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Introduction to Soil Classification
Debbie Surabian State Soil Scientist CT/RI
USDA - NRCS
Soil Taxonomy was developed by the USDA Soil Conservation Service staff
between 1951 and 1975.
Soil Taxonomy
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
• 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
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)
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
The Soils That We Classify
For purposes of classification, the lower boundary of soil is arbitrarily set at 200 centimeters.
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
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
• Entisol • Inceptisol • Andisols • Spodosols • Mollisols • Alfisols • Ultisols • Oxisols • Aridisols • Vertisols • Histosols • Gelisols
Twelve Soil Orders
• Highest level of classification • Broadest group • 12 orders • Based on diagnostic horizons and climate
Soil Orders
Degree of weathering and soil development in the different soil orders
Global Distribution
Entisols – 18%
Inceptisols – 15%
Mollisols – 7%
Spodosols – 4%
Histosols – 1%
Soil Taxonomy
• 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
• 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?
Entisols – Six Suborders
Manchester Series
Hadley Series
TYPIC UDIFLUVENTS
TYPIC UDORTHENTS
• 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
Inceptisols – Seven Suborders
Sutton Series
AQUIC DYSTRUDEPTS TYPIC ENDOAQUEPTS
Scitico Series
• 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
Mollisols – Eight Suborders
TYPIC ENDOAQUOLLS
Menlo Series
TYPIC ARGIUDOLL
Sharpsburg Series
• 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
Spodosols – Five Suborders
AQUODS AQUODS
• histos “tissue”
• Organic soils derived from hydrophytic plants
• Low bulk density and very high water holding capacity
Histosols
Histosols – Five Suborders
Alfisols
• pedalfer • Naturally fertile soils • High base saturation • Clay-enriched subsoil horizon
Aridisols
• arid (Latin, dry) • Form in aridic soil moisture
regimes (dry) • Low OM • Little leaching
Ultisols
• -ultimate (Latin, last) • Old, highly weathered, high
degree of development • Low fertility
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)
Andisols
• ando (Japanese, black soil) • Form from P.M. of volcanic
origin • Young soils • Can have unusually low bulk
densities • High water holding capacity
• oxide (French, oxide) • Very highly weathered soils • Intense red or yellow color
(high in oxidized iron) • Low native fertility
Oxisols
Gelisols
• gelid (Greek, cold) • Young soils with little profile
development • High organic carbon content
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
Great Group
Family Sub Group
Sub Order
Order
Series
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
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