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Introduction to Soil Classification
Debbie Surabian State Soil Scientist CT/RI
USDA - NRCS
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Soil Taxonomy was developed by the USDA Soil Conservation Service staff
between 1951 and 1975.
Soil Taxonomy
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Soil Taxonomy was designed to classify all the world's soils because expanding soil survey programs demanded more precise definitions of soil properties.
– Organize knowledge of soils with similar properties – Facilitate communication among pedologists – Transfer information and technology
Soil Taxonomy
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• Adopted in 1965 by Dr. Guy D. Smith, USDA
• Official soil classification system of the United States
• Has been recognition as a possible universal system for classifying soils.
• To date, no system of classification has world wide acceptance – World Reference Base for Soil Resources (WRB)
Soil Taxonomy
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Criteria Used in Soil Taxonomy
• Based on soil properties that can be observed and measured.
• Chemical, physical, and biological properties such as moisture, temperature, texture, structure, pH, soil depth
• Presence or absence of certain diagnostic horizons (surface and subsurface horizons)
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The upper limit of soil is the boundary between soil and air [or] shallow water…[not] too deep (typically more than 2.5 meters) for the growth of rooted plants.
The Soils That We Classify
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The Soils That We Classify
For purposes of classification, the lower boundary of soil is arbitrarily set at 200 centimeters.
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Order (12) – Most general, based on soil forming processes.
Sub-Orders (70) – Based on similarities in soil formation (moisture/temp/other).
Great Groups (344) – Based on differences between soil horizons (diagnostic horizons).
Sub-Groups (2,664) – Profile characteristics.
Family (~11,332) – Based on properties that effect management (mineralogy, temperature)
Series (~23,600) – Most specific, based on kind and arrangement of horizons.
Phase – Field mapping units (stony, slope, other), not a category in soil taxonomy.
Hierarchical System of Soil Taxonomy
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Great Group
Humid Climate
Mixed, mesic
Family Mixed Mineralogy
Mean Annual Temp. 8 – 15c
Sub Group Reflects
Central Concept
Sub Order Sandy
Order Entisol
Typic Udipsamments
Windsor Series
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• Entisol • Inceptisol • Andisols • Spodosols • Mollisols • Alfisols • Ultisols • Oxisols • Aridisols • Vertisols • Histosols • Gelisols
Twelve Soil Orders
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• Highest level of classification • Broadest group • 12 orders • Based on diagnostic horizons and climate
Soil Orders
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Degree of weathering and soil development in the different soil orders
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Global Distribution
Entisols – 18%
Inceptisols – 15%
Mollisols – 7%
Spodosols – 4%
Histosols – 1%
Soil Taxonomy
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• Recent • Very recent or young soils • Little or no profile development • No diagnostic subsurface horizons • Form on
• resistant parent materials, • mine spoils, • steep slopes, • floodplains
Entisols
Windsor Series
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• Simple soils to very productive alluvial soils
• Pedogenically featureless (between “not soil” and horizonated soil)
• Generally associated with human transported materials
Entisols
No: active gravel pit
No: pavement and buildings
Yes: vegetated area, Fortress Series.
Is it Soil?
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Entisols – Six Suborders
Manchester Series
Hadley Series
TYPIC UDIFLUVENTS
TYPIC UDORTHENTS
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• inception (Latin, beginning) • Slightly more development than
Entisols • Young soils but beginning of profile
development is evident • May have a cambic horizon
Inceptisols
Gloucester Series
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Inceptisols – Seven Suborders
Sutton Series
AQUIC DYSTRUDEPTS TYPIC ENDOAQUEPTS
Scitico Series
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• mollis (Latin, soft) • Very dark-colored soils • Naturally fertile soils • Typically form under grasslands • Has a mollic epipedon:
• Thick, humus-rich surface horizon
• High % base saturation throughout profile
Mollisols
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Mollisols – Eight Suborders
TYPIC ENDOAQUOLLS
Menlo Series
TYPIC ARGIUDOLL
Sharpsburg Series
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• From Greek spodos, “wood ash”
• Acid soils with low fertility • 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.
• Has a spodic horizon • Accumulations of organic
matter and/or iron/aluminum oxides in the subsoil
Spodosols
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Spodosols – Five Suborders
AQUODS AQUODS
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• histos “tissue”
• Organic soils derived from hydrophytic plants
• Low bulk density and very high water holding capacity
Histosols
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Histosols – Five Suborders
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Alfisols
• pedalfer • Naturally fertile soils • High base saturation • Clay-enriched subsoil horizon
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Aridisols
• arid (Latin, dry) • Form in aridic soil moisture
regimes (dry) • Low OM • Little leaching
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Ultisols
• -ultimate (Latin, last) • Old, highly weathered, high
degree of development • Low fertility
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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)
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Andisols
• ando (Japanese, black soil) • Form from P.M. of volcanic
origin • Young soils • Can have unusually low bulk
densities • High water holding capacity
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• oxide (French, oxide) • Very highly weathered soils • Intense red or yellow color
(high in oxidized iron) • Low native fertility
Oxisols
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Gelisols
• gelid (Greek, cold) • Young soils with little profile
development • High organic carbon content
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Series
• Soil series predates Soil Taxonomy – used since 1899 • In 1975 there were 10,500 series in the U.S. and its territories;
compared to about 23,600 series today • Lowest level of classification – can be subdivided into Phases • Typically name based on location where first identified
Period of Soil Taxonomy
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Great Group
Family Sub Group
Sub Order
Order
Series
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Great Group Accumulation of Organic
Matter in the Surface
Loamy, mixed, active, acid, mesic, shallow
Family Particle Size Class Mixed Mineralogy Clay Activity Class
Reaction Class Mean Annual Temp. 8-15c
Soil Depth class (to Cd)
Sub Group Reflects
Central Concept
Sub Order
Aquic
Order Inceptisol
Typic Humaquepts
Whitman Series
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Deb’s Tips and Tricks to using Soil Taxonomy
• Start from the beginning and work your way through the keys
• If the criteria are not met, continue in order until they are met
• Read and re-read each sentence
• Use the Official Series Descriptions (OSDs) to learn how to key out pedons
• Don’t assume the pit you dig will be a perfect match to any OSD pedon
• Learn what you need to describe in the pit to classify your soil
• Review Ch. 18 - nice explanation on designation for horizons and layers
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