Transcript Document

Soil Classification in the
United States
Soil 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
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Black cotton soils
Rice soils
Olive soils
Limestone soils
Piedmont soils
Alluvial soils
Suitability for different uses
Method 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 Soils
• This 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 Polypedon
• There 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 Model
Hierarchical soil classification
groups soils into increasing
levels of generality between
these two concepts –pedons
and polypedons
 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 series.
Major Soil Classification
Systems of the World
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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
The United States Soil Taxonomy
 Soil 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 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.
Diagnostic Horizons
 Soil 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)
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.
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Mollic epipedon (A)
Umbric (A)
Histic (O)
Ochric (A)
Melanic
Anthropic
(A)
Plaggen (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 2030% 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.
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Melanic
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thick, black horizon that contains high concentrations of OC and
short-range-order minerals such as allophane and imogolite.
Anthropic
o
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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.
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Argillic (Bt)
Natric (Btn)
Spodic (Bhs)
Oxic (Bo)
Cambic (B)
Kandic
Albic (E)
Agric (A or B)
Calcic (K)
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Duripan (m)
Fragipan (x)
Gypsic (y)
Salic (z)
Petrocalcic
Petrogypsic
Placic
Sombric
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.)
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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)
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Cambic
o slightly altered layer - not weathered enough to be
argillic, Bw horizon designation or development of
color and or structure
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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.
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Kandic
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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
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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
 Categories of the Soil Taxonomy
 Description of the Soil Orders
 Key to the Soil Orders
 Suborders
 Great Groups and Sub Groups
 Soil Families
 Soil Series
Categories of the Soil Taxonomy
1. Orders (12) (surface and subsurface diagnostic
horizons)
2. Suborders (55) (Soil temperature. and moisture
regimes)
3. Great group (238) (subsurface diagnostic horizon)
4. Subgroup (1243) (drainage, lithic contact, PM, clay
type)
5. Family (7504) (Texture of
diagnostic surface
horizon)
6. Series (about 19,000) in U.S.
Soil Orders
• Every 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
-recent
• No diagnostic subsurface
horizons.
• Very recent or young soil
• Little if any profile
development
• Form on
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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
• 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)
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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
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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
• 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.
• 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.
 Ustolls –dry Mollisols (Mollisols with Ustic moisture regime)
 Udults –moist Ultisols (Ultisols with Udic moisture regime)
 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
Soil Series
• Series is the most specific unit in the Soil
Taxonomy. It is a subdivision of the family.
• It is defined by a specific range of soil
properties such as kind, thickness and
arrangement of horizons.
• Series names are usually given to reflect their
locations. There are about 19,000 series in
today’s Soil Taxonomy.
• Examples of soil series names
– Cordova series, Kokomo Series, Westland Series,