Interactions between soil animals and their environment can be described in terms of positive and negative feed-back loops taking place in the build-up and steady-state of soil ecosystems, respectively. The size of animals determines the scale at which they interact with their physical and biotic environment. Nevertheless varying scales at which animals intervene in functional processes is not relevant to any hierarchical position within the ecosystem, due to symmetrical patterns in the relationships between microbes, animals, humus forms and vegetation types. The present knowledge has been reviewed and discussed to the light of an integrated view of the soil ecosystem, with a particular accent put on soil acidity.