The short answer. Soil organic carbon is the carbon held in the organic part of the soil: decomposed plant and animal residues, living organisms, and substances made by soil organisms. It governs soil structure, how water gets in and is held, and how nutrients are made available to the crop. When it declines it declines invisibly, which is why it is the early warning system for the condition of the one asset on the farm whose condition is not routinely monitored.
What soil organic carbon is made of
Soil organic carbon, often shortened to SOC, is a complex mix of decomposed plant and animal residues, living organisms, and substances synthesised by soil organisms. It gets there through photosynthesis: crops and green cover draw carbon down from the atmosphere, pass some of it into the soil through their roots and return more as residue. Microbial and fungal life then decomposes that organic matter and cycles its nutrients.
Soil organic carbon is closely related to soil organic matter, the term many farmers know from a standard soil test. Organic matter is the material; organic carbon is the carbon within it, and it is the carbon that is measured and reported when you want to know what your management is doing to the soil.
Why it matters on a farm
Soil is your core asset, underpinning business performance and future land value. Soil organic carbon governs four things that decide how well that asset performs:
- Structure. How the soil holds together, drains and resists compaction and erosion.
- Infiltration. How quickly rain gets into the soil rather than running off it.
- Water holding. How much water the soil keeps for the crop through a dry spell.
- Nutrient availability. How much of what the crop needs is held and released by the soil itself.
Across our monitoring programmes we are seeing an emerging relationship between soil organic carbon stock change and yield stability. Stable and increasing carbon stocks protect the following year’s yield against too much water or too little. In the UK, five annual monitoring cycles have included two droughts, and the fields that held their carbon held their yield. The detail is in soil organic carbon and yield.
Concentration and stock
Soil organic carbon can be expressed two ways. Concentration is the share of the soil that is organic carbon, as a laboratory reports it for a sample. Stock is the amount of carbon held in the soil across an area, reported in tonnes of carbon per hectare. Stock is what matters when you want to know whether a field is gaining or losing carbon, and it is what we report: the change in stock against your own baseline, field by field, with the accuracy stated.
How soil organic carbon is measured
Real soil samples are taken on every farm we monitor and analysed by dry combustion in an ISO-accredited laboratory, which measures the carbon directly rather than estimating it. Our AI then ties those samples to satellite imagery at 100 readings a hectare, a value every 10 metres, so the result is a map and a field total rather than a list of sample points. The error of that step is measured against samples the AI never saw and deducted in full. The full method is in how soil carbon is measured, and how the error is handled is in accuracy stated.
Why it has to be measured every year
Soil carbon loss does not appear in a yield map, a soil test or a set of accounts. It appears in a poor season, as yield loss you cannot explain, and by then the decline may have been running for years. Measured once a year, at the same point in the year, against your own baseline, a bad year shows up in that year while the fix is still cheap. It works the way an annual blood-pressure check works for your health: it catches decline before the symptoms arrive.
Questions we are asked about soil organic carbon
Is soil organic carbon the same as soil organic matter?
They are closely related but not the same. Organic matter is the material in the soil that came from living things; organic carbon is the carbon within it. Carbon is what is measured and reported when you want to track what your management is doing to the soil.
Can a standard soil test tell me my soil carbon?
A standard soil test averages a field into one number, and the variation disappears: the headland, the light patch, the corner that goes first in a dry year. To see change you can act on, you need samples analysed for carbon, tied to imagery across the field, measured at the same point each year against a baseline.
How do I increase soil organic carbon?
Keep living roots and green cover for as much of the year as possible, add diversity to the rotation, cultivate less, and return residue and organic manures to the soil. Which of these works on your fields is what measurement shows. See how to increase soil organic carbon.
How do I get my soil organic carbon measured?
Send us your SBI number and your hectares and we will send a written quote within two working days. Minimum project area is 150 hectares. Get a quote.
Part of our guide to soil carbon measurement for farms. Related: how to increase soil organic carbon · how soil carbon is measured · what a baseline is.
