1. Soil Classification in the United States
Soil Morphology, Genesis, and Classification (SPS 350)

2. 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 uses
Method of soil formation

3. 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.

4. 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

5. 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.

6. 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.

7. Major Soil Classification Systems of the World
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

8. 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

9. 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.

10. 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

11. 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)
Umbric (A)
Histic (O)
Ochric (A)
Melanic
Anthropic (A)
Plaggen (A)

12. 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.

13. Melanic Anthropic Ochric
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

14. 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)
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
Sulfuric

15. Soil subsurface diagnostic horizons
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.)

16. Calcic - contains an accumulation of CaCO3
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.

17. 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.

18. 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

19. 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.

20. 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.

21. Degree of weathering and soil development in the different soil orders

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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.

29. 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

30. 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)

31. 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

32. 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

33. 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

34. Simplified Key to the 12 Soil Orders of The Soil Taxonomy

35. Soil map of North America showing distribution of 12 soil orders of the Soil Taxonomy

36. Suborders 1. Soil Moisture Regimes
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

37. 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

38. 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)

39. Suborders in the Soil Taxonomy

40. 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)

41. 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

42. 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.

43. 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, etc
(See next figure for complete structure of Soil Taxonomy)