Bases for Soil Classification in Soil Taxonomy

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Soil Classification in the United StatesSoil Morphology, Genesis, and Classification (SPS 350)


Why classify soils?Whenever we call things by group names, or give them labels that inform us of their important properties, we are doing classification. The names or labels could provide information about their appearance, make up, functions or their other known important characteristics Same applies to soils when we use names like Black cotton soils Rice soils Olive soils Limestone soils Piedmont soils Alluvial soils Suitability for different usesMethod of soil formation


Such classification terms may convey some valuable meaning to local users However, they do not help us to organize our knowledge of soils or for defining the relationships among different soils For classification to address these concerns, soils are classified as natural bodies on the basis of their soil profile characteristics By using a system of classification of soils, we create a universal language of soils that enhances communication among users of soils around the world.


Concept of Individual SoilsThis concept recognizes the existence of individual entities each of which we call a soil. Soil individuals having one or more characteristics in common may be grouped together. The groups are aggregated into higher-level categories of soils, each having some characteristic that sets them apart from the other. Therefore broad soil groups are defined as one moves up the classification pyramid


Pedon and PolypedonThere are no sharp demarcations between one soil individual and another. There is a gradation in soil properties as one moves from one soil individual to an adjacent one. Soil individuals are characterized in terms of an imaginary three dimensional unit called pedon. A pedon (1 to 10 m2) is the smallest sampling unit that displays the full range of properties that characterize a particular soil. The pedon serves as the fundamental unit of soil classification. A group of similar pedons that are closely associated in the field are called polypedons, and are recognized as soil individuals. All soil individuals that have in common a suite of soil profile properties and horizons that fall within a particular range are said to be in the same soil series.


Pedon-Polypedon-Landscape ModelHierarchical soil classification groups soils into increasing levels of generality between these two concepts –pedons and polypedonsA soil unit in a landscape consists of a group of very similar pedons (polypedon). A polypedon is a soil individual. Soil individuals that have in common a suite of soil profile properties and horizons that fall within a particular range are said to belong to the same soil series.


Major Soil Classification Systems of the WorldUSSR (Russian) Soil Classification System Natural Soil System of Kubiena French Soil Classification System Belgium Soil Classification British System of Classification Soil Classification of Canada Australian Soil Classification System Brazilian Soil Classification System FAO/UNESCO Soil Map of the World US Soil Taxonomy


The United States Soil TaxonomySoil Taxonomy, the official soil classification system of the United States was officially adopted in 1965 Since then, it has gained recognition as a possible universal system for classifying soils. However, to date, no system of classification has world wide acceptance Soil Taxonomy is based on soil properties that can be objectively observed and measured. Soil Taxonomy makes use of nomenclature which gives definite connotation of the major characteristics of soils


Bases for Soil Classification in Soil TaxonomyThe physical, chemical and mineralogical, and biological properties of soils as they are today, are the basis for classifying soils in the Soil Taxonomy. E.g. moisture, temperature, color, texture, structure, organic matter, pH, % base saturation, soil depth, etc. are important criteria for classification. Some of these properties are observed in the field, but others require precise measurements in the lab. These precise measurements are used to define certain diagnostic soil horizons, the presence or absence of which determine the place of a soil in the classification system.


Diagnostic HorizonsSoil Taxonomy makes use of diagnostic soil horizons for classification. Diagnostic horizons have specific characteristics that are indicative of certain classes of soils There are two types of diagnostic horizons Soil Surface diagnostic horizons (epipedons) Sub surface diagnostic horizons


Soil surface diagnostic horizons (epipedons)Mollic epipedon (A) Umbric (A) Histic (O) Ochric (A)Melanic Anthropic (A) Plaggen (A) An epipedon is a diagnostic horizon that forms at the surface. Any horizon may be at the surface of a truncated soil. There can be only one epipedon for mineral soils.


Soil surface diagnostic horizons (epipedons)Mollic Epipedon thick, dark, soft, surface layer. Characteristics: Thick - greater than 10 inches, High base saturation, Mineral soil, Soils formed under prairie vegetation Umbric like mollic, but low base saturation Histic Organic Soil - saturated with water, with more than 20-30% organic matter Plaggen surface layer made by humans that is > 50 cm thick that has been produced by manuring. It usually contains artifacts, such as brick and pottery, and spade marks throughout.


