Soils and Water, Spring 2008
Soil Classification: Outline
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Introduction
Soil moisture regimes
Soil temperature regimes
The Soil Taxonomy
– Diagnostic horizons
– soil orders
Also see this article in the New Yor Times:
http://www.nytimes.com/2006/08/11/opinion/11fri4.html?_r=1&oref=slogin
Why Classify?
• Organize knowledge in an attempt to find
order in soils.
• Help dealing with large number of
objects/concepts/ideas.
• Enable the collection of new knowledge.
• Facilitate the optimal use of soils (applied
objective).
• Understand the distribution and functioning
of soils (basic objective)
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Precursors of Present Classification Systems
• The development of soil classification
systems were based on the concepts of
Zonal, Intrazonal, and Azonal soils.
– Zonal soils reflect the climate and
vegetation under which they developed.
– Intrazonal soils have properties dominated
by some local factor (e.g., drainage, salts).
– Azonal soils have properties corresponding
to the parent material (young or highly
eroded soils).
Relationship Between Annual
Precipitation and C.E.C. for Central USA
Modified from Jenny and Leonard (1934, Soil Sci. 38: 363-381)
C.E.C.
cmolc/kg
Exchangeable
bases
Exchangeable H
10
20
30
40
Annual precipitation, inches
Relationship Between N and Rainfall
After Jenny and Leonard (1934, Soil Sci. 38: 363-381)
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Lecture 10, Soil Classification
medium-textured soils
soil sampled from 0-25 cm
sampled along a isotherm (11°C)
vegetation: mostly pastures
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Soils and Water, Spring 2008
Relationship Between Organic Carbon and Clay
Nichols (1984, Soil Sci. Soc. Am. J. 48: 1382-1384
• Sixty-five pedons from Oklahoma, Texas, and Eastern New Mexico.
• % OC could be predicted from % clay and mean annual precipitation (R2=0.90).
• %OC increase with an increase of clay content and precipitation.
Depth to Calcium Carbonate Accumulation as
a Function of Precipitation
After Jenny and Leonard (1934, Soil Sci. 38:
363-381)
Depth = 2.5 (R-12) inches
Soil Moisture Regimes
• Aquic: soils with a perched or real
groundwater table (gley phenomena).
• Udic: soils mostly moist. There may be a
short dry season.
• Ustic: soils that are moist for more than half a
year but have a distinct dry season.
• Xeric: soils that are moist for more than half a
year but have a dry summer.
• Aridic: soils that are dry for more than half a
year and never moist for long periods.
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Soil Moisture Regimes
temperature
PE
Udic
precipitation
Ustic
Soil Moisture Regimes
temperature
PE
Xeric
precipitation
Aridic
Relationship Between N and Temperature
After Jenny (1928, J. Amer. Soc. Agronomy 20: 900-921)
• Sampled along a North-South transect of the U.S.
Great Plain Region.
• Soils developed under prairie grasses.
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Temperature Effect on Amount and Type
of Clay Minerals
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Sampled from NJ to GA.
Clay content increases with increasing temperatures.
The ratio silica/alumina decreases from 4 to 2 for the sequence NJ-GA.
Soil Temperature Regimes
• Air and soil temperatures are related. Below depths
of 20 m (Alaska), 15 m (mid-latitude), and 10 m
(tropics), soil temperature and mean air temperature
are similar:
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Soil Temperature Regimes
• Soil temperature regimes are based on mean
annual, mean summer, and mean winter soil
temperatures, measured at 50 cm.
• There are six soil temperature regimes. Ordered
from lower to higher mean annual temperature:
Pergelic, Cryic=Frigid, Mesic, Thermic, and
Hyperthermic.
• Soil temperature regimes are typically used to
classify at the lower levels of the Soil Taxonomy.
Types of Soil Classification
Systems
• Taxonomic systems: based on the natural
characteristics of soils, e.g., texture, color,
C.E.C., soil temperature and moisture.
– Soils are separated into multiple categories
(levels) and within each level they are separated
into taxa (classes).
• Interpretive systems:based on rearrangement
of taxonomic systems for particular uses
(e.g., septic tanks, houses with basement,
road and streets).
Soil Taxonomy
• Soil Taxonomy was developed in the USA for making
and interpreting soil surveys mainly for agricultural
purposes.
• http://soils.usda.gov/technical/classification/tax_keys/
contains the “Keys to Soil Taxonomy” (2006).
