The short answer. Soil organic carbon rises when more carbon goes into the soil than comes out. On a working farm that means keeping living roots and green cover for as much of the year as possible, more diversity in the rotation, less cultivation, and returning crop residue and organic manures to the soil. Which combination works on your farm, in your soil and your weather, is something only measurement can tell you, field by field.
Where soil carbon comes from
Carbon moves from the atmosphere into the soil through photosynthesis. Plants draw it down, pass some of it to the soil through their roots, and return more as residue. Soil life, including microbial and fungal biomass, then decomposes that organic matter and cycles its nutrients. Every day a field carries a growing crop or green cover is a day that carbon is still being drawn down. Every day it lies bare, or is cultivated, is a day carbon can be lost.
That is why the practices that build soil carbon all do one of two things: put more carbon in, or stop it coming out.
Five practices that build soil organic carbon
1. Living roots and green cover for as many days as possible
The single biggest lever is time under green cover. Catch and cover crops close the gap between cash crops, so the field keeps drawing down carbon into late summer and autumn, and the soil is protected from over-winter rainfall, which can erode and leach carbon stocks.
2. More diversity in the rotation
A more diverse rotation puts different root systems and residues into the soil across the years, and supports a broader base of soil life to process them.
3. Less cultivation
Cultivation breaks up soil structure and exposes organic matter to the air, where it is broken down and lost. Minimising cultivation and bare soil keeps more of the carbon you have built where it is.
4. Returning residue to the soil
Crop residue left on or in the field is carbon returned rather than removed. Maximising returned organic matter is one of the most direct ways to feed the soil.
5. Organic manures and amendments
Organic manures and other organic amendments add carbon directly and feed soil life. In the published two-farm comparison below, the farm that gained carbon in a wet year was applying organic nitrogen.
What it looks like when it works, and when it does not
Heavy rain is when soil is most exposed. Water moving across bare or low-cover ground carries carbon-rich topsoil with it, and a wet winter can undo years of careful work in a single season. In one such year, two farms on the same clay loam went in opposite directions.
- Farm one, no regenerative practices: no cover cropping, very little catch cropping, no organic nitrogen. Soil organic carbon change: minus 28 tonnes a hectare.
- Farm two, regenerative practices: cover cropping, organic nitrogen applied. Soil organic carbon change: plus 7 tonnes a hectare.
One farm lost ground in the year that tested it. The other gained. That is what a regenerative system looks like under stress, and it was only visible because both farms were measured. These results were published in the soil health measurement paper we were commissioned by WWF to write for Codex Planetarius in December 2025. More on what the measurements show is in soil organic carbon and yield.
Why the payback is more than carbon
Rising soil carbon reflects better structure, more water infiltrating and being held, and more organic nutrients available to the crop. That is what protects yield and cashflow through the weather events that punish everyone else. 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.
Why you have to measure it
These principles are sound, but soils, rotations and weather differ. A practice that builds carbon on one farm can do little on another, and a system can look regenerative on paper while a field quietly goes backwards. Soil carbon loss is invisible: it 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.
Measuring your fields once a year, against your own baseline, shows which of your practices are working and catches decline while the fix is still a rotation change rather than a rebuild. Because we relate soil carbon change to management practice across every project we monitor, we can also tell you what turned the same pattern around on farms like yours. How the measurement is done is in how soil carbon is measured.
Questions we are asked about building soil carbon
How quickly can soil organic carbon increase?
It depends on the soil, the starting point, the practices and the weather in each year, which is why a single number from a textbook is not much use on your farm. What we can say is that change shows up in annual measurement, in both directions: in one wet year a farm without regenerative practices lost 28 tonnes a hectare, while a regenerative farm on the same clay loam gained 7.
Do cover crops increase soil carbon?
Catch and cover crops are one of the clearest ways to close the green-cover gap between cash crops: they keep carbon being drawn down into late summer and protect the soil from over-winter rainfall. Whether they build carbon on your fields, and by how much, is what annual measurement shows.
Can soil carbon go down even if I farm regeneratively?
Yes. A wet winter, a dry year or a single field managed differently can push carbon down. That is exactly why measuring every year matters: a bad year shows up in that year, while you can still act.
How do I find out whether my soil carbon is rising?
Measure it, field by field, against a baseline, at the same point every year. 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: what soil organic carbon is · soil carbon and yield · why once a year.
