water-holding capacity, and nutrient cycling.
Climate
Additional SOC represents CO₂ drawn down and stored in the ground.
Resilience
Better soils buffer drought and heavy rainfall, supporting yields.
Permanence and reversal risk
Soil carbon is durable but not irreversible. If a farm returns to practices that oxidise organic matter, stored carbon can be released again. Credible standards manage this with buffer pools (a shared reserve of credits), ongoing monitoring, and rules that account for reversal risk — so a claimed removal reflects durable, not fleeting, storage.
How SOC is measured (in brief)
SOC is quantified by sampling soil, measuring carbon concentration and bulk density in an accredited lab, and increasingly combining those measurements with satellite remote sensing and calibrated process models. Well-designed MRV targets ≤±15% uncertainty on stock change, with annual verification.
Measurement is where projects earn trust. Explore the detail:
Frequently asked questions
What is soil organic carbon in simple terms?+
It is the carbon stored in the organic matter of soil — the remains of plants, microbes, and other living things at various stages of decomposition. It is distinct from soil inorganic carbon, which is held in minerals like carbonates.
What is the difference between SOC and SIC?+
SOC (soil organic carbon) is carbon in organic matter and is strongly influenced by land management. SIC (soil inorganic carbon) is carbon in soil minerals such as carbonates, which changes far more slowly. Carbon projects almost always target SOC.
Is soil carbon permanent?+
Soil carbon is durable but reversible: gains can be lost if practices revert (for example, a return to intensive tillage). Credible programmes manage this with buffer pools, monitoring, and reversal-risk rules.
How is soil organic carbon measured?+
Through soil sampling and lab analysis of carbon concentration and bulk density, increasingly combined with satellite remote sensing and process models. Leading MRV targets ≤±15% uncertainty on stock change.
Ready to build lasting soil carbon?
Start with a free SOC feasibility assessment — a fast, honest read on whether a project makes sense for your land and goals.