eluviation

Eluviation, in soil science, is the downward movement and removal of dissolved and suspended materials from the upper part of the soil profile by percolating water. As water infiltrates, fines such as clay minerals, organic matter, and oxides of iron and aluminum are leached from the upper horizons, often leaving a lighter, bleached eluviated layer known as an E horizon. The process contrasts with illuviation, where leached materials are deposited in lower horizons, commonly enriching a B horizon with clays, oxides, or organic matter.

Formation and characteristics of eluviation are influenced by climate, drainage, and soil properties. It is most evident in well-drained soils with sufficient rainfall and active leaching. Sandy and loamy textures promote eluviation because water moves more readily, whereas dense, strongly structured, or acidic horizons can slow or limit translocation. In some soil orders, notably podzols, eluviation is pronounced enough to create a distinct E horizon that is light-colored or gray, reflecting the loss of iron, aluminum, and organic matter. The materials removed can contribute to illuviation in lower horizons, where clays, oxides, and organic matter accumulate.

Practical effects include changes in soil color and structure, and often a reduction in available nutrients in the upper horizons due to preferential removal of soluble and exchangeable elements. Understanding eluviation helps explain vertical differentiation in soils and the development of characteristic horizon sequences observed in many humid temperate and boreal landscapes.