Eluviation is a fundamental process in soil science defined by the removal of materials from one soil layer to another by the downward movement of water. Derived from the Latin word eluere (“to wash out”), the process is a type of leaching. It involves the transport of fine particles and dissolved substances from the upper horizons to the lower ones. Understanding this natural phenomenon is key to comprehending how distinct layers, known as horizons, develop in the soil over time.
The Mechanism of Material Removal
Eluviation is primarily driven by percolation, which occurs when the volume of water infiltrating the soil exceeds the amount that can be held or evaporated. This downward-moving water acts as a transporting agent, carrying soil constituents through the pore spaces. The materials removed are typically those that can be dissolved or remain suspended as tiny, colloidal particles.
The percolating water strips the upper soil layers of specific components, leading to a loss of mass and color. Fine clay particles are the most commonly transported material, alongside organic matter (humus) and metal oxides, such as iron and aluminum. These components are often released from minerals in solution, especially in acidic soil conditions.
The Role of the Eluvial Horizon
The physical manifestation of eluviation is the E horizon, often designated as the eluvial horizon. This distinct layer is positioned beneath the topsoil (A horizon), which is rich in organic matter. The E horizon represents the zone where the loss of material is at its maximum intensity.
This layer is characterized by its notably lighter color, frequently appearing grayish, whitish, or bleached. This pale appearance results from the removal of darker staining agents like iron oxides and organic matter, leaving behind a higher concentration of resistant minerals. Quartz, which is less susceptible to dissolution and transport, is often the dominant mineral remaining. As a result of this stripping, the eluvial layer is often less fertile and has a coarser texture than the horizons above and below it.
Eluviation Versus Illuviation
Eluviation is inextricably linked to its reciprocal process, illuviation; together, these two forces define the structure of many soil profiles. Eluviation involves the washing out of material from an upper layer, while illuviation is the accumulation of that same material in a lower layer.
The materials stripped from the E horizon are carried downward by the water until they precipitate or are filtered out in the underlying B horizon, the zone of illuviation. The B horizon accumulates the transported clay, humus, and iron and aluminum oxides. The deposition of these fine particles results in the B horizon becoming significantly denser and often having a higher clay content than the E horizon above it. This accumulation completes the cycle of vertical material movement, forming a cohesive profile where the loss from one layer directly contributes to the gain in the next.