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