Soil Classification A classification system is needed to organize knowledge on 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 is a soil individual. Each soil individual is classified as belonging to one of more than 18,000 soil series. Soil Taxonomy 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. Order Suborder Great group Subgroup Family Series Order Differentiation largely based on presence or absence of diagnostic horizons. Soils within same order have been subjected to similar pedogenesis. Suborder Differ in moisture, temperature, chemical or textural features. Soil formation is more similar within same suborder. Great group Differentiated on basis of horizon sequence and other features. Subgroup Consists of soils that are typical of the great group and soils that are intergrades to other orders, suborders or great groups. Family Based on properties affecting plant growth or engineering use such as texture, mineralogy, temperature and other features. 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) Water regime Temperature regime Texture and other physical properties Chemical and mineralogical properties Surface Diagnostic Horizons Epipedons 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. Umbric Like mollic but lower base content. Forest vegetation under warm and wet climate. Ochric Light color and low organic matter content. Associated with weakly developed soils (Aridisols, Entisols and Inceptisols).Melanic Thick, black and high organic matter content. Developed in volcanic debris. Melanic Histic Organic horizon over a mineral soil. 20 to 30 + cm thick and 20 to 30 % + organic matter, depending on clay content. Mollic Histic Umbric Subsurface Diagnostic Horizons Albic Light colored and highly leached E horizon. Depleted of clay and oxides. Relatively coarse texture. Ochric Argillic 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.Spodic Secondary 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. Oxic Accumulations of solubilized materials such as Calcic CaCO3 accumulation. Hardpan horizons such as Fragipan Dense and brittle but weakly cemented horizon. Spodic Albic 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). Cambic Aquic Saturated for long enough to cause anaerobic conditions (gleying / mottling). Udic Sufficient water for plant needs. Perudic Ustic Xeric Torric Aridic Usually low moistureGelisols Permafrost or cryoturbation limit development. Soil Temperature Regimes Based on mean annual, summer and summer minus winter soil temperatures. Entisols From recent. Little profile development. No subsurface diagnostic horizons. Overview of Soil Orders Entisols Little profile development. Have A and C only. 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. Inceptisols More developed than Entisols. Have weak B (A, B and C). Andisols Form in volcanic debris. Have melanic epipedon. Entisol Aridisols Arid soils. Carbonate or salt layers common. Mollisols Grassland soils with mollic epipedon. Vertisols Deep cracks / pedoturbation due to shrink-swell clays. Alfisols Forest soils with an E and Bt (argillic). Therefore, A, E, Bt and C. Inceptisols From inception. Cambic horizon usually present. Causes for limited development are the same as with Entisols but there has been greater development. 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.Inceptisol Andisols 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. Andisol Aridisols 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. Aridisol Mollisols 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. MollisolVertisols Alfisol Inverting of surface and subsurface or pedoturbation by alternate shrinking and swelling