FUNCTIONING OF THE SOIL SYSTEM - Soil Ecology - page 485

Selection of particles
Most endogeic earthworms exhibit some degree of selection in their choice of both
organic and mineral soil particles. In temperate regions, Bouché and Kretzschmar (1974)
found concentrations of plant debris of up to 15 % thus demonstrating that these
endogeic earthworms actively select for this type of material. Most earthworm casts
actually have greater concentrations of organic matter than the surrounding soil in field
conditions and in cultures with homogeneous soil (Lee, 1985). The rate of relative
concentration, however, varies with the species, and with the soil organic matter con-
centration (Barois et al., 1999) (Figure IV.53).
All earthworms select mineral materials of particular size classes, depending on
species and soils. Standard measurements made for nine species of anecic and endogeic
tropical species show a clear preference for clay-sized materials in soils with clay
contents less than 30 %; a few cases of significant selection for sand particles have been
indicated in highly clay-rich soils (Figure IV.54).