Soil Classification I
ü Reasons for classification
ü Some concepts of soil
ü Examples of soil classification systems
ü Basis for classifying soils
ü Diagnostic horizons
§ We classify things so that we can make sense out of
§ 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
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
§ 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
§ A polypedon is a soil
§ 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
§ 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
§ 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
§ 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
§ 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
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
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
§ 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
§An epipedon is a diagnostic horizon that forms at the
§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
o like mollic, but low base saturation
o Organic Soil - saturated with water, with more than 20-30%
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.
o thick, black horizon that contains high concentrations of OC and short-
range-order minerals such as allophane and imogolite.
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
o thin, light colored - surface layers that do not fit any of the
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
§ 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
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%).
o same as argillic but with > 15% exchangeable sodium (Na)
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.)
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)
o slightly altered layer - not weathered enough to be argillic,
Bw horizon designation or development of color and or
• 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
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
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 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
§ Great group (238) (subsurface diagnostic horizon)
§ Subgroup (1243) (drainage, lithic contact, PM, clay
§ Family (7504) (Texture of diagnostic surface horizon)
§ Series (about 19,000) in U.S.
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
• No diagnostic subsurface
• Very recent or young soil
• Little if any profile development
• Form on
• resistant P.M.,
• mine spoils,
• steep slopes,
inception (Latin, beginning)
• Slightly more development
• Young soils but beginning of
profile development is
• Well-defined profile
characteristics of mature soils
are yet to be developed.
• May have a cambic horizon
mollis (Latin, soft)
• Typically form under
• E.g., Central US
• Soils with a mollic epipedon:
• Thick humus-rich surface horizon
§ High % base saturation
§ Slightly leached
§ Very fertile soils
• Do have an argillic horizon
• About 35% base saturation
• Develop in humid, temperate
• Vegetation is usually
deciduous (forests, savanna)
• Gray to brown surface horizon
• Good for grain production
-spodos (Greek, wood ashes)
• Have a spodic horizon
• Form in humid, cool climates
and occur most often in conifer
forests (New England, Mich.,
• Form in acid, coarse, quartz
(sandy) bearing P.M.
• Low fertility
-ultimate (Latin, last)
• Have Argillic or Kandic with
<35% Base saturation.
• Common in S.E. US, humid
tropical, and subtropical
• Warm and humid environs.
• Old, highly weathered, high
degree of development (ult)
• Low fertility
-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
• Intense red or yellow (high
in oxidized iron).
• Low % Base Saturation.
• extensively leached
• Low fertility.
-histos (Greek, tissue)
• Organic soils (peats and
• Mainly found in cold
climates (Alaska, Canada,
• Low-lying swampy areas
• Virtually all Histosols
occur in wetland
• Have very low bulk
densities and very high
water holding capacity
-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)
-arid (Latin, dry)
• Form in aridic moisture
• Dry at least 6 months
• Low OM
• Little leaching
• High base sat.
• Can be very productive if
-ando (Jap., black soil)
• Form from P.M. of volcanic
• Soil forms by rapid weathering of
volcanic ash to produce poorly
(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
-gelid (Greek, cold)
• Presence of permafrost layer
within 100 cm of soil surface
defines this soil class
• Young soils with little profile
• Cold temperatures and frozen
conditions for much of the year
slow the process of soil formation.
• May show evidence of
• 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
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
• 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
(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
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
• 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
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)
• 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
• 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
§ 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.
is the most specific unit in the Soil
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
v Examples of soil series names
v Cordova series, Kokomo Series, Westland Series, etc
(See next figure for complete structure of Soil Taxonomy)