Soil carbon calculator: from organic matter to tonnes of carbon

How it works

Soil tests often report organic matter (SOM), but carbon programs and research count soil organic carbon (SOC). Organic matter is only partly carbon; the rest is mostly oxygen, hydrogen and nitrogen. So the first step divides by a conversion factor:

SOC (%) = SOM (%) / factor

The default factor is 1.724, traditionally credited to the Dutch chemist J. M. van Bemmelen in the late nineteenth century. It assumes organic matter is 58% carbon. In a 2010 review in Geoderma, D. W. Pribyl argued that this is too low for most soils and that a factor of about 2, meaning 50% carbon, is usually closer. The tool offers both. If your lab report already gives organic carbon, often measured by dry combustion, you can skip this step.

The second step turns a percentage into a stock, the mass of carbon under one hectare down to the depth you sampled:

t C/ha = SOC (%) × bulk density (g/cm³) × depth (cm) × (1 - stone fraction)

The units work out neatly. A hectare of soil 1 cm deep, at a bulk density of 1 g/cm³, weighs 100 tonnes, so 1% of it is 1 tonne.

Bulk density is the dry mass of soil per unit of volume, pores included. Without a lab figure, the tool offers typical starting values: 1.55 for sandy soil, 1.35 for loam, 1.3 for silt loam, 1.2 for clay and 0.6 for organic or peaty soil. Stones and gravel hold almost no organic carbon, so the stone fraction is taken out of the soil volume. The tool caps it at 90%.

Last, tonnes of carbon convert to tonnes of CO2 by multiplying by 44/12, and per hectare converts to per acre by dividing by 2.471. Per-acre results come in metric tonnes and in US short tons.

Worked example

The tool opens with 3.448% organic matter. Divided by 1.724, that is 2.0% organic carbon.

With a bulk density of 1.3 g/cm³, a depth of 30 cm and no stones:

2.0 × 1.3 × 30 = 78 t of carbon per hectare.

In CO2 terms, 78 × 44 / 12 = 286 t per hectare. Per acre, 78 / 2.471 is about 31.6 metric tonnes of carbon, or 34.8 US short tons.

Now suppose management raised organic matter by one percentage point in that same 30 cm layer, from 3.448% to 4.448%, with bulk density unchanged. With Pribyl's factor of 2.0, organic carbon rises by 0.5 points, and the stock by 0.5 × 1.3 × 30 = 19.5 t of carbon per hectare, about 71.5 t of CO2. With the van Bemmelen factor, the same change works out to about 22.6 t of carbon. The choice of factor alone moves the answer by about 16%.

A full point of organic matter is a large change for most farmed soil, and usually a slow one. Our piece on what soil carbon science can promise looks at how hard such changes are to measure and to pay for.

What it can't tell you

Depth. The tool counts only the layer you enter. Many programs sample the top 30 cm, but carbon below that counts too, and some practices move carbon between layers rather than adding it. No-till, for example, can raise carbon near the surface while deeper soil holds less.

Bulk density. It weighs as much as the carbon percentage, because the stock is a straight product of the two. A table value is only a starting guess for a given field, and bulk density itself changes with tillage, traffic and compaction.

Equivalent soil mass. If bulk density changes between two sampling dates, a fixed 30 cm core holds a different mass of soil each time, and the carbon stock can appear to rise or fall for that reason alone. Careful studies compare equal masses of soil rather than equal depths. This tool works with a fixed depth, so use it to compare like with like.

Variability. Soil carbon varies a lot across one field, and a year's change is small next to that spread. Detecting a real change takes many samples and usually several years.

Permanence. Carbon stored in soil can be lost again if a practice stops, the land is plowed or the climate warms. Gains also slow as the soil settles toward a new balance. A stock figure says nothing about how long the carbon will stay.

Organic only. Soils on limestone or in dry regions can hold inorganic carbon as carbonates. That is not organic carbon and does not belong in this calculation.

Building organic matter on a working farm, season by season, is the subject of our piece on agroecology on the farm. To put other greenhouse gases on the same CO2 footing, use the CO2e calculator.

Sources

• J. M. van Bemmelen's late nineteenth century work, the traditional origin of the 1.724 factor (organic matter taken as 58% carbon).
• D. W. Pribyl, "A critical review of the conventional SOC to SOM conversion factor", Geoderma 156 (2010), which argues for a factor of about 2.
• B. H. Ellert and J. R. Bettany, Canadian Journal of Soil Science (1995), the usual reference for comparing carbon stocks on an equivalent soil mass basis.

Questions

How do you convert organic matter to organic carbon?

Divide the organic matter percentage by a conversion factor. The traditional van Bemmelen factor is 1.724, which assumes 58% carbon; Pribyl's 2010 review suggests about 2.0 for most soils. So 3.448% organic matter is 2.0% organic carbon with the first factor and about 1.7% with the second.

How do you calculate soil carbon stock?

Multiply organic carbon (%) by bulk density (g/cm³), by depth (cm) and by the share of the soil that is not stones. The result is tonnes of carbon per hectare: 2 × 1.3 × 30 = 78 t C/ha.

How much carbon is in an acre of soil?

It varies widely. For a soil with 2% organic carbon, a bulk density of 1.3 and a 30 cm sampling depth, it comes to about 31.6 metric tonnes, or 34.8 short tons, of carbon per acre.

How much CO2 is one tonne of soil carbon?

About 3.67 tonnes of CO2, from the ratio of the molecular masses, 44/12.

What is bulk density in soil?

The dry mass of soil in a given volume, pores included, usually given in g/cm³. Most mineral soils fall roughly between 1.0 and 1.6, and peaty soils are much lighter.