Within soil there is a very wide range of sizes of organisms from large animals such as earthworms which can be seen with the naked eye to the microscopic such as bacteria and viruses which need high magnification (1000x and beyond).  Body sizes can be grouped into three main groups: macro-, meso- and micro-biota (Wallwork, 1970; Swift et al., 1979). Unfortunately, as can be seen in the figure below, the ranges that determine each size group are not exact for all members of each group, often leading to considerable confusion as to whether a particular organism should be considered macro or meso, and so on. In addition there is often confusion as to what constitutes and “organism”. For example the slime moulds have a life cycle in two parts as either single cells or as a unified organised mass/colony of single cells acting as one “organism”.

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