Soil is a complex mixture of organic matter, minerals, gases, and water that forms the uppermost layer of the Earth’s crust, supporting plant life and numerous organisms. When viewed in a vertical cross-section, the soil reveals a layered structure known as the soil profile. These distinct layers are called soil horizons, each possessing unique physical, chemical, and biological characteristics. Soil scientists assign capital letters to these master horizons to classify the composition of the soil profile.
Understanding the Soil Profile
The formation of these visible layers results from a long process called pedogenesis, which is the natural evolution of soil from its parent material. This development involves four major processes: additions, losses, translocations, and transformations of material over time. Weathering breaks down the original rock, and water movement causes substances to be lost from one layer and accumulated in another.
The differentiation of horizons is based on observable properties like color, texture, structure, and composition. Darker colors indicate higher organic matter content, while changes in texture reflect the movement of clay particles. The entire profile provides a chronological record, reflecting the history of the soil’s development and the environmental factors that influenced it.
The Primary Diagnostic Horizons (O, A, E, B, C)
The uppermost layer, the O horizon, is dominated by organic material derived from decomposing plant and animal litter. Since this layer is not composed primarily of mineral soil, its thickness can vary dramatically, being thick in forests but often absent in cultivated fields. This layer is further subdivided based on the degree of decomposition.
Beneath the organic layer is the A horizon, or topsoil, a mineral horizon rich in accumulated organic matter called humus. This incorporation gives the A horizon a characteristically dark color and makes it the most biologically active layer, supporting the majority of plant roots and soil organisms. It is soft and porous, allowing for excellent water infiltration and aeration.
The E horizon lies below the A horizon, representing a zone of maximum eluviation, or leaching. Water moving downward strips this layer of clay, iron, aluminum oxides, and organic matter, leaving behind resistant sand and silt particles. This loss of darker materials results in a pale or whitish-gray layer.
The B horizon, or subsoil, is the zone of illuviation, where materials leached from the A and E horizons accumulate. This layer becomes enriched with clay, iron oxides, aluminum, or calcium carbonate, often giving it a denser, blocky structure and a reddish or brownish hue. The accumulation of these translocated substances results in this horizon having a higher density and lower organic content.
The C horizon sits directly below the subsoil and consists of the parent material from which the soil developed. This layer is composed of weathered or unconsolidated rock and sediment that has experienced little alteration from the physical, chemical, and biological processes that form the upper horizons. It lacks the structure and organic enrichment of the layers above it.
The Underlying Bedrock and Transitional Layers
Completing the vertical profile is the R horizon, which is not technically soil but represents the hard, unweathered bedrock underlying the entire regolith. This layer consists of solid rock, such as granite, sandstone, or basalt, that is too hard to be excavated by hand. The R horizon serves as the ultimate base of the soil profile, with the C horizon being the weathered transition zone just above it.
The boundaries between master horizons are rarely sharp, leading to the designation of Transitional Horizons to describe zones where properties blend. These are named using the two capital letters of the adjacent master horizons, such as AB or BC. The first letter indicates the dominant characteristics; for example, an AB horizon is transitional between A and B, but its properties are more similar to A.