Mineral particles < 0.002 mm. When dry, clay is hard; when wet it swells and becomes pliable and sticky. Clay colloids are finely divided clays, dispersed in water, with a negative surface charge that attracts positively charged ions; they are among the most reactive constituents of a soil (W. van Olphen 1991). Clay micelles are individual particles, platey in form, < 2 µm in diameter, having a negative charge, and therefore able to attract cations within a soil; try Murrmann and Koutz in S. Reed, ed. (1972). The clay-humus complex is a mixture of clay particles and decaying organic material that attracts and holds the cations of soluble salts within the soil profile.