The E horizon is a specific layer within the vertical cross-section of soil, known as the soil profile, which is composed of distinct horizontal layers called soil horizons. These horizons develop over time, creating layers with differing physical, chemical, and biological characteristics. The E horizon is particularly notable because its defining features result from a natural “washing out” process that physically and chemically alters its composition.
Placement Within the Soil Profile
The standard arrangement of a comprehensive soil profile includes master horizons labeled O, A, E, B, C, and R, although not all soils will exhibit every layer. The E horizon is typically situated beneath the A horizon (topsoil) and directly above the B horizon (subsoil). Its position reflects its formation history, as materials are moved out of the layer above and deposited into the layer below.
The presence of a distinct E horizon is not universal; it is frequently missing in many soil types. It tends to be more pronounced in older soils, particularly those found in forested regions with high rainfall. Even when present, the thickness of the E layer can vary significantly depending on local climate, vegetation, and the age of the soil itself.
The Process of Eluviation
The formation of the E horizon is defined by a soil-forming process called eluviation, a term derived from the Latin word meaning “washing out.” Eluviation is the removal of dissolved or suspended material from a soil layer by the downward movement of water, which occurs when rainfall exceeds the rate of evaporation. As water filters through the upper A horizon and into the E horizon, it acts as a transporting agent, carrying fine particles and soluble components with it.
This percolating water mechanically transports tiny colloidal-sized particles, such as silicate clay minerals, which are suspended in the moving liquid. Simultaneously, the water dissolves and removes chemical components, most notably iron and aluminum oxides, which often give soil its darker or reddish hues. This continuous removal of suspended and dissolved materials from the E layer is what gives the horizon its unique, depleted characteristics.
The materials that are removed are then carried deeper into the soil profile. This downward transport continues until the materials are deposited and accumulate in the layer below, a process known as illuviation, which forms the B horizon. Therefore, the E horizon serves as the zone of exit for these materials, making it a mirror image of the B horizon, which is the zone of accumulation.
The intensity of eluviation is often linked to the acidity of the water moving through the soil. This is frequently the case in forested areas where decaying organic matter produces weak organic acids. These acids enhance the stripping of iron coatings from mineral grains, accelerating the removal of materials and the development of the E horizon.
Identifying Characteristics and Composition
The constant removal of materials by eluviation results in a layer that is visually and texturally distinct from the horizons above and below it. The most striking visual characteristic of the E horizon is its pale color, often appearing gray, white, or light yellowish-gray. This bleached appearance is a direct consequence of the loss of darkening agents, specifically the organic matter and the iron and aluminum oxides that have been washed out.
Because the finer components like clay and organic matter have been transported away, the E horizon is left with a high concentration of materials that are more resistant to weathering. Its composition is dominated by coarser particles, primarily sand and silt, often composed of the mineral quartz. This gives the layer a coarser texture compared to the B horizon below it, which has accumulated the finer clay particles.
The texture of the E horizon is often loose, and its structure may be platy or massive. The loss of fine clay, organic matter, and metal oxides means the E horizon is lower in nutrients and less fertile than the overlying A horizon. This depletion makes it a layer of limited biological activity.