The A-horizon (topsoil) is the zone of eluviation (leaching) — soluble salts, clay, and colloidal material are washed downward by percolating water. It is typically dark-coloured due to organic matter, has the most biological activity, and is depleted of soluble minerals. The B-horizon (subsoil) is the zone of illuviation — materials leached from above accumulate here. The C-horizon is weathered parent material; the O-horizon is organic litter.