Particle size fractionation of organic matter
The granulometric fractionation of organic matter (OM) distinguishes it according to its size, into coarse fractions (> 50 µm) and fine fractions (< 50 µm), within which nitrogen and carbon are distributed.
The fine fraction corresponds to humified OM, that is to say the most stable part of the OM. It is present in greater proportion in temperate climate.
The coarse fractions, also called “particulate OM”, correspond to plant debris in the process of decomposition. These fractions constitute a source of energy for living organisms in the soil. They constitute the so-called “labile” or active part of the OM.
The analysis can be performed on two fractions (<50 µm, 50 to 2000 µm), or three fractions (<50 µm, 50 to 200 µm, 200 to 2000 µm). Fractionation thus separates the different qualitative compartments of OM according to a size criterion:
- 200 - 2000 µm = the fresh fraction which groups the coarsest and youngest OM (2 to 5 years). It ensures the roles of element reserve, energy reserve, and physical protection of the soil.
- 50 - 200 µm = the labile fraction which groups OM in the process of decomposition. It represents the reserve of energy and nutrients available in the medium term (10 to 12 years) for soil living organisms and crops.
- 0 - 50 µm = the stable fraction which groups the finest and most degraded OM (>50 years). It plays a role in soil structuring and stabilization in the long term, water retention as well as in its cation exchange capacity (thus nutrients are better retained and slowly released to plants).
Standard
NF X31-516: Soil quality - Granulo-densimetric fractionation of particulate organic matter of soil in water
Sampling and logistics
Analysis on dry soil.
Description of the measurement method
The measurement consists of a physical fractionation of fine earth (dispersion of soil in aqueous medium, sieving, sedimentation, centrifugation), a weighing of fractions and a measurement of C and N contents in the fractions.
Example ranges of variation
For the first 20 centimeters of soil occupied by arable crops and polyculture-livestock (183 observations):
| Soil depth studied | Min | Max | Median | |
| Amount of C | 0 - 50 µm | 6 g/kg dry matter | 27 g/kg dry matter | 11 g/kg dry matter |
| 50 - 200 µm | 0.5 g/kg dry matter | 4.2 g/kg dry matter | 1.7 g/kg dry matter | |
| Share of organic carbon | 0 - 50 µm | 72% | 94% | 88% |
| 50 - 200 µm | 6% | 28% | 12% | |
| Amount of N | 0 - 50 µm | 0.5 g/kg dry matter | 2.7 g/kg dry matter | 1.2 g/kg dry matter |
| 50 - 200 µm | 0 g/kg dry matter | 0.3 g/kg dry matter | 0.1 g/kg dry matter | |
| Proportion of nitrogen | 0 - 50 µm | 80% | 97% | 93% |
| 50 - 200 µm | 3% | 20% | 7% |
For the first 20 centimeters of soil occupied by viticulture (42 observations):
| Soil depth studied | Min | Max | Median | |
| Amount of C | 0 - 50 µm | 5 g/kg dry matter | 26 g/kg dry matter | 12 g/kg dry matter |
| 50 - 200 µm | 2 g/kg dry matter | 10 g/kg dry matter | 4 g/kg dry matter | |
| Share of organic carbon | 0 - 50 µm | 49% | 85% | 76% |
| 50 - 200 µm | 15% | 51% | 25% | |
| Amount of N | 0 - 50 µm | 0.4 g/kg dry matter | 2.5 g/kg dry matter | 1 g/kg dry matter |
| 50 - 200 µm | 0.1 g/kg dry matter | 0.7 g/kg dry matter | 0.3 g/kg dry matter | |
| Proportion of nitrogen | 0 - 50 µm | 56% | 93% | 81% |
| 50 - 200 µm | 7% | 44% | 19% |
Interpretation
The evolution of the share of the different fractions is known to inform about the evolution of the soil organic status over a shorter time frame than the total soil carbon and nitrogen content.
An increase in the share of C or N in the coarse fraction is considered a precursor sign of soil enrichment in organic matter.
The proportion of the fine fraction allows to identify the long-term storage of carbon (soil residence time of several decades). It corresponds to the stable fraction and influences water retention and cation exchange capacity.
A high value of carbon in the < 50 µm fraction indicates a high chemical stabilization of C in soils (organo-mineral bonds), linked to a carbon storage function. Carbon in fractions larger than 50 µm, easily available to soil organisms, is related to OM mineralization and macroaggregation.
A high value of nitrogen in fractions > 50 µm seems favorable to nitrogen supply for crops.
An optimal distribution of OM fractions would be: 85% stable OM, 10% labile OM, and 5% fresh OM, however, this distribution should be reviewed according to established objectives.
Advantages and limitations
Advantages
- Coarse fractions (particulate organic matter) respond quickly to changes in practice (4 to 5 years)
- Flexibility in sampling periods and sample preservation (analysis on dry soil)
- Usable in advisory situations
Limitations
- One limitation is its cost (80-120€) because these indicators multiply the number of analyses, and thus the service fees. The link between fractions and carbon dynamics remains to be clarified.
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 status of soils. https://agriculture.gouv.fr/tour-dhorizon-des-indicateurs-relatifs-letat-organique-et-biologique-des-sols
- Deschamps T. et al. 2022. Interpretation guide for bioindicator analysis. https://www.arvalis.fr/infos-techniques/douze-indicateurs-pour-evaluer-la-fertilite-biologique-du-sol
