2 Soil Horizons and Profiles

Horizons and profiles

A soil horizon is a layer, generally parallel to the land surface, that differs from neighboring layers in properties such as color, texture, structure, or composition. A soil profile is the vertical sequence of horizons at one location. Profiles are not identical everywhere: horizons may be absent, thin, thick, or interrupted.

The main horizons

Soil scientists use capital letters to identify master horizons. A common simplified sequence is O–A–E–B–C, sometimes above R. This sequence is a guide rather than a required arrangement, and horizon identification depends on observed properties and evidence of development—not position alone. For example, an E horizon is often above a B horizon, but its label refers to its characteristics rather than simply its depth.

• O: Organic material, such as leaf litter and partly decomposed plant remains. It may occur at the surface or be buried, and it is not present in every soil.

• A: Mineral soil near the surface, commonly mixed with organic matter by organisms and plant roots. It is often darker than underlying mineral layers.

• E: A zone of eluviation, where moving water removes some materials, often clay, iron, or organic matter. The loss can leave the horizon relatively pale; E horizons do not occur in all profiles.

• B: A subsurface zone altered by soil-forming processes. Materials moved from above may accumulate through illuviation; minerals may also weather, and soil structure or color may develop.

• C: Relatively little-altered parent material beneath the more developed soil. It may consist of weathered rock or transported deposits.

• R: Hard bedrock beneath the soil. It is included as a layer in the profile description but is not itself soil.

How layers develop and vary

Soil forms as parent material changes over time. Weathering breaks down minerals, organisms add and transform organic matter, and water moving through the profile can remove, alter, or deposit materials. These processes create contrasts between layers.

In a developing profile, surface organic matter may become incorporated into mineral soil. Downward-moving water can leach some substances from an upper layer and deposit or transform them deeper down. The resulting horizons vary with local conditions, including climate, organisms, landscape position, parent material, and time.

A forest soil may have a thin O horizon, a darker A horizon, a pale E horizon, and a more strongly developed B horizon. Another soil may have no O or E horizon and pass from A to B to C. A recently deposited soil may show only weak layering. Erosion or disturbance can remove or mix surface horizons, changing the profile that remains.

Reading a profile

When comparing horizons, observe their depth and thickness, color, texture, and structure. Also note roots, stones, and whether boundaries between layers are abrupt or gradual. These visible traits help distinguish layers; chemical and mineral properties may require laboratory analysis.

Horizon labels can include lowercase suffixes and numbers to show additional distinctions. For instance, Ap commonly indicates an A horizon affected by plowing, while Bt indicates a B horizon with accumulated silicate clay. These labels provide a compact description, but soils with the same label are not necessarily identical.