Melanic thick, black horizon that contains high concentrations of OC and short-range-order minerals such as allophane and imogolite. Anthropic disturbed by human activity. Meets all of the requirements for a mollic except (A) it has >250 ppm PsO5 soluble in 1% citric acid with or without the base saturation requirements or (B) the duration of available moisture. Ochric thin, light colored - surface layers that do not fit any of the above


Soil subsurface diagnostic horizonsArgillic (Bt) Natric (Btn) Spodic (Bhs) Oxic (Bo) Cambic (B) Kandic Albic (E) Agric (A or B) Calcic (K) Duripan (m) Fragipan (x) Gypsic (y) Salic (z) Petrocalcic Petrogypsic Placic Sombric SulfuricDiagnostic subsurface horizons form below the soil surface. Usually, they are B horizons but diagnostic subsurface horizons may include parts of A or E horizons. Some soils do not have a diagnostic subsurface horizon.


Argillic contains illuvial clay -Bt Must contain a significant clay increase. If eluvial horizon has <15% clay, must have at least a 3% absolute increase (e.g., from 10 to 13%). Natric same as argillic but with > 15% exchangeable sodium (Na) Spodic illuvial accumulation of oxides of Al and Fe and OM, red or dark red color - only found in acid sandy soils, with high rainfall- generall found below E horizon. Contains a Bhs or Bs horizon subsurface horizons (cont.)Soil subsurface diagnostic horizons


Oxic very weathered layer of only Fe and Al oxides and 1:1 clay minerals, low pH and not very fertile (found in tropical soils) Cambic slightly altered layer - not weathered enough to be argillic, Bw horizon designation or development of color and or structure Calcic - contains an accumulation of CaCO3 Has CaCO3 equivalent 15% and contains 5% more CaCO3 equivalent than the C horizon or Has CaCO3 equivalent 15% and contains 5% identifiable pedogenic CaCO3 forms such as concretions, soft powdery forms, etc.


Kandic a highly weathered horizon with low cation exchange capacity (CEC). It usually meets all of the classification criteria of an argillic horizon along with the following CEC requirements in 50% or more of its thickness from the top to the Bt to a depth of 100 cm: CEC < 16 cmol/kg of clay Effective cation exchange capacity (ECEC) <12 cmol/kg of clay where ECEC = Ca + Mg + K + Na + extractable Al Fragipan a "hardpan" that is brittle when moist and very hard when dry. Peds will slake or fracture when placed in water. Very difficult to dig with a spade when dry.


Soil Classification IICategories of the Soil Taxonomy Description of the Soil Orders Key to the Soil Orders Suborders Great Groups and Sub Groups Soil Families Soil Series


Orders (12) (surface and subsurface diagnostic horizons) Suborders (55) (Soil temperature. and moisture regimes) Great group (238) (subsurface diagnostic horizon) Subgroup (1243) (drainage, lithic contact, PM, clay type) Family (7504) (Texture of diagnostic surface horizon) Series (about 19,000) in U.S. Categories of the Soil Taxonomy


Soil OrdersEvery soil in the world is assigned to one of 12 orders that reflect major course of development. In the orders, there is considerable emphasis placed on the presence or absence of major diagnostic horizons. Bold letters in the soil order names indicate formative element used as ending for lower taxa.


Degree of weathering and soil development in the different soil orders


1. Entisols -recentNo diagnostic subsurface horizons. Very recent or young soil Little if any profile development Form on resistant P.M., mine spoils, steep slopes, floodplains


2. Inceptisols inception (Latin, beginning)Slightly more development than Entisols Young soils but beginning of profile development is evident. Well-defined profile characteristics of mature soils are yet to be developed. May have a cambic horizon


3. Mollisols mollis (Latin, soft)Typically form under grasslands. E.g., Central US Soils with a mollic epipedon: Thick humus-rich surface horizon High % base saturation throughout profile Slightly leached Very fertile soils


4. Alfisols pedalferDo have an argillic horizon About 35% base saturation Develop in humid, temperate regions Vegetation is usually deciduous (forests, savanna) Gray to brown surface horizon Good for grain production


5. Spodosols -spodos (Greek, wood ashes)Have a spodic horizon Form in humid, cool climates and occur most often in conifer forests (New England, Mich., Canada) Form in acid, coarse, quartz (sandy) bearing P.M. Low fertility

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Last Updated: 8th March 2018

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