Soil activated carbon
The carbon oxidized by potassium permanganate, sometimes called active or labile carbon, represents a less stable fraction of carbon. It is rapidly degradable (from a few days to a few months) and constitutes a source of oxidizable carbon: a fraction of energy for the organisms living in the soil.
Sampling and logistics
Analysis on dry soil.
Description of the measurement method
This is a measurement by cold mild oxidation of the soil organic matter using a reagent, potassium permanganate (KMnO4).
The method was adapted from Weil et al., (2003) and Culman et al., (2012):
- extraction of air-dried soil sieved at 2 mm with concentrated KMnO4 at 0.02 mol/L (ratio of one part soil to 8 parts solution, according to a mass/volume ratio)
- then measurement by UV spectrophotometry. This last step allows measuring the amount of KMnO₄ consumed to determine the content of oxidizable organic matter in the soil.
Example ranges of variation
Context:
- Climate: temperate (mainland France)
- Texture: all
| Indicator | Min | Max | Median |
|---|---|---|---|
| Carbon oxidized by KMnO4
(Land use: Large crops and polyculture-livestock - 183 observations at soil depth: 0-20cm) |
390 mg C/kg soil | 1414 mg C/kg soil | 715 mg C/kg soil |
| Carbon oxidized by KMnO4
(Land use: Vineyards - 32 observations at soil depth: 0-20cm) |
325 mg C/kg soil | 1223 mg C/kg soil | 759 mg C/kg soil |
| 4.6% of total organic C | 7.9% of total organic C | 6.6% of total organic C |
Interpretation
A high value of carbon will translate into structural stability as well as potentially high water and air circulation. The mineralization of organic matter will then also be high. This indicator can be combined with in situ respiration/mineralization potential measurements.
Advantages and limitations
Advantages
- Low-cost analysis, feasible on dry samples
- Sensitive to agricultural practices
Limitations
- Measurement seems less relevant in environments with significant carbon inputs into the system (e.g., livestock effluent systems)
- Current scientific questioning about the type of carbon targeted by the method
- The measurement protocol may differ between laboratories. For monitoring, ensure to use the same technique
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 [Available online: https://agriculture.gouv.fr/tour-dhorizon-des-indicateurs-relatifs-letat-organique-et-biologique-des-sols].
- Cusset, E., Bennegadi-Laurent, N., Recous, S., Bernard, P. Y., Perrin, A. S., Tscheiller, R., ... & Riah-Anglet, W. (2024). Which soil microbial indicators should be included in routine laboratory tests to support the transition to sustainable management of arable farming systems? A meta-analysis. Ecological Indicators, 167, 112706.
- Culman, S. W., Snapp, S. S., Freeman, M. A., Schipanski, M. E., Beniston, J., Lal, R., ... & Wander, M. M. (2012). Permanganate oxidizable carbon reflects a processed soil fraction that is sensitive to management. Soil Science Society of America Journal, 76(2), 494-504.
- Deschamps T. et al. (2022). Guide to interpreting bioindicator analysis. [Online: https://www.arvalis.fr/infos-techniques/douze-indicateurs-pour-evaluer-la-fertilite-biologique-du-sol]
- Weil, R. R., Islam, K. R., Stine, M. A., Gruver, J. B., & Samson-Liebig, S. E. (2003). Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use. American Journal of Alternative Agriculture, 18(1), 3-17. OAD project MO (2017-2019), funded by FranceAgrimer and led by IFV [Unpublished results]
