Organic matter
The different categories of organic matter, techniques for increasing soil OM content, ...
In agriculture, soil organic matter (SOM) is a complex mixture of animal and plant residues at all stages of decomposition, living and decomposing microbial tissues and heterotrophic biomass, as well as relatively stable humic substances. It plays an essential role in the structurefertility and thechemical and biologicalactivity of soils, as well as providing various ecosystem services (carbon sequestration, support for biodiversity, etc.).
Definition
Soil is created when plant and animal life settle in the decomposition debris of a parent rock. When these living beings die, their matter becomes incorporated into the soil, mixing with the mineral substances. They then represent the"organic constituents" or"organic matter" (OM).
Various indicators can be used to assess the quality of an organic amendment.
C/N
The C/N ratio is one of the quality indicators for organic products. It is the ratio of grams of carbon to grams of nitrogen. Depending on their origin, nature and age, OMs have highly variable proportions of carbon and nitrogen (as well as oxygen and hydrogen).[1]
| Product | C/N |
|---|---|
| Protein | 3 à 4 |
| Bacteria | 4 à 10 |
| Faba bean | 12 à 15 |
| Vine shoot | 50 à 90 |
| Wheat straw | 70 à 100 |
| Wood bark | 100 à 300 |
| Sure powder | infinite (no nitrogen) |
C/N increases with plant age (transformation of mineral carbon (CO2) into organic carbon (sugars)). Plants accumulate more carbon (which is produced) than nitrogen (which is absorbed by the roots). The older the plant, the more organic carbon it will have produced and therefore the higher its C/N will be. However, this rate of accumulation differs greatly between 2 different plant species.
A common misconception is that a low C/N corresponds to high mineralisation. This is true in a number of situations, but not all. For example, powdered sugar is more degradable than wood bark, even though the C/N of sugar is higher. The degradability of a product is determined by its molecules and not by its carbon and nitrogen content. This is why a new, more reliable indicator has been developed : theISMO (Organic Matter Stability Index).
- An organic product with a high C/N corresponds to a young OM.
- Conversely, if the C/N of the product is low, this means that the OM is older.
In soils, a high C/N can be explained in 2 ways :
- Recent inputs of fresh OM with a high C/N : high soil C/N and high mineralisation.
- Environmental conditions unfavourable to soil life : acid pH <5, low temperature, waterlogging, compaction.
A high C/N is not necessarily a sign of slow mineralisation, but rather of young OM or OM that is blocked in its mineralisation.
Nor does a high C/N imply nitrogen starvation. This phenomenon can be explained by a combination of conditions :
- Organic matter with a high C/N.
- Highly digestible OM (e.g. : plant cover, straw, etc.).
- The OM has been integrated into the soil and not left on the surface.
- Too little nitrogen available in the soil to feed both plants and organisms.
Nitrogen hunger is therefore soil and plant dependent.
Conversely, a low C/N does not always provide good nitrogen fertilisation. This is because fertilisation depends on two conditions :
- Low C/N
- High digestibility of the organic product by soil micro-organisms
If the organic product is stable, the C/N will have no impact on nitrogen availability (e.g. : compost, methanisation digestates, etc.).
Used on its own, this quality criterion has its limits : research shows that nitrogen availability depends more on the location of this element in the various biochemical fractions (solubles, hemicellulose, cellulose, lignin) than on the overall C/N ratio of the product.
ISMO
The ISMO (Indice de Stabilité des Matières Organiques - Organic Matter Stability Index) is an indicator, developed by INRA, which represents the percentage of stable organic matter in relation to its total organic matter content. It therefore gives an indication of the quantity of organic matter needed to reconstitute the soil's humic stock. The higher the ISMO, the more stable the amendment will be in the soil.
An essential component of conservation agriculture, increasing soil organic matter levels is an agro-ecological practice whose results can be seen over the long term.
