Soil organic carbon

Technical articles

Biological indicators of soil quality carbon Soil analyses

In the soil, carbon is mainly present in two forms:

• inorganic, constituting carbonate minerals (limestones) of the soil
• organic, constituting organic matter of the soil

The analysis of organic carbon is used to estimate the total organic matter content of the soil.

Standard

In France, there are two standardized analysis methods that serve as references:

• NF ISO 10694 : Soil quality - Determination of organic carbon and total carbon after dry combustion (elemental analysis), also known as the “Dumas method”
• NF ISO 14235 : Soil quality - Determination of organic carbon by sulfochromic oxidation, also known as the “Anne method”

Sampling and logistics

The analysis is performed on a dry, sieved, and ground soil.

Description of the measurement method

Dry combustion method

The standard allows the use of two different techniques.

First method

The approach is based on the measurement of total carbon followed by subtraction by calculation of carbonate carbon.

All carbon present in the soil is oxidized to CO₂ by heating above 900°C under an oxygen flow. The measurement consists of quantifying the released CO₂.

The organic carbon content is then calculated by subtracting the mineral carbon (measured by calcimetry) from the total carbon.

Second method

This approach consists of eliminating carbonates prior to measurement by acid treatment of the sample, so the organic carbon content is measured directly.

Sulfochromic oxidation method

The determination of organic carbon content in soils is done by spectrophotometry, after hot oxidation in a sulfochromic medium in the presence of potassium potassium. Indeed, this dichromate is orange in the form Cr₂O₇²⁻ but becomes green when reduced (Cr³⁺). The color change detected by spectrophotometry thus allows quantifying the amount of organic carbon that has been oxidized.

Comparison of the two methods

The results provided are comparable with similar uncertainties for non-carbonated soils (10%) and carbon contents up to 5-6%.

• For highly carbonated soils, the dry combustion method can be somewhat less relevant due to variability in carbonate measurement within the same soil sample.
• For very organic soils (more than 5-6% organic carbon), the sulfochromic oxidation method gives lower contents than the dry combustion method. It is then necessary to verify the method used by the interpretation reference being referred to.

Example of variation ranges

Indicator	Land use	Number of observations	Min	Max	Median
Organic carbon Dry combustion	Major crops	878	2.58 g C/kg soil	58.2 g C/kg soil	14.61 g C/kg soil
	Grasslands	521	6.78 g C/kg soil	145 g C/kg soil	25.15 g C/kg soil
	Forests	672	0.59 g C/kg soil	170 g C/kg soil	28.2 g C/kg soil
	Vineyards and orchards	59	3.41 g C/kg soil	39.3 g C/kg soil	9.22 g C/kg soil

Source: Data from the RMQS network on 0-30 cm (Saby et al. 2019)

Interpretation

Organic carbon allows estimating the soil organic matter via a conventional multiplication coefficient generally of 1.72. So OM = 1.72 x organic carbon content.

However, the NF ISO 14235 standard specifies that this coefficient can vary between 1.7 and 2.5, without giving criteria for choice (a multiplication coefficient of 2 seems indicated in many situations such as well-humified horizons or forest soils).

The ratio: OM content/carbon content varies with the degree of humification.

Organic carbon should be interpreted according to soil characteristics such as clay content and limestone content (linked to the mineralization kinetics which decrease with clay and limestone content).

The main mechanism of protection of OM is complexation at the surface of clays.

Advantages and limitations

Advantages

• Low cost analysis (20-30 € if the analysis is done alone and less than 15 € if done in a more complete package), routinely performed by all laboratories
• In association with bulk density, stoniness, and sampling depth, it allows to estimate the organic carbon stock
• Fundamental indicator to qualify soils, both as a monitoring or diagnostic indicator

Limitations

• Slow evolution over time, at least 5 years are needed between two analyses to start seeing a significant change
• Global approach, does not provide information on the quality of the materials

Source

• RMT Bouclage. 2025. Biological functioning indicators of agricultural soils. [03/11/25]. https://www.rmt-fertilisationetenvironnement.org/moodle/pluginfile.php/5041/mod_resource/content/3/Recueil%20indicateurs%20de%20fonctionnement%20biologique%20des%20sols%20agricoles_vf251009.pdf
• Balloy B. et al., (2017). Overview of indicators related to the organic and biological state of soils. https://agriculture.gouv.fr/tour-dhorizon-des-indicateurs-relatifs-letat-organique-et-biologique-des-sols]
• Baize D., 2017. From carbon rate to organic matter rate in soils. The words of Agronomy - History and critique under the direction of P. Morlon, INRAe-ACT
• Saby, Nicolas; Bertouy, Benoit; Boulonne, Line; Bispo, Antonio; Ratié, Céline; Jolivet, Claudy. 2019. Summary statistics from RMQS on agronomic data and trace elements of French soils from 0 to 50 cm. https://entrepot.recherche.data.gouv.fr/dataset.xhtml?persistentId=doi:10.15454/BNCXYB