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Soil Organic Carbon Stock Calculator

Convert a soil test into carbon stock — soil organic matter or organic carbon percentage, bulk density, sampling depth and stone content to tonnes of carbon and CO₂-equivalent per hectare, the change between two samplings as a sequestration rate, and what a 0.4% per year (4 per 1000) target means for your field.

Soil organic matter (loss on ignition)

Soil organic carbon, if measured directly (0 = derive from SOM)

Bulk density

Sampling depth

Coarse fragments (> 2 mm) by volume

Field area

Previous SOC % at the same depth (0 to skip the change)

Years between samplings

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How it works

SOC % ≈ SOM % × 0.58 · stock (t C/ha) = SOC % × bulk density (g/cm³) × depth (cm) × (1 − stone fraction) · CO₂e = C × 44 ÷ 12 · 4-per-1000 target = 0.4% of stock per year

Soil holds more carbon than the atmosphere and vegetation combined, and turning a laboratory percentage into tonnes per hectare needs two more numbers most soil tests skip: how dense the soil is and how deep the layer goes, with stones subtracted because they hold no carbon. The units line up conveniently so that one percent carbon in one centimetre of soil at a density of one is one tonne per hectare. Loss-on-ignition tests report organic matter, of which carbon is roughly 58%. Comparing two samplings gives a sequestration rate, but soils change slowly and vary enormously across a field, so detecting a real change demands many cores, consistent depth and measured density — otherwise a difference in compaction masquerades as gained or lost carbon.

Worked example

A loam with 3.4% organic matter (≈ 1.97% carbon), bulk density 1.3 g/cm³ and 5% stones holds 1.97 × 1.3 × 30 × 0.95 ≈ 73 t C/ha in the top 30 cm — 268 t CO₂e per hectare, 1,460 t C across a 20 ha field. The 4-per-1000 aspiration would mean adding about 0.29 t C/ha a year (1.07 t CO₂e); if a sampling five years earlier had shown 1.85% carbon, the field has gained about 4.5 t C/ha, 0.90 t C/ha/yr.

Frequently asked questions

Why 0.58?

The van Bemmelen factor assumes organic matter is 58% carbon. Modern work suggests 0.5 is often closer; if your lab measured carbon directly (dry combustion), use that and skip the conversion.

Can soil carbon be sold as offsets?

Some schemes pay for verified increases, but they require baseline and repeat measurements to protocol, permanence commitments and buffers for reversal — and the annual gains per hectare are small in money terms. Treat it as a co-benefit of good soil management first.

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