Composting is a natural process in which organic material is decomposed by microorganisms in a moist and warm environment. There are two common ways of categorising the composting process into specific phases. The first is based on the temperature development of the material during composting, the other on the condition of the material itself.

I. Categorisation into phases based on temperature development

During composting, the temperature of the compost material changes greatly depending on the stage. A distinction is made between the following phases:
Mesophilic phase
This phase is dominated by the growth of mesophilic microorganisms (such as fungi and bacteria), which break down the organic material. The temperature is between 20-60°C.

• Thermophilic phase
  In this phase, the temperature rises to 40-60°C, which means that thermophilic microorganisms (such as yeasts and actinomycetes) dominate. These microorganisms decompose the compost material faster than mesophilic microorganisms and contribute to a reduction in volume and the release of nutrients.
• Cooling phase
  In this phase, the temperature drops and mesophilic microorganisms begin to dominate again. The nutrients that were released in the thermophilic phase are now processed further by the mesophilic microorganisms.
• Maturation
  In this phase, the remaining organic substances are broken down and the nutrients are stabilised. The compost temperature drops to the ambient temperature and the compost material becomes humus-like.

It is important to note that the phases can vary and do not always take place in a precisely defined sequence. This depends on factors such as moisture, air circulation and the composition of the compost material.

II Classification according to the condition of the material

As composting is a decomposition process in which organic material is processed by microorganisms into humus, the second classification option is based precisely on the condition of the compost material:

• Intensive phase
  In the intensive phase, strong biological activity takes place in which microorganisms (such as bacteria, fungi and moulds) decompose organic substances and release nutrients. The temperature rises due to the high metabolic activity of the microorganisms, which leads to accelerated decomposition of the compost material. The intensive phase is the most important phase in the composting process, as organic substances are converted into humus-like material here.
• Plateau phase
  In the plateau phase, the decomposition rate of the compost material is lower, although microorganisms are still active. The temperature of the compost material has stabilised and the nutrients are distributed more evenly. In this phase, it can be assumed that the compost material is sufficiently mature to be used.
• Maturation phase (post-rotting)
  In the maturing phase, the compost material is completely stabilised, the nutrients are fully balanced and there is no longer any strong decomposition. The microorganisms are inactive or only slightly active. The maturing phase is also referred to as post-rotting because sometimes a second rotting is required to fully mature the compost material.

It should also be noted here that these phases are not always clearly defined and can overlap depending on factors such as moisture, air circulation and the composition of the compost material. However, the terms intensive phase, plateau phase and maturing phase are useful reference points for the monitoring and evaluation process of the composting process.