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					     Soil Classification I
ü Introduction
ü Reasons for classification
ü Some concepts of soil
ü Examples of soil classification systems
ü Basis for classifying soils
ü Diagnostic horizons
                   Introduction
§   We classify things so that we can make sense out of
    them.
§   Soils are classified so we may study just a few similar
    groups rather than many individuals.
§   Names such as black cotton seed soils, rice soils, olive
    soils, limestone soils piedmont soils, alluvial soils
    etc., are still in common use today
§   In present day classification, soils are classified as
    natural bodies on the basis of their profile
    characteristics and not just on the basis of their
    suitability for a particular use.
Reasons for Classifying Soils
v       To organize our knowledge of soils
v       To understand relationships among soil individuals
        and classes
v       To remember the properties of the soils
v       To establish subdivisions of soils in a manner useful
        for practical and applied purposes
    •     predicting soil behavior
    •     identifying their best uses
    •     and estimating their productivity.
          Concept of individual soils
§   Through classification, soil is perceived as being
    composed of a large number of individual units or natural
    bodies called soils.
§   Pedon is the smallest volume that can be considered as
    soil.
    §   Pedon refers to the smallest three-dimensional body of soil
        layers large enough to illustrate the nature and arrangement of
        soil horizons and their variability. It varies in size from 1-10 m2.
§   A group of very similar pedons that are closely associated
    in the field is called polypedons.
§   Polypedons can be large enough to be recognized as a
    landscape component termed a soil individual.
Natural body concept of soils
                §   A 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
          Examples of Soil
       Classification systems
§   USSR (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
                    Soil Taxonomy
§   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 Taxonomy
§   The 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.
Øphysical, chemical and mineralogical
properties of pedons are the basis for
classifying soils.

ØSome of these properties are observed
in the field e.g., moisture, temperature,
color, arrangement of soil horizons etc.

Øothers properties require precise
measurements in the lab., e.g., texture,
structure, organic matter, pH, % base
saturation, etc.

ØThe precise measurements are used to
define diagnostic soil horizons, the
presence or absence of which determine
the place of a soil in the classification
system
        Diagnostic Horizons
§ Soil Taxonomy makes use of diagnostic soil
  horizons for putting a soil into a class.
§ 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)
§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.

   §   Mollic epipedon (A)     §Melanic
   §   Umbric (A)              §Anthropic (A)
   §   Histic (O)              §Plaggen (A)

   §   Ochric (A)
Soil surface diagnostic horizons (epipedons)

§   Mollic Epipedon
    o   thick, dark, soft, surface layer. Characteristics: Thick - greater
        than 10 inches, High base saturation, Mineral soil, Soils
        formed under prairie vegetation
§   Umbric
    o   like mollic, but low base saturation
§   Histic
    o   Organic Soil - saturated with water, with more than 20-30%
        organic matter
• Plaggen
    o 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
    o     thick, black horizon that contains high concentrations of OC and short-
          range-order minerals such as allophane and imogolite.


•       Anthropic
    o     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
    o     thin, light colored - surface layers that do not fit any of the
          above
   Soil subsurface diagnostic horizons
Diagnostic 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 (Bt)                  §   Duripan (m)
   §   Natric (Btn)                   §   Fragipan (x)
   §   Spodic (Bhs)                   §   Gypsic (y)
   §   Oxic (Bo)                      §   Salic (z)
   §   Cambic (B)                     §   Petrocalcic
   §   Kandic                         §   Petrogypsic
   §   Albic (E)                      §   Placic
   §   Agric (A or B)                 §   Sombric
   §   Calcic (K)                     §   Sulfuric
     Soil subsurface diagnostic horizons
§   Argillic
    o contains illuvial clay -Bt
    o Must contain a significant clay increase.
    o If eluvial horizon has <15% clay, must have at least a 3%
      absolute increase (e.g., from 10 to 13%).
§   Natric
    o   same as argillic but with > 15% exchangeable sodium (Na)
§   Spodic
    o   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.)
§   Oxic
    o   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
        o   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
    o   Has CaCO3 equivalent 15% and contains 5% more CaCO3
        equivalent than the C horizon or
    o   Has CaCO3 equivalent 15% and contains 5% identifiable
        pedogenic CaCO3 forms such as concretions, soft powdery
        forms, etc.
•    Kandic
    o   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:
    o   CEC < 16 cmol/kg of clay
    o   Effective cation exchange capacity (ECEC) <12 cmol/kg of
        clay where ECEC = Ca + Mg + K + Na + extractable Al


•    Fragipan
    o   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 II
ü Categories of the Soil Taxonomy
ü Description of the Soil Orders
ü Key to the Soil Orders
ü Suborders
ü Great Groups and Sub Groups
ü Soil Families
ü Soil Series
    Categories of the Soil Taxonomy
§    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.
                      Soil Orders
v Every soil in the world is assigned to one of 12 orders that
  reflect major course of development.
v In the orders, there is considerable emphasis placed on the
  presence or absence of major diagnostic horizons.
v 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
             -recent


• No 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
           pedalfer

• Do 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
   6. Ultisols
 -ultimate (Latin, last)

• Have Argillic or Kandic with
  <35% Base saturation.
• Common in S.E. US, humid
  tropical, and subtropical
  regions

• Warm and humid environs.