• The Soil Taxonomy has six levels, each with a variable
number of classes:
Order
Suborder
12
63
Great group
~250
Subgroup
~1,400
Family
Series
~8,000
~19,000
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Soil Orders
See http://soils.ag.uidaho.edu/soilorders/
Name
Formative
element
Alfisols
Andisols
Aridisols
Entisols
Gelisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
alf
and
id
ent
el
ist
ept
oll
ox
od
ult
ert
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Soil Type and Population
Name
Alfisols
Andisols
Aridisols
Entisols
Gelisols
Histosols
Inceptisols
Mollisols
Oxisols
Spodosols
Ultisols
Vertisols
%Global
Population
(1990)
1700
Population (millions)
15.9
1.6
5.1
14.4
0.4
0.5
25.7
5.9
3.5
1.5
16.7
5.3
142.7
11.1
34.3
129.6
11.4
4.1
291.1
32.1
25.1
17.6
119.5
40.2
% Increase
1990
918.3
93.3
292.0
830.8
22.5
31.4
1480.4
342.2
199.2
86.8
959.4
307.2
85
88
88
84
50
87
80
91
87
80
87
87
Soil Type and People
Population Density (persons/km2)
140
Alfisols
120
Andisols
Aridisols
100
Entisols
Gelisols
80
Histosols
Inceptisols
60
Population density in NJ
Mollisols
Oxisols
40
Spodosols
Ultisols
20
0
1650
Vertisols
1700
1750
1800
1850
1900
1950
2000
Year
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Pressure on the Soil Resource
Population by Soil Order, % of total
30
Andisols
Inceptisols
25
20
Ultisols
15
5
Entisols
Alfisols
10
Histosols
Mollisols
Aridisols
Vertisols
Gelisols
0
0
Spodosols
5
10
15
20
25
30
Are a by Soil Orde r, % of tota l
Oxisols
Other Soil Classification Systems
• Highly man-influenced soils. Examples of
materials of interest include:
– Garbic: organic wastes of human activities.
– Spolic: inorganic material moved during
surface mining, highway construction, etc.
– Scalped land surfaces: freshly exposed
materials.
– Dredge materials: from waterways.
– Old garden/farm soils.
Aridisols
• Aridisols are soils of arid regions that exhibit at least a minimal amount of
subsurface horizon development. They are characterized by being dry most of
the year.
• Aridisols contain subsurface horizons in which clays, calcium carbonate, silica,
salts, and/or gypsum have accumulated.
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Mollisols
• Mollisols are the soils of grassland ecosystems. They are characterized by a
thick, dark surface horizon ( mollic epipedon) and high base saturation.
• Mollisols primarily occur in the middle latitudes and are among some of the
most important and productive agricultural soils in the world.
Alfisols
• Alfisols are well developed forest soils with a subsurface horizon in which
clays have accumulated.
• Alfisols are mostly found in temperate humid and subhumid regions of the
world.
• Alfisols are very productive soils for agriculture and silviculture.
Ultisols
• Ultisols are acid soils formed under forest vegetation. They are
found in humid temperate and tropical areas of the world, typically
on old, stable landscapes.
• Ultisols exhibit very strong horizon development and have a
subsurface horizon in which clays have accumulated.
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Spodosols
• Spodosols often occur under coniferous forest in cool, moist climates. Usually of
coarse texture.
• Spodosols are acid soils characterized by a subsurface accumulation of humus and
Al & Fe oxides.
• Spodosols typically have a light-colored E horizon overlying a reddish-brown
spodic horizon.
Oxisols
• Oxisols are very highly weathered soils that are found primarily in the inter
tropical regions of the world.
• These soils contain few weatherable minerals and are often rich in Fe and Al
oxide minerals. Extreme low levels of native fertility.
• Very productive soils with inputs of lime and fertilizers.
Inceptisols
• Inceptisols are soils that exhibit minimal horizon development and have
diagnostic horizons that form rapidly, e.g.cambic horizon.
• Inceptisols occur under a wide range of environmental settings (e.g., steep
slopes, young geomorphic surfaces, and on resistant parent materials).
Lecture 10, Soil Classification
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Entisols
• Entisols are soils developed in unconsolidated parent material with usually no
genetic horizons except an A horizon.
• All soils that do not fit into one of the other eleven orders are Entisols.
• The result is that the Entisol order is characterized by great diversity.
Vertisols
• Vertisols are clay-rich soils that shrink and swell with changes in moisture
content (usually located in areas with a seasonal dry period).
• Because of the shrink/swell activity of these soils, they often do not have
distinct, well-developed horizons.
Andisols
• Andisols are soils that have formed in volcanic ash or other volcanic ejecta.
• As a result, Andisols possess unique chemical and physical properties:
– very high water-holding capacity
– ability to 'fix' (and make unavailable to plants) large quantities of
phosphorus.
Lecture 10, Soil Classification
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Soils and Water, Spring 2008
Histosols
• Soils composed mainly of organic materials. They contain at least 20-30%
organic matter by weight .
• Histosols typically form in settings where poor drainage inhibits the
decomposition of plant and animal remains, allowing these organic materials to
accumulate over time. Histosols are often referred to as peats and mucks.
Lecture 10, Soil Classification
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