Soils

From the perspective of sedimentary rock formation, soils represent the interface between weathering and the rock record. The materials that make up sedimentary rocks are either siliciclastic grains derived from land as a result of weathering or chemical minerals precipitated from ocean or lake water, and the elements that make up those chemical minerals were released from parent rocks by chemical weathering processes operating on land and in the ocean. Both types of sedimentary material therefore begin with weathering, and weathering products initially form soils over weathered bedrock before being eroded and transported away. ^[1]

Subaerial weathering products initially form soils of varied thickness over weathered bedrock. Throughout geologic time, most of these soils have ultimately been stripped away and transported as sediment to sedimentary basins; however, some soils are preserved to become part of the sedimentary record. ^[1] A preserved ancient soil is called a paleosol. Most soils in the geologic past were eroded, but those preserved at unconformities or within alluvial successions provide a direct window into the climate and weathering conditions of the past.

The characteristics and thickness of a soil mantle are a function of the bedrock lithology, the climate (rainfall and temperature), and the slope of the bedrock surface. These factors govern the intensity of weathering, determine which minerals survive to become part of the soil profile, determine what new minerals are created, and control how long soil materials remain before being removed by erosion. On very steep slopes, for example, the weathered mantle may be removed so rapidly by erosion that little soil accumulates. ^[1]

Soil-Forming Processes

In addition to the chemical and physical weathering processes that break down bedrock, several other biological and chemical processes operate within soils over time to modify their characteristics. These include: additions to the ground surface (precipitation of dissolved ions from rainwater; influx of windblown dust; addition of organic matter from vegetation); transformations (decomposition of organic matter to produce organic compounds; weathering of primary minerals and formation of secondary minerals including iron oxides); transfers (downward movement of solid or suspended material from one horizon to a lower one by groundwater percolation, called eluviation; accumulation of soluble or suspended material in a lower horizon, called illuviation; transfer of ions upward by capillary water movement); removals (removal of substances still in solution into groundwater or surface water); and bioturbation of soil by animals such as ants and termites and by plants. ^[1] These processes generate the distinct layered structure of soils - the soil profile.

Soil Horizons and the Soil Profile

Soils are classified on the basis of the characteristic horizontal layers - called horizons - that are visible in road cuts and excavations. The thickness and character of these horizons reflect the soil-forming processes that have operated and vary widely depending on climate, parent rock, and time. ^[1] Soil profiles can be divided into five major horizons: O, A, E, B, and C. ^[1]

The O-horizon is the surface accumulation of mainly organic matter. ^[1] The A horizon, occurring at the surface or below the O-horizon, consists of a dark-coloured accumulation of organic matter - such as leaf litter - that is decaying and mixing with mineral soil. ^[1] The E horizon, which underlies an O or A horizon, is a light-coloured eluvial horizon characterised by less organic matter, fewer iron and aluminum compounds, and/or less clay than the underlying horizon - material has been removed from it by downward movement. ^[1] The B horizon underlies an O, A, or E horizon and may contain illuvial concentrations of fine organic matter, clay, and similar materials; most of the original rock structures have been obliterated by soil-forming processes in this zone. ^[1] The C horizon lies above bedrock and consists of partly altered bedrock that can be deeply weathered but is relatively unaffected by soil-forming processes. ^[1]

In practice, soil layers are commonly much more complex than this five-horizon scheme. As many as 24 different kinds of soil horizons have been described. ^[1]

Soil Classification

Several systems exist for the more detailed classification of soils. One of the more widely used classifications in the United States appears in the U.S. Soil Taxonomy system, which recognises 12 major classes or orders of soils. Examples include aridosol (soils of arid regions) and ultisol (leached soils of warm, humid regions). These soil types are differentiated on the basis of complex criteria such as the amount of contained organic material, the presence of clay layers, and the presence of oxic (iron-rich) horizons. ^[1] Other classification systems used internationally include the Australian handbook classification and the FAO (UNESCO) world map classification. ^[1]

The factors that influence soil formation - and thus the kind of soil that forms in any given place - are the parent rock material, the length of the soil-forming process, climate (wet or dry), topography (steep or gentle slopes), and organisms (vegetation cover and soil fauna such as earthworms). Climate plays a particularly important role in soil formation. ^[1] The sensitivity of soil type to climate makes soils powerful proxies for paleoclimate when they are preserved in the rock record as paleosols.

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