Illuviation

Illuviation is a pedogenic process in soils where dissolved or suspended materials are transported downward by percolating water and deposited in a lower horizon. It is the counterpart to eluviation, the removal or leaching of materials from a horizon.

During illuviation, substances such as clay minerals, humus (organic matter), iron oxides, and aluminum oxides are carried from upper horizons into a subordinate, illuvial horizon. The illuviated horizon is commonly designated as Bt when clay is deposited, Bh for humus illuviation, or other related subhorizons that reflect the dominant deposited material. The resulting horizon differentiation is a key feature of many mature soils.

Illuviation typically occurs in climates with sufficient leaching, such as humid temperate to tropical regions, where rainfall or irrigation drives downward movement of solutes. The process often leads to changes in texture and color in the illuvial horizon, such as increased clay content and distinct mineral assemblages, which can influence soil structure, fertility, and cation exchange capacity.

In soil classification, illuviation is one of the translocation processes that shape horizon development. It helps explain the presence of distinct illuvial horizons beneath a surface eluviated layer and is commonly observed in well-developed soils where long-term leaching has occurred.