The basic needs of the compost

The basic needs of the compost

The basic needs of the compost


A good compost never comes about by chance. The decomposing micro-organisms have very specific demands concerning the conditions in the compost. The basic requirements for efficient decomposing are sufficient supplies of oxygen and nutrients, as well as suitable humidity and temperature.


What distinguishes a compost from a heap of waste:

  • A heap of waste is an anaerobic dense formation that rots and stinks.
  • A compost is an airy heap that "burns" and does not smell.

The micro-organisms that decompose the compost need oxygen to respire. To ensure the supply of oxygen, the compost must remain airy. To ensure the lightness of the compost, use coarser substance, so called dry bedding, among the compact waste and toss the compost as required. Your compost may also run out of oxygen if it gets too wet. The denser the compost is, the more easily it soaks and becomes an anaerobic heap of waste.


The micro-organisms of the compost are primarily swimmers, which can only live in the water. Therefore, dryness is the worst enemy of the compost. A compost which is not working properly is usually too dry. The dryness may result from scanty watering or a too-loose structure of the compost. For example, whole branches or flower stems leave the compost too loose, which makes it dry out easily. Although the compost should be light, it must not be too loose. 

There is a good rule of thumb for testing the wetness of the compost: if you clench some compost in your hand, a few drops of water should be extracted from it. If water runs freely between your fingers, the compost is too wet.

Nutrients and energy

The micro-organisms in the compost use energy from the organic waste for maintenance of their vital functions. The micro-organisms need carbon compounds to generate energy and nitrogen to build up their cells. They also use other nutrients, but the ratio between carbon and nitrogen is vital for successful composting. 

All organic waste is rich in carbon. For example, autumn leaves, wood and branch chips, straw, peat and hay are "poor" waste that is rich in carbon.

Nitrogen, on the other hand, is generally considered as "powerful". Green parts of vegetation, such as cut grass and weeding waste as well as household waste, dung and urine, are rich in nitrogen. On the other hand, dead vegetation from which chlorophyll has disappeared is poor in nitrogen. For example, Biolan Compoststarter and Biolan Natural Fertilizer are industrially fabricated nitrogen products.
A compost rarely lacks carbon. On the other hand, garden composts in particular are often too poor in nitrogen. Lack of nitrogen slows down reproduction of the micro-organisms. As a result, the compost does not warm up, its decaying slows down and the end-product is too meagre for soil conditioner.

If the compost is too rich in nitrogen, the compost "burns" hot, but smells of ammonia. The reason for this is that the micro-organisms waste the excess nitrogen, which evaporates as ammonia into the atmosphere.

It is difficult to give universally applicable advice concerning the nutrient ratio, because the composted waste is so diverse. Garden composts are usually poor in nitrogen. Household waste composts are often too rich in nitrogen. In particular, the large composters of housing companies often smell of ammonia, if bedding rich in carbon is used too sparsely. Garden and household waste complete each other in the compost, so the best compost can be achieved by mixing different kinds of waste.


The micro-organisms in the compost generate heat. In Finnish conditions, too much heat evaporates from the compost, which slows down the composting process. 

In a large compost heap, the outermost layer of the heap acts as insulation and the inner part composts. To ensure that sufficient heat is generated, an non-insulated compost must be at least one cubic metre in size.
Smaller composts require an insulating jacket on the outside. According to the rule of thumb, the smaller the compost, the thicker the insulation should be. In Finnish conditions, the insulation should be at least 5 cm thick