Composting is a controlled decomposition – a process that has been occurring in nature for millions of years on both small and large scales. In nature, this process is called decay or decomposition, during which microorganisms such as bacteria, fungi, etc. first break down dead plant matter. Later, small organisms like worms, woodlice, and others continue the breakdown into humus, forest soil, or mould. This biomass is reduced to its basic components, which are then made available to plants again as nutrients.
In horticulture, agriculture, and waste management, composting is deliberately used to quickly and efficiently turn organic materials into garden soil. A compost heap must provide a suitable habitat for microorganisms, fungi, and other small organisms, which are essential for the decomposition process. These organisms require specific conditions, i.e., appropriate food, before they can establish themselves. For example, coarse organic materials such as tree cuttings must be shredded. The mix of wet and dry organic waste should also be balanced. Additionally, sufficient moisture and a consistent supply of air (i.e., oxygen) are crucial.
The decomposition process is usually initiated by fungi and bacteria. During what is called the “hot rotting” phase (decomposition phase), they successively break down carbohydrate compounds and woody materials. The bacteria release minerals and nitrogen compounds, which become available to plants again when the compost is used as fertiliser.
The typical heat generated in this phase comes from the excess energy released as microorganisms break down the molecules. The increased temperature, in turn, encourages the activity of these organisms, kills off pathogens, and speeds up the decomposition process.
During this period, the volume of the compost decreases significantly – at the end of the first phase, which lasts around three to four weeks, “fresh compost” is available, which can already be spread on beds as mulch.
In the so-called main rotting phase (conversion phase), woodlice, mites, and springtails join in, further breaking down the plant material. The temperature in the compost drops to about 25 degrees. This phase lasts between two and six weeks.
During the after-rotting phase (build-up phase), organisms such as earthworms or millipedes mix the organic and mineral components through their digestion and burrowing activities. This phase lasts approximately three to six months. The final result is dark, crumbly, and above all, nutrient-rich finished compost.
The technical steps of industrial composting, for example in composting plants, vary depending on the method used. These typically include the preparation of composting raw materials, a largely controlled intensive decomposition process, followed by several weeks of after-rotting for biological stabilisation of the compost, and finally, sieving and packaging.
Simply explained: The contribution of composting to the carbon cycle (video in German).
September 2024