SOIL CLASSIFICATION

• Soil Classification (SC)


  The systematic arrangement of soils into groups or categories based on their characteristics
• Soil Taxonomy


  A grouping or classification of soils into categories or levels based on how they relate morphologically, physically, and chemically; refers to the system of classification developed by the USDA
• Soil Taxonomy system


  • Comprehensive system developed by the Soil Survey Staff of the U.S. Dept. Of Agric. (USDA) headed by G.D. Smith in 1960, adopting an improved version in 1975
  • Organizes all soils into 12 Orders, 64 Suborders, 250 Great groups, 1400 Subgroups, 8000 Families, and 19,000 Series (the last level, in the U.S. only)
• Other countries may have different classification systems, but most are similar to the system used in the USA. The United Nations FAO Soil Map of the World uses a different system and has 31 major soil groups
• Order


  The most general or broadest category in the soil taxonomy system. All soils belong to one of the 12 soil orders. The presence or absence of the diagnostic features determines the emphasis for placing soil into a soil Order
• Suborders


  Differentiated largely based on properties and horizons resulting from differences in soil moisture, soil temperature, and dominating effects of chemical or textural features
• Great groups


  Subdivisions of suborders based on differentiating soil horizons and soil features. Differentiating horizons: Bt, Bs, Bh (those that have accumulated clay, Fe&Al, and humus respectively) and those that have pans (hardened or cemented layers). Soil features: cracking of clays, soil temperature, and major differences in content of Ca, Mg. K. Na, gypsum, and other salts
• Subgroups


  Each great group is divided into three kinds of subgroups: one representing the central (typic) segment of the soil group; a second which has properties that intergrade toward other orders, suborders, or other great groups; and a third (extra grade), which has properties that prevent its classification as typic or an intergrade to another soil category
• Families


  Soil families are separated within a subgroup mainly based on soil properties important to plant growth or the behavior of soils when used for engineering purposes. These properties include texture, mineralogy, pH, average soil temperature, the area's moisture regime, permeability, thickness of horizons, structure, and consistency
• Series


  The most detailed category of soil taxonomy; each family contains several similar soil series. The series' name represents a prominent geographic name of a river or town where the series was first recognized, e.g., Jasaan series; in the Philippines, this is the basis for mapping the distribution of soils. Soils in the same series have about the same color, texture, structure, consistency, thickness, pH, the same # and arrangement of horizons, and similar chemical and mineralogical properties
• The 12 soil orders

  • GELISOLS
  • HISTOSOLS
  • SPODOSOLS
  • ANDISOLS
  • OXISOLS
  • VERTISOLS
  • ARIDISOLS
  • ULTISOLS
  • MOLLISOLS
  • ALFISOLS
  • INCEPTISOLS
  • ENTISOLS
• Wetness features


  • Anthrepts - anthropic or plaggen epipedon
  • Cryepts - cryic soil temp. regime
  • Ustepts - ustic soil moist. regime
  • Xerepts - xeric soil moist. regime
  • Udepts - other Inceptisols (i.e., udic SMR)
• INCEPTISOLS - Soils with no illuvial horizon, with weakly developed B; young soil
• ENTISOLS - Mineral soils with little or no Morphological development
• Entisol suborders


  • Aquents - exhibit wetness features
  • Arents - distinctive plow layer
  • Fluvents - formed in alluvial deposists
  • Orthents - loamy or clayey textures
  • Psamments - sandy textures
• Epipedons


  Diagnostic Surface Horizons
• Histic epipedon


  Usually wet organic horizons, OM> 20%. A histic horizon has a thickness requirement.
• Mollic epipedon


  Thickness > 25 cm thick, value & chroma not >3, BCSP>50%, O.C.> 0.6 %, strong structure. Concept: Deep, dark-colored, OM-rich, fertile, mineral topsoil formed on grasslands.
• Umbric epipedon


  Like mollic except BCSP < 50 % (lower natural fertility)
• Ochric epipedon


  Weakly developed: thin, light-colored, weak structure, low OM, hard, and massive when dry.
• Anthropic epipedon


