A classification system is needed to organize knowledge about the thousands of natural bodies known as soils.
Pedon is the smallest unit of soil that embodies the essential characteristics of a soil.
Polypedon is a laterally contiguous group of similar pedons. It constitutes a soil individual.
Each soil individual is classified as belonging
to one of more than 18,000 soil series.
Various classification systems, past and present, throughout the world. In the US, we use Soil Taxonomy. It is based on measurable properties and uses nomenclature that is descriptive of soil properties. There are 6 categories in Soil Taxonomy.
Differentiation largely based on presence or absence of diagnostic
horizons. Soils within same order subjected to similar pedogenic
Differ in moisture, temperature, chemical or textural features.
Pedogenesis is more similar within same suborder.
Great group Differentiated on basis of horizon sequence and other features.
of soils that are typical of the great group and soils that are
intergrades to other orders, suborders or great groups.
Based on properties affecting plant growth or engineering use such
texture, mineralogy, temperature and others.
Series Most specific category.
There are also subdivisions of series known
as soil phases, however, soil phase is not part of Soil Taxonomy.
Phases describe differences within a series as to surface texture, solum
depth, slope, extent of erosion and so forth. Soil phase is equivalent
to mapping unit.
Properties Used in Classification
Diagnostic horizons (surface and subsurface)
Texture and other physical properties
Chemical and mineralogical properties
Surface Diagnostic Horizons
These include the upper portion of the soil profile darkened by accumulation of organic matter. A partial list includes:
Mollic Thick, dark and high base content. Associated with prairie vegetation.
mollic but lower base content. Forest vegetation under warm and
Light color and low organic matter content. Associated with weakly
developed soils (Aridisols, Entisols and Inceptisols).
Thick, black and high organic matter content. Developed in volcanic
Organic horizon over a mineral soil. 20 to 30 + cm thick and 20 to 30 %
organic matter, depending on clay content.
Subsurface Diagnostic Horizons
Light colored and highly leached E horizon. Depleted of clay and oxides.
Relatively coarse texture.
Secondary accumulation of clay. Whether a horizon is an argillic depends
on clay content of eluvial horizon.
Cambic Color or weakly developed B horizon. Common in Inceptisols.
Oxic Highly weathered and thick. Low fertility. Common in humid tropics.
accumulation of organic matter usually with Al and Fe oxides.
Common in coarse textured soils in cool climates and under coniferous
forest. But also found in warm climates where water table is close to surface.
Accumulations of solubilized materials such as
Calcic CaCO3 accumulation.
Hardpan horizons such as
Fragipan Dense and brittle but weakly cemented horizon.
Soil Moisture Regimes
Presence or absence of groundwater or water available to plants in the control section (defined as the distance between wetting depths of 2.5 and 7.5 cm of water within 24 h added to dry soil, 10 - 30 cm in clay but 30 - 90 cm in sand).
for long enoubh to cause anaerobic conditions (gleying or
Udic Sufficient water to meet plant needs.
Aridic Low moisture except for short periods.
Soil Temperature Regimes
Based on mean annual, summer and summer
minus winter soil temperatures.
Overview of Soil Orders
Little profile development -A only.
Inceptisols More developed than Entisols -weak B.
Andisols Form in volcanic debris. Have melanic epipedon.
Aridisols Arid soils. Carbonate or salt layers common.
Mollisols Grassland soils with mollic epipedon.
Vertisols Deep cracks and pedoturbation due to shrink-swell clays.
Alfisols Forest soils with an E and Bt (argillic).
Spodosols Forest soils with an E and Bh or Bs (spodic).
Ultisols Similar to but more weathered than Alfisols.
Oxisols More weathered than Ultisols. Have an oxic horizon.
Histosols Organic soils.
Gelisols Permafrost or cryoturbation limit development.
From recent. Little profile development. No subsurface diagnostic horizons. Development limited by any of several causes such as: 1) wetness whereby saturation inhibits horizon development, 2) dryness with little water movement and sparse vegetation, 3) deposition that continually buries the developing profile, 4) erosion at a rate nearly equal to downward profile development, 5) parent material that is highly resistant to weathering or 6) lack of time for development.
