Illuviation

Illuviation is the accumulation, in a lower soil horizon, of clay particles, organic matter, iron and aluminium oxides or dissolved minerals that percolating water has carried down from the horizons above. It is the paired counterpart to eluviation, the leaching that removes the same material from the upper horizon, and the two together are the mechanism that builds the layered profile, an organic-poor, leached upper horizon over a clay-enriched or iron-stained lower one, used to classify and identify soil types in the field. This description is adapted from Wikipedia contributors under CC BY-SA 4.0; changes were made. https://creativecommons.org/licenses/by-sa/4.0/