Soil is a highly organized natural body with a distinct layered structure that records its unique history. Standard soil classification recognizes six primary layers, or master horizons, designated by the capital letters O, A, E, B, C, and R. While these six layers form the foundation for understanding nearly all soils globally, the actual number of specific variations found in nature is significantly larger, involving dozens of possible distinctions. These foundational layers are a direct result of the long-term interaction between climate, living organisms, parent material, topography, and time.
Defining the Soil Profile
The soil profile is the vertical cross-section of the soil from the surface down to the underlying bedrock. Within this profile, distinct layers develop, which are known as soil horizons. A soil horizon is a layer that runs generally parallel to the land surface and possesses physical, chemical, and biological properties that set it apart from adjacent layers. Horizons are genetically related, meaning they form as a result of active soil-forming processes. These layers are differentiated by characteristics such as color, texture, structure, and the presence of accumulated materials. Examining the sequence and characteristics of these layers allows scientists to classify the soil and understand its environmental history.
The Six Primary Horizons
The master horizons are designated by capital letters and represent the broadest classification of soil layers. The sequence O-A-E-B-C-R represents an idealized, fully developed soil profile, though many soils contain only a subset of these layers.
O Horizon (Organic)
The O horizon is the uppermost layer, dominated by organic material in various stages of decomposition. It is primarily composed of plant litter, leaves, and other residues that accumulate on the surface. Material ranges from slightly decomposed to highly decomposed humus, depending on biological activity and environmental conditions.
A Horizon (Topsoil)
Directly beneath the organic layer is the A horizon, or topsoil, which is the first mineral horizon. It is characterized by a significant accumulation of humified organic matter intimately mixed with mineral soil particles. This mixing typically gives the A horizon a darker color than underlying layers. It is the zone of highest biological activity, supporting the majority of plant roots and soil organisms.
E Horizon (Eluviation)
The E horizon is a mineral layer characterized by the loss of materials, a process called eluviation. Water percolating through the A horizon leaches or washes away clay, iron, aluminum oxides, and organic matter. This removal leaves behind a concentration of resistant minerals, often quartz. The E horizon typically results in a lighter, ash-gray or white color and a sandier texture.
B Horizon (Illuviation)
The B horizon, or subsoil, is the primary zone of accumulation, known as illuviation. Materials leached from the E or A horizons are deposited and concentrate here. These accumulated materials include silicate clay, iron, aluminum oxides, and sometimes carbonates. This accumulation results in a denser structure and a distinct color, often reddish or yellowish brown due to iron oxides.
C Horizon (Parent Material)
The C horizon consists of unconsolidated rock or sediment that has experienced little change due to major soil-forming processes. This layer is below the zone of major biological activity and root penetration. It represents the raw material from which the overlying A and B horizons may have developed through long-term weathering.
R Horizon (Bedrock)
The R horizon is hard, unweathered, consolidated bedrock, such as granite or limestone. It cannot be excavated by hand tools. Though technically not soil, the R horizon is included in the profile because its weathering is the ultimate source of the mineral content found in the C horizon.
How Soil Horizons Develop
The development of distinct soil layers is the result of pedogenesis, driven by four primary mechanisms that reorganize soil material over centuries.
Additions
Additions involve the input of material to the soil profile from external sources. The most visible addition is organic matter, such as leaf litter and dead roots, which contribute carbon and nutrients to the surface horizons. Other additions include dust from the atmosphere and soluble salts brought in by rainwater.
Losses
Losses involve the removal of materials from the soil profile, often through water movement. Leaching is a significant loss, where soluble components are dissolved or suspended in water and moved out of the soil profile entirely. Erosion is another form of loss, physically removing surface soil and organic matter via wind or water.
Transformations
Transformations are the in-place chemical and physical changes that alter soil components. A primary example is the chemical weathering of primary minerals, breaking them down to form new secondary minerals like clay. The decomposition of complex organic matter into stable humus is another important transformation that enriches the A horizon.
Translocations
Translocations refer to the movement of materials from one horizon to another within the soil profile. The movement of clay and iron from the E horizon down to accumulate in the B horizon is a classic example. This internal movement, mediated largely by water, is responsible for the sharp differentiation between the layers of loss (eluviation) and accumulation (illuviation).
The Concept of Subordinate Distinctions
While the six master horizons provide the basic framework, they are a significant simplification of the complexity found in nature. To accurately classify the variation within these layers, soil scientists use subordinate distinctions, represented by lowercase letters added as suffixes to the master horizon letters. These detailed symbols allow for a highly specific description of a soil profile, reflecting its unique history and environmental conditions. Because these distinctions can be combined and applied to any master horizon, the total number of specific horizon types is much greater than six.
Examples of subordinate distinctions include:
- The symbol ‘t’ denotes the accumulation of silicate clay (e.g., Bt horizon).
- The suffix ‘k’ indicates the accumulation of pedogenic carbonates (e.g., Ck layer).
- The suffix ‘g’ signifies gleying, which indicates strong wetness and reduction of iron.
- The suffix ‘b’ denotes a buried genetic horizon.