Composting is a technique of decomposition and transformation of organic matter resulting from a biological process through the combined action of soil micro-organisms and macro-organisms. In general, the process takes place over a period ranging from a few weeks to 9 months under controlled aerobic conditions (aeration, temperature, humidity). Raw materials of plant and/or animal origin are thus stabilized and sanitized. The final product, compost, is a stable organic amendment rich in humus, which can be applied to the soil to improve fertility, structure, and biological activity.

Composting unfolds in several successive phases, each characterized by specific microbial and thermal dynamics. The initial activation phaserapidly degrades easily degradable organic materials (sugars, cellulose), causing a rise in temperature to 30-40°C and the main loss of mass. Thethermophilic phase increases the temperature up to 70-80°C, promoting the degradation of more complex compounds (waxes, hemicelluloses) and ensuring sanitization that eliminates undesirable bacteria and contaminants. Then, during the cooling phase, the temperature decreasestoward 40°C and the degradation of more resistant materials continues over several weeks. Finally, the maturation phase stabilizes the compost atambient temperature, leading to the progressive degradation of lignin and the formation of stable humus over several months. To ensure thesetransformations, humidity (around 50-60%) and oxygenation must be maintained within optimal ranges. The recommended composition is basedon a ratio of 70% nitrogen-rich material (grass clippings, animal feces) and 30% carbon-rich material (straw, branches, ramial chipped wood, etc.). The result obtained is stabilized organic matter. There are different valorization pathways: from individual households to municipalities orcompanies that commercialize it, as well as the farmers themselves.

Compost improves the structure, and more generally the health, of the soil by increasing its stability, its water retention capacity, and its richness innutrients and micro-organisms. By reintroducing organic matter into soils, it contributes to closing nutrient cycles, reducing but not replacingmineral and organic fertilizers, and storing carbon, playing a key role in the agroecological transition.