Soil is a complex, three-dimensional body structured into distinct layers that extend vertically beneath the surface. This vertical cross-section, known as the soil profile, is a record of the soil’s history and development. Scientists categorize these layers using designations called horizons, which differ in physical, chemical, and biological properties. The concept of “the five layers” refers to the primary or master horizons.
Defining the Master Horizons
The five most commonly referenced master horizons are designated by the letters O, A, B, C, and R, representing a typical sequence from the surface downward. The topmost layer is the O horizon, which is dominated by organic material, such as leaves, twigs, and other detritus at various stages of decomposition. This layer is most prominent in forested areas and is the only master horizon not primarily composed of mineral matter.
Directly beneath the organic layer lies the A horizon, often called topsoil, which is a mineral layer darkened by an accumulation of decomposed organic matter, known as humus. This mixing of mineral particles—sand, silt, and clay—with humus makes the A horizon the most biologically active layer and the one most associated with plant root growth. Water percolating through the A horizon begins the process of removing soluble materials and fine particles, a process called eluviation.
The B horizon, or subsoil, is characterized by the accumulation of materials that have moved down from the layers above, a process known as illuviation. These accumulated substances include silicate clay, iron and aluminum oxides, and sometimes organic matter, often giving the layer a distinct color or dense structure. In some soils, a separate E horizon may exist between the A and B layers, which is a pale, heavily-leached zone characterized by the maximum loss of clay and minerals.
The C horizon consists of the parent material from which the soil developed, representing unconsolidated, partially weathered rock or sediment. This layer shows minimal evidence of the biological and chemical processes that shape the layers above it. The R horizon is the underlying layer of hard, unweathered bedrock, such as granite, basalt, or sandstone, which forms the deepest boundary of the soil profile.
Factors Driving Horizon Differentiation
The development of these distinct layers, a process known as pedogenesis, is driven by a complex interplay of environmental factors acting over long periods. Climate, particularly temperature and precipitation, strongly influences the rate of mineral weathering and the extent of water movement through the profile. Warm, humid climates accelerate chemical reactions that break down parent material, while high rainfall increases the amount of material transported downward.
A primary mechanism for layer separation is the movement of materials through the profile via water. The material removed during eluviation is deposited in a lower layer through illuviation, which causes the B horizon to become enriched and often denser with accumulated clay or iron.
The initial composition of the C and R horizons, the parent material, dictates the soil’s starting point, influencing its texture, mineral content, and rate of development. For instance, soil derived from volcanic ash will develop differently than soil formed from limestone bedrock. Organisms, including plant roots, earthworms, and microorganisms, also play a substantial role, contributing organic matter to the O and A horizons and physically mixing and aerating the upper profile.
Over geological timescales, these processes of addition, loss, transformation, and translocation of material create the unique chemical and physical properties that define each horizon. The length of time the soil has been forming allows these processes to progress, leading to a more pronounced and vertically differentiated soil profile in older landscapes. Topography, or the slope of the land, further modifies horizon development by controlling water runoff and erosion, which can limit the depth of the upper layers.
Ecological Functions of the Soil Profile
The layered structure of the soil profile performs a range of functions that support terrestrial ecosystems and global biogeochemical cycles. The uppermost A and O horizons support plant life, providing anchor for root systems and a readily available supply of nutrients from decomposed organic matter. These layers also house the vast majority of soil biodiversity, including microbes, fungi, and invertebrates, which drive nutrient cycling and decomposition.
The entire profile acts as a natural water filtration and storage system, regulating the earth’s water supplies. Water percolates through the porous upper layers, where impurities are filtered out, and is then stored in the profile for plant use or slowly released to recharge groundwater reserves. The texture and structure of the B horizon, particularly its clay content, strongly influence the water-holding capacity and drainage characteristics of the landscape.
The O and A horizons play a significant role in global carbon sequestration, storing organic carbon derived from the atmosphere in the form of humus. By stabilizing this carbon, the soil profile helps to mitigate atmospheric carbon dioxide concentrations. The profile also serves as a recycling center, where microorganisms break down dead organic material and release mineral elements that plants can absorb.