Composting is a biochemical process by which organic matter is decomposed and a humus called compost is obtained. The phases of the composting process are very specific and have to be controlled frequently so that the temperature, humidity, oxygen, and other factors are ideal and develop correctly. As a general rule, specific facilities are required to carry out the whole composting process, precisely to ensure greater control over how the raw material that is to be converted into compost evolves stage by stage.

Below we explain the different stages of the composting process:

1. Mesophilic phase

The mixture of primary materials is still at room temperature and not yet humidified. “Mesophilic” micro- organisms (micro-organisms that grow at 20-45 °C) start to reproduce by breaking down organic molecules. The metabolic activity of the micro-organisms raises the temperature to 40-45 °C over a period of two to eight days. It also decreases pH of the mixture due to the production of organic acids.

2. Thermophilic phase

Also called the “sanitisation” phase, in this stage the temperature rises above 45 °C and mesophilic micro-organisms are replaced by “thermophiles” (micro-organisms that thrive at temperatures between 45 °C and 70 °C). Thermophiles break down more complex carbon sources, such as cellulose and lignin. Another important change is that nitrogen is transformed into ammonia and the pH of the mixture becomes alkaline.
At 60° the thermophilic stage of the process begins. Here, organisms capable of living in very hot conditions degrade complex molecules.
Subsequently, for several days or even months (depending on the primary materials used), the temperature remains high and the biological activity of the micro-organisms decreases. A pasteurisation process then takes place in which harmful bacteria and contaminants (such as Salmonella or Escherichia coli) are killed. This process allows the product to be sanitised. During this phase, the mixture has to be frequently aerated to provide oxygen to the micro-organisms so that they can continue to decompose.

3. Cooling phase

When the carbon and nitrogen have been consumed, the temperature drops again to 40-45 °C. Mesophiles reappear and decompose the remaining cellulose and lignin material. The pH drops again slightly.

4. Thermophilic phase

This stage of the process requires the mixture to be kept at room temperature. During this time a series of secondary reactions take place which cause condensation and polymerisation of the humus. At the end of this period, we obtain a product that can be called compost.

The composting process brings great benefits to organic amendments. It offers sanitisation, a reduction of bad smells, and reduces the volumes that have to be applied. Composting is undoubtedly one of the most complete and beneficial fertilisers for plant life.