Structure

The pedosphere has its own specific structure. Vertical variability is the result of internal processes altering parent materials in situ into pedogenic features and properties that make up horizons and soil profiles; see Figure 3. These processes are usually called soil-forming, or specific pedogenic, processes. Many variations are possible due to the wide range of environmental conditions and scope of the factors themselves interacting to form and

Figure 3 Vertical variability revealed as genetic horizons (layers) in a drained and cultivated Chernozem soil derived from calcareous glacial till in Iowa, USA. Photo credit: R. W. Arnold.

Wet soil in tall grass prairie (steppe)

Figure 3 Vertical variability revealed as genetic horizons (layers) in a drained and cultivated Chernozem soil derived from calcareous glacial till in Iowa, USA. Photo credit: R. W. Arnold.

develop soils. The natural sequence of in situ-formed layers in a natural soil is a system of genetic soil horizons - a genetic profile or soil body. During the Anthropocene, human activities have already modified most of the land, so that few truly natural soils exist (in tundra and boreal taiga zones, high mountains, tropical rainforests, and extreme deserts). New kinds of anthropic features and soil horizons are being identified, described, and recognized as significant features of the pedosphere. The World Reference Base for Soil Resources now reflects such changes. Refinements of the concept of the pedosphere will be, and are, being made as improved techniques for their examination and measurement become available.

Soil, as a multiphase body in the pedosphere, has several kinds of depth distributions at any moment. There are temperature profiles, moisture profiles, gaseous ones, soil solution and nutrient profiles, macro- and microbiota ones, and solid-phase profiles. The first three or four are mainly functional, that is, they are very labile and change quickly (10_1-101 years). The solid-phase profile is more stable, changes slowly (101-2-105-6 years), and is characterized by interrelated horizons with variable texture, structure, and mineralogical and chemical composition (Figure 4). Many kinds of diagnostic features and horizons

Pedosphere

Regional physiography

Toposequence

Toposequence

Pedon

Soil horizon

Aggregate

Pedon

Soil horizon

Soil profile

Soil profile

Mineral-organic complex

Mineral structure (molecular)

Figure 4 Schematic of hierarchal scales involving soil solidphase components that combine to form horizons, profiles, local and regional landscapes, and the global pedosphere. Adapted from Sposito G and Reginato RJ (eds.) (1992) Opportunities in Basic Soil Science Research, p. 11. Madison, WI: Soil Science Society of America.

are recognized, and their combinations give rise to a large number of unique soils throughout the pedosphere. Classification systems such as the World Reference Base for Soil Resources and Soil Taxonomy are based on combinations of defined pedogenic properties, mainly solid-phase ones. The organization of these systems facilitates small-scale representations of the pedosphere, as noted by the color patterns in Figure 5.

The lateral combinations of individual soil bodies comprise the continuous soil cover of land, the pedosphere. Spatial patterns or structures of soil cover exist at all scales of observation; however, there are differences of opinion about what and how to define the combinations at different scales. Soil surveys of portions of the pedosphere are made at different scales, usually depending on the nature and genesis of soil patterns and need for detailed information about land use and management.

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