All organisms which live, during all or part of their life cycle, on the soil surface or in the soil, and which, through their various lifestyles, improve its properties (the following terms are used: ecosystem services of resource supply and biological regulation) and generally facilitate plant growth and development. Their strong interactions with one another, along with the physical and chemical component of the soil (in particular bedrock), with plants or with necromass (lato sensu, all dead organisms and their waste), contribute to the formation of soils (which are a mixture of 93 to 99% of more or less altered mineral matter and 1 to 7% of more or less degraded organic matter) and the maintenance of their fertility.
The essential processes involved are, on the one hand, the dissolution of mineral materials (of the bedrock in particular) and, on the other hand, the fragmentation, decomposition and humification of the necromass, then the humus mineralisation thus created, which together represent the various biogeochemical cycles allowing the provision to soil and plant organisms of various nutrients in different chemical forms.
Certain soil organisms grow at the expense of crops; animals are therefore called pests and fungi and bacteria are called pathogens. Those species which develop at the expense of pests and pathogens are called auxiliaries.
Soil organisms are classified into four main groups: (i) microflora, i.e., bacteria, archaea, cyanobacteria, algae, diatoms, fungi, (ii) microfauna, i.e. mycetozoa, protozoa, rotifers, tardigrades, nematodes, (iii) mesofauna, i.e. collembola, mites, potworms, protura, diplura, pauropoda, symphyla, and (iv) macrofauna, i.e. lumbricina or earthworms, diplopods, chilopoda, isopods, ground beetles, staphylinids, spiders, molluscs, rodent mammals such as voles or insectivores such as moles.