Beneath your feet lies a complex, layered history of the Earth's surface
In short: Soil is not just dirt; it is a structured record of geological and biological time. By identifying specific layers known as horizons, scientists can decode the history of a landscape. These distinct zones, defined by their unique physical and chemical properties, reveal how the ground beneath us has evolved over centuries.
A soil horizon is a layer parallel to the surface, distinguished from the material above and below it by physical, chemical, or biological traits. These layers are the result of pedogenesis, or soil-forming processes. While some soils, such as the USDA's Entisols, show little development, most soils exhibit a sequence of master horizons. These are typically labeled with capital letters, such as O for organic surface layers, A for mineral horizons, B for subsoil horizons, C for poorly developed material, and R for hard bedrock.
The classification of these horizons is not universal; various systems exist, including the World Reference Base for Soil Resources (WRB), which lists 40 diagnostic horizons. Because these systems are artificial constructs designed to describe local conditions, they cannot be mixed. A layer is only considered a horizon if it shows evidence of soil-forming processes. If no such processes have occurred, it is simply called a layer. Human activities, such as deep ploughing or major earthworks, can disrupt or remove these distinct layers entirely, making it essential to consider local history when analyzing soil profiles.
Diagnostic horizons are often used to define soil types. For instance, an A horizon might contain modified organic matter and evidence of cultivation, while an E horizon is characterized by the loss of materials like clay or iron through downward movement. B horizons, conversely, often show the accumulation of these materials. The WRB manual provides precise criteria for these designations, such as defining a litter layer as containing over 90% recognizable dead plant tissues. Understanding these nuances allows researchers to interpret the complex, often buried, history of the land.
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- Summary by Intellect · source: Soil horizon
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