Because soil-forming processes operate from the surface downward, soil composition, texture, structure, and color gradually evolve differently at varying depths.
These vertical differences, which usually become more pronounced as time passes, divide the soil into zones or layers known as horizons. If you were to dig a pit in soil, you would see that its walls are layered. Such a vertical section through all of the soil horizons constitutes the soil profile.
Figure presents an idealized view of a welldeveloped soil profile in which five horizons are identified.
From the surface downward, they are designated as O, A, E, B, and C. These five horizons are common to soils in temperate regions; not all soils have these five layers. The characteristics and extent of horizon development vary in different environments. Thus, different localities exhibit soil profiles that can contrast greatly with one another:
- The O soil horizon consists largely of organic material, in contrast to the layers beneath it, which consist mainly of mineral matter. The upper portion of the O horizon is primarily plant litter, such as loose leaves and other organic debris that are still recognizable. By contrast, the lower portion of the O horizon is made up of partly decomposed organic matter (humus) in which plant structures can no longer be identified. In addition to plants, the O horizon is teeming with microscopic life, including bacteria, fungi, algae, and insects. All these organisms contribute oxygen, carbon dioxide, and organic acids to the developing soil.
- The A horizon is largely mineral matter, yet biological activity is high, and humus is generally present— up to 30 percent in some instances. Together the O and A horizons make up what is commonly called the topsoil.
- The E horizon is a light-colored layer that contains little organic material. As water percolates downward through this zone, finer particles are carried away. This washing out of fine soil components is termed eluviation. Water percolating downward also dissolves soluble inorganic soil components and carries them to deeper zones. This depletion of soluble materials from the upper soil is termed leaching.
- The B horizon, or subsoil, is where much of the material removed from the E horizon by eluviation is deposited. Thus, the B horizon is often referred to as the zone of accumulation. The accumulation of the fine clay particles enhances this horizon’s ability to hold water. In extreme cases, clay accumulation can form a very compact, impermeable layer called hardpan.
- The O, A, E, and B horizons together constitute the solum, or “true soil.” It is in the solum that soilforming processes are active and that living roots and other plant and animal life are largely confined.
- The C horizon is characterized by partially altered parent material. Whereas the parent material is difficult to see in the O, A, E, and B horizons, it is easily identifiable in the C horizon. Although this material is undergoing changes that will eventually transform it into soil, it has not yet crossed the threshold that separates regolith from soil.