This section contains questions and answers on five topics related to different groups of soil organisms:
Most organisms are in the top layers of soil, usually the top 2-3 centimetres, because this is usually where most of the organic matter is. Organisms occur to depths of several kilometres below the soil surface, but the types of organisms that occur this far down are not the same as those close to the surface. The organisms in soil are often commonly found close to root surfaces, in living roots, in dead roots, on soil particles, or amongst aggregates of soil particles.
Within the top layers of the soil, the earthworms and soil animals are able to travel through most parts of the soil. Fungi extend to cover soil particles and aggregates. They can form a mat of hyphae, which can extend centimetres or even metres in the soil. Fungi can also grow into soil aggregates and form a network of hyphae inside the aggregate. Bacteria tend to accumulate inside soil aggregates because they are less likely to be eaten by soil animals such as protozoa and mites in this environment. Bacteria can be carried down further into the soil in water that is percolating downwards.
Soils that are clayey often have many bacteria because these soils have lots of small pores (spaces) which protect the bacteria. Sandy soils with few aggregates are less suitable habitats for bacteria and fungi unless there is lots of organic matter, because in sands the bacteria have fewer small pores to live in.