This part of ISO 23611 specifies a method for sampling and handling earthworms from field soils as a prerequisite for using these animals as bioindicators (e.g. to assess the quality of a soil as a habitat for organisms). Basic information on the ecology of earthworms and their use as bioindicators in the terrestrial environment can be found in the references listed in the bibliography. This part of ISO 23611 applies to all terrestrial biotopes in which earthworms occur. The sampling design of field studies in general is specified in ISO 10381-1 (see also Reference [38]) and guidance on the determination of effects of pollutants on earthworms in field situations is given in ISO 11268-3. These details can vary according to the national requirements or the climatic/regional conditions of the site to be sampled (see also Annex C). This part of ISO 23611 is not applicable for semi-terrestrial soils and it can be difficult to use under extreme climatic or geographical conditions (e.g. in high mountains). Methods for some other soil organism groups, such as collembolans, are covered in other parts of ISO 23611. This part of ISO 23611 does not cover the pedological characterization of the site which is highly recommendable when sampling soil invertebrates. ISO 10390, ISO 10694, ISO 11272, ISO 11274, ISO 11277, ISO 11461 and ISO 11465 are more suitable for measuring pH, particle size distribution, C/N ratio, organic carbon content and water-holding capacity.