  Any epipedon altered by human influence, e.g., plowing, farming, building projects, urban areas, etc.
• Plaggen epipedon


  Man-made, sod-like horizon due to years of manuring
• Melanic epipedon


  Dark (value and chroma #2), thick (30 cm), high OC (> 6%), with andic materials
• Endopedons


  Diagnostic Subsurface Horizons–All are mineral horizons, though some have accumulations of organic matter.
• E albic horizon


  Eluvial, light-colored, clay, Fe & Al oxides removed
• Bt argillic horizon


  Clay accumulation horizon with evidence of illuviation, e.g., clay films, etc.
• Btn natric horizon


  Like argillic but with high Na; often has a columnar or prismatic structure
• Bt kandic horizon


  An argillic horizon of low-activity clays (kaolinite-like); the presence of clay films not required.
• Bhs spodic horizon


  Illuvial OM, Fe and Al oxide accumulations; an acidic, cool area, humus and/or sesquioxide accumulation
• Bo oxic horizon


  Highly weathered B horizon; mainly a mixture of kaolinite, hydrated Fe and Al oxides, quartz, and very little water-dispersible clay.
• Bk calcic horizon


  > 15% CaCO3 or CaCO3 + MgCO3 accumulation
• Bkk calcic horizon


  > 50% carbonates plugging pores
• Bkm petrocalcic horizon


  Cemented calcic horizon
• By gypsic horizon


  >5% gypsum accumulation, CaSO4
• Bym petrogypsic horizon


  Cemented gypsic horizon
• Bss slickensides horizon


  Shear failure planes and surfaces due to high shrink-swell clays – vertic properties
• Bw cambic horizon


  A "color" or weakly developed B horizon (common in Inceptisols).
• Other Subsurface diagnostic horizons


  • agric- OM and clay accumulation below the plow layer due to tillage
  • Bqm duripan - hardpan, silica-cemented plus some carbonates
  • Bx fragipan - brittle pan, loamy and dense; weakly cemented.
  • Bs placic –thin-hardpan cemented by Fe, Fe + Mn, or Fe + OM.
  • Bh sombric - organic matter accumulation
  • Bj sulfuric - strongly acid horizon high in sulfides with Jarosite mottles, when drained oxidizes to sulphuric acid (pH <3.5 – toxic to plants)
  • Bz salic - soluble salt accumulation horizon
  • g gleying – Fe has been reduced (and possibly removed) due to anaerobic conditions (saturation), indicated by high values (5 or more) and low chromas (2 or less), and possible redox concentrations
• Soil Moisture Regimes


  • Aquic conditions– saturated with water; the soil has visual evidence (mottles) of poor drainage conditions.
  • Udic– adequate water all year round
  • Perudic– precipitation exceeds evapotranspiration in all months, but soil is not saturated for long periods.
  • Ustic– sufficient water during the growing season only, usually in summer
  • Xeric– deficient water and with a dry cropping season; precipitation comes in the wintertime; typical of Mediterranean climate.
  • Aridic – very water deficient; long dry periods, short wet periods
  • Torric– same as aridic but used at specified locations in the classification system.
• Soil Temperature Regimes


  • Pergelic – Soils with mean annual temperature (MAT) < 0°C and have permafrost (perennially frozen horizon)
  • Cryic (Gr. kryos, coldness; meaning very cold soils) - MAT < 8°C but does not have permafrost.
  • Frigid -warmer in summer than soil with a cryic regime; MAT < 8°C
  • Mesic – MAT is 8°C or higher but lower than 15°C, and the difference between mean summer and mean winter soil temperatures is >6°C
  • Thermic - MAT is 15°C or higher but < 22°C, and the difference between mean summer and mean winter soil temperatures is >6°C
  • Hyperthermic - MAT is 22 °C or higher, and the difference between mean summer and mean winter soil temperatures is > 6°C
  • Isofrigid – MAT < 8° C.
  • Isomesic – MAT is 8° C or higher but < 15° C.
  • Isothermic – MAT is 15° C or higher but < 22° C.
  • Isohyperthermic – MAT is 22° C or higher.