The different categories of OM
Based on a functional approach, we can classify different types of organic matter for different functions :
| Type of OM | Function |
| Living organic matter :
living plants and animals |
Transformation/mineral |
| Fresh organic matter :
plant and animal debris |
Energy substrate and
chemical growth/fertility |
| Transitional organic matter :
advanced matter (cellulose lignin, proteins) |
Energy substrate/
physical fertility (soil structure) |
| Humic (or stable) matter :
lignin, cellulose nitrogenous matter |
Physical fertility
(long-term stability) |
The organic matter mineralisation cycle
Biological activity (in soils or in composting or methanisation facilities) breaks down organic matter and transforms it into mineral matter : methane and carbon dioxide, water and nutrients(nitrogen, phosphorus, etc.).
This breakdown provides plants with a large quantity of nutrients to which they do not have access in organic form.
Plants absorb mineral elements through their roots. These elements must therefore be in soluble form.
Minerals present in the soil are only partially in soluble form. They can be reorganised by soil microfauna into organic or organo-mineral form (nitrogen, phosphorus), adsorbed by soil complexes, immobilised in mineral form (phosphorus), or transformed into gas (nitrogen). Each mineral has its own geobiochemical cycle.
Soil biodiversity
Soil biodiversity representsall the species of living organisms found in our soils.[2]. This includes animals, fungi and micro-organisms. Soil organisms can be classified into 5 main categories :
- Megafauna : "large" animals such as mice and shrews, which provide the soil with organic matter and use it as shelter and food.
- Macrofauna : mainly represented by earthworms, the "engineers" of the soil, they aerate the soil and make it permeable.
- Mesofauna : springtails and mites play a major role in the decomposition of organic matter.
- Microfauna : fungi and nematodes, the former of which can be mycorrhizal and the latter, although often plant parasites, in some cases help to decompose organic matter.
- Microorganisms : mainly bacteriaThese are the most numerous and diverse soil organisms. They recycle nutrients and help to develop and structure the soil.
Benefits and importance of a high OM content
A high level of OM in the soil has many benefits. Through its various roles and actions, OM helps to maintain soil quality.[3]
| Role | Action | Benefit |
|---|---|---|
| Physical role
(cohesion) |
Structure, porosity and
waterretention |
-Penetration and
storage of water - Erosion control -Reduced compaction -Warming |
| Biological role
(energising) |
Stimulation of
biological activity |
-Humification,
mineralisation, etc. -Aeration -Root growth |
| Chemicalrole
(nutritive) |
Mineralization, cation exchange
cation exchange capacity, retention of micropollutants and pesticides |
-Supply ofelements
mineral elements - Storage and availability of mineral elements -Limiting toxicity -Water quality |
Increase and maintain the level of organic matter in the soil
- Leave residues from the previous harvest on the plot after harvesting.
- Establish intercroppingplant cover.
- Add animal waste by grazing or spreading manure, slurry, compost or other carbon-based amendment.
- Apply ramial fragmented wood (RCW) [4].
- Accelerate the degradation of biomass by burying it in the soil using tillage techniques such as ploughing, which will distribute the organic matter evenly over the entire height of the tilled surface. On the other hand, direct sowing under plant cover will encourage an accumulation of organic matter on the surface of the soil.
- Avoid crops thatconsume humus.
- Avoid tillage, stubble ploughing and soil compaction.
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Source
- ↑ Thibault Déplanche, 2023 : Understanding everything about C/N https://www.linkedin.com/posts/thibaut-deplanche_id%C3%A9e-re%C3%A7ue-cn-haut-implique-faim-dazote-activity-7091681634449506304-HsWO/?utm_source=share&utm_medium=member_desktop
- ↑ Vincent et al, 2023 : Soil biodiversity https://aube-haute-marne.chambres-agriculture.fr/fileadmin/user_upload/National/FAL_commun/publications/Grand-Est/67_fertilite_des_sols_importance_matiere_organique_2011.pdf
- ↑ CA Aube Haute Marne, 2011 : The importance of organic matter https://aube-haute-marne.chambres-agriculture.fr/fileadmin/user_upload/National/FAL_commun/publications/Grand-Est/67_fertilite_des_sols_importance_matiere_organique_2011.pdf
- ↑ The " ramial fragmented wood " A tool for boosting soil organic matter(https://transgal.projet-agroforesterie.net/pdf/TCSBRF.pdf)