• Old, highly weathered, high
  degree of development (ult)
• Low fertility
     7. Oxisols
  -oxide (French, oxide)
• Have an oxic horizon.
   • Dominated by residual
     accumulations of Fe and Al
     oxides and kaolinite
• Most extensively weathered
  soils (occur in tropical
  regions).
• Intense red or yellow (high
  in oxidized iron).
• Low % Base Saturation.
   • extensively leached
• Low fertility.
   8. Histosols
 -histos (Greek, tissue)
• Organic soils (peats and
  mucks)
• Mainly found in cold
  climates (Alaska, Canada,
  Finland, Russia)
• Low-lying swampy areas
• Virtually all Histosols
  occur in wetland
  environments
• Have very low bulk
  densities and very high
  water holding capacity
   9. Vertisols
   -invert (Latin, turn)

• High clay soils (>30%
  sticky shrink-swell clays).

• Deep cracks upon drying

• Materials from soil surface
  fill cracks and become part of
  the subsurface when the
  cracks close (inverted soil)
  10. Aridisols
     -arid (Latin, dry)

• Form in aridic moisture
  regimes (dry)
• Dry at least 6 months
• Low OM
• Little leaching
  • High base sat.
  • Can be very productive if
    irrigated
    11. Andisols
   -ando (Jap., black soil)
• Form from P.M. of volcanic
  origin
  • Soil forms by rapid weathering of
    volcanic ash to produce poorly
    crystallized aluminosilicates
    (allophane & imogolite)

• Andisols are young soils ( and
  they have high OM)
• In dry climates, can be
  susceptible to wind erosion
• Can have unusually low bulk
  densities
    12. Gelisols
    -gelid (Greek, cold)
• Presence of permafrost layer
  within 100 cm of soil surface
  defines this soil class

• Young soils with little profile
  development
   • Cold temperatures and frozen
     conditions for much of the year
     slow the process of soil formation.

• May show evidence of
  cryoturbation
   • physical disturbance of soil material
     caused by freezing and thawing
Simplified Key to the 12 Soil Orders of The Soil Taxonomy
Soil map of North America showing distribution of 12 soil
              orders of the Soil Taxonomy
                           Suborders
v   Subdividing Soil Orders into Suborders is based on
     • soil moisture regimes
     • soil temperature regimes


1.Soil Moisture Regimes
(refers to the presence or absence of water saturated conditions or
   plant-available soil water at a defined section of the soil (control
   section)
    •   Aquic = soils with saturated horizons
    •   Aridic = arid climates; dry in all portions of profile most of the time
    •   Udic = humid climates; moisture usually exceeds evapotranspiration
    •   Ustic = between aridic and udic (sufficient moisture for crop growth)
    •   Xeric = Mediterranean climates; cool, moist winters and hot dry summers
2.Temperature regimes

(Based on mean tempt. differences of soils at 50 cm depth)
   • Pergelic   = MAST < 0 ºC permafrost
   •   Cryic   = 0 ºC – 8 ºC, summer Temp <15 ºC
   •   Frigid = 0 < 8 ºC (has warmer summers)
   •   Mesic = 8 < 15 ºC
   •   Thermic = 15 to 22 ºC
   •   Hyperthermic = MAST > 22 ºC
                           Suborders
v   Therefore within each soil order, soils are grouped into
    suborders on the basis of soil properties that reflect
    major environmental controls on soil forming processes.
v    Many soil suborders are indicative of the moisture
    regime or temperature regime under which the soils are
    formed
    •   E.g., soils formed under wet conditions are identified under
        separate suborders as being wet soils.
    à   Ustolls –dry Mollisols (Mollisols with Ustic moisture regime)
    à   Udults –moist Ultisols (Ultisols with Udic moisture regime)
    à   Aquents – wet Entisols (Entisols with aquic moisture regimes)
Suborders in the Soil Taxonomy
            Soil Great Groups
Great groups are subdivisions of suborders.
Great groups are defined largely by the presence or
  absence of diagnostic horizons and the arrangement of
  those horizons.
Formed by adding formative elements to the suborder
  names (Table 3.7 of Textbook, pp. 113)

Examples of Great group names:
   • Hapl + Aquoll = Haplaquoll (hapl ~ minimum horizon)
   • Argi + Udoll = Argiudaoll (argi ~ argillic horizon)
   • Natra + Aqualf = Natraqualf (natr ~ natric horizon)
                 Soil Subgroups
• Subgroups are subdivisions of the great groups.
• The central concept of the great group makes up one
  subgroup, termed typic.
• E.g., Typic Hapludoll subgroup typifies the Hapludolls
  great group.
• Other subgroups may have characteristics that intergrade
  between those of the central concept and soils of other
  orders, suborders or great groups.
• Fluventic Haplaquoll
   • -oll means Mollisol; aqu means aquic moisture (poor or very
     poor drainage); hapla means min. B horizon development; fluv
     flood plain; entic means entisol (young soil).
• Mollic Albaqualf
   • -alf means alfisol; aqu means aquic moisture regime; alba
     indicates a strongly developed E horizon
              Soil Families
§ Soils fall into the same family if at the same
  depth they have similar physical and chemical
  properties that affect plant root growth.
§ The soil properties include soil texture,
  mineralogy, cation exchange activity of the
  clay, temperature and depth of the soil
  penetrable by roots.
§ A family classification such as Typic Argiudolls
  loamy, mixed, active, mesic conveys very
  detailed information to a soil surveyor.
                      Soil Series
       is the most specific unit in the Soil
v Series
  Taxonomy. It is a subdivision of the family.

v Itis defined by a specific range of soil properties
  such as kind, thickness and arrangement of horizons.

v Series names are usually given to reflect their
  locations. There are about 19,000 series in today’s
  Soil Taxonomy.
v Examples of soil series names
   v   Cordova series, Kokomo Series, Westland Series, etc
   (See next figure for complete structure of Soil Taxonomy)

				
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