From inception. Cambic horizon usually present. Causes for limited development are the same as with Entisols but there has been greater development.
Formed in volcanic ejecta. Have melanic epipedon. More profile development than Entisols but not so much as to mask properties of parent material. Andic properties in upper profile, including low bulk density and potential for wind erosion.
Soils formed under arid conditions. Evapotranspiration > precipitation during most of the year and little water percolates through the soil. Pedogenesis is retarded by lack of water. Since there is little leaching, Aridisols contain a high concentration of basic cations. Lack of water keeps vegetation sparse, therefore, organic matter is low. Translocation has occurred only to the extent of moving soluble materials such as carbonates downward in the profile. Accumulation of carbonates as caliche is common.
Fertile, grassland soils with a mollic epipedon. Melanization or darkening by accumulation of organic matter is the dominant pedogenic process. Organic matter added at surface and in subsurface by dense mat of roots.
Inverting of surface and subsurface or pedoturbation by alternate shrinking and swelling of expanding clays is the dominant process. Requires sufficiently long dry period to cause wide and deep cracks to develop. Compression due to inverted surface soil forces subsurface peds to slide with respect to one another. Produces slickensides or pressure faces on peds and rippled soil surface topography called gilgai.
Developed under forest vegetation. Characterized by occurrence of an argillic horizon without mollic or spodic horizons. Alfisols are distinguished from the more highly weathered Ultisols by having higher content of basic cations. The dominant pedogenic process is translocation of clay. It is eluviated from upper profile to deeper positions by drainage water. Movement of clay particles is stopped by evapotranspiration, clogging of pores by translocated clay or flocculation, as by Ca2+ or Mg2+. Alfisols are productive soils but somewhat less fertile and more acidic than Mollisols.
Sandy and acidic forest soils with a spodic horizon. These form in parent material low in clay. Therefore, high conductivity and low base content facilitates acidic leaching. Two processes are necessary for spodic horizon to form: 1) mobilization of surface organic matter and Al and Fe oxides and 2) accumulation of these materials in the subsurface. The former occurs under rapidly infiltrating water and is aided by acidic litter under conifers. Movement is stopped by evapotranspiration, pore blockage, precipitation reactions or presence of shallow ground water. Spodosols are not fertile soils.
Term from ultimate development. Similar to Alfisols, these developed under forest vegetation and contain an argillic horizon. However, Ultisols are more weathered than Alfisols and have a lower content of basic cations. Accordingly, Ultisols are also less fertile.
Highly weathered soils of tropical regions that contain an oxic horizon. Oxisols contain few weatherable minerals. Instead, Al and Fe oxides are dominant. Oxisols occur on stable landscape positions where subject to intense weathering. Fertility is low and largely due surface organic matter. Therefore, nutrient cycling is very important to soil fertility in Oxisols.
From histo- or tissue, these are
organic soils that contain > 20 % organic matter to > 30 % if clay 60 %
or more. These develop where the rate of organic matter accumulation exceeds
its rate of decomposition. Typically, this is under wet and cold conditions
but Histosols may develop under higher temperatures as in Louisiana.
Bulk density is only about 0.2 - 0.3 g / cm-3, therefore, wind erosion is a problem. So too is oxidation. Therefore, if Histosols are drained and used for agricultural production, these must be keep wet near to the surface to limit erosion and subsidence when not used for growing a crop.
From gelid. These exhibit permafrost or evidence of cryoturbation. Therefore, development is limited by little water movement and development is retarded by profile mixing.
Example Series Name
Olivier fine-silty, mixed, thermic Aquic Fragiudalf
fine-silty, mixed, thermic
texture mineralogy temperature regime
differs from typical (Typic) of great group
presence of mottles indicating wetness
Great group Fragiudalf
Udalfs that have a fragipan
Alfisols that have a udic moisture regime
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