Carbon farming

Carbon farming is a bundle of agronomic practices aimed at raising the stock of organic carbon in soil: cover crops and lengthened rotations, reduced or zero tillage, residue retention, organic amendments, perennials and integrated grazing. The mechanism behind all of them is the same. Soil organic matter is the difference between carbon entering as residues and root exudates and carbon leaving as microbial respiration. Practices act either on the input, raising the share of assimilated carbon that reaches the soil, or on the output, slowing mineralisation by preserving aggregate structure and the physical protection of organic matter inside aggregates.

Saturation#

The first property of the result is that it is finite. The gap between input and output does not stay constant: as the stock grows, so does the substrate available for respiration, so the accumulation rate declines and the soil approaches a new equilibrium asymptotically. The global meta-analysis of conversion to no-till (West and Post, 2002) gives a mean rate on the order of 0.57 tonnes of carbon per hectare per year, peaking in the first 5–10 years and approaching a new equilibrium within 15–20. Which means the carbon flux sold as annual is intrinsically transitional: it exists in the passage between two steady states and ends of its own accord.

There is a stoichiometric limit as well. Stabilised soil organic matter has a fairly narrow ratio of carbon to nitrogen, phosphorus and sulphur; carbon cannot be accumulated without immobilising a proportionate quantity of nitrogen and phosphorus. A good share of claimed carbon gains implicitly assumes nitrogen that has to come from somewhere.

Reversibility#

The second property is harsher. Carbon protected inside soil aggregates is protected mechanically. One tillage operation breaks the aggregates, exposes the organic matter to decomposition, and can return years of accumulation to the atmosphere. There is no equivalence between storing carbon in soil and storing it in a geological reservoir: the soil stock is held only by continuing the practice. A change of owner, of crop price or of rotation, and it leaves.

Hence a third point that is often missed: reduced tillage redistributes carbon towards the surface. Sampling at 0–30 cm shows a gain; sampling to a metre, in many studies, does not confirm it. That is a methodological rather than a rhetorical problem, and it is handled on the soil carbon MRV page, which owns the measurement question. Baselines, additionality and permanence as market design belong to the carbon markets page.

What remains true#

Neither saturation nor reversibility cancels the agronomic value of these practices. More organic matter means better structure, infiltration and water retention, less erosion and more drought resilience — and that benefit accrues to the farm whether or not the carbon is ever sold. The defensible position is to treat the climate effect as secondary and bounded, and the soil effect as the main one.