Soils hold more carbon than the atmosphere and all vegetation combined, which has made increasing that store an attractive climate proposal. The attraction rests on a real mechanism: ploughing exposes organic matter to oxidation, and practices that avoid it — no-till planting, cover crops, returned residues — can rebuild what was lost. The difficulty is that the store saturates. A soil accumulating carbon does so at a declining rate for two or three decades and then stops, so the total available is a one-off recovery of what previous management released rather than an ongoing sink. It is worth having and it is not a substitute for reducing emissions. Estimates of the global potential differ by an order of magnitude depending on how long accumulation is assumed to run.