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Ch12
- 2. Soils
• Soil and Regolith
• Soil Forming Factors
• Soil Components
• Soil Properties
• Soil Chemistry
• Soil Profiles
• Pedogenic Regimes
2© 2011 Pearson Education, Inc.
- 4. Soil and Regolith
• Thin layer on top of Earth’s crust
(about 15 cm deep)
• Complex combination of minerals,
organisms, gases, liquids, etc.
• Nurtures life
• Fundamental interface between the
four spheres
• Regolith—inorganic material that
results from rock weathering
• Top layer of regolith is different in
amount of biochemical weathering
4© 2011 Pearson Education, Inc.
Figure 12-2
- 5. Soil and Regolith
• Surface of lithosphere
almost always represented
by soil
• Soil often recognized by
color
• Depth also important, but
is only obvious when
exposed through
excavation
5© 2011 Pearson Education, Inc.
Figure 12-3
- 6. Soil-Forming Factors
• Geologic factor
– Source of rock fragments
in soil is parent material
– Nature of parent material
influences soil
characteristics
– Young soils very reflective
of the rocks or sediments
from which they resulted
6© 2011 Pearson Education, Inc.
Image courtesy of www.albany.edu
- 7. Soil-Forming Factors
• Climatic factor
– Temperature and moisture
significant to soil formation
– Chemical and biological
processes accelerated by
high temperatures and
moisture
• Topographic factor
– Slope and drainage
significant (Figure 12-5)
– Lowering of top and bottom
of soil
7© 2011 Pearson Education, Inc.
Figure 12-5
- 8. Soil-Forming Factors
• Biological Factor
– All soils contain some
quantity of living
organisms
– Plant and animal
contributions to soil
development
– Pedoturbation
8© 2011 Pearson Education, Inc.
Figure 12-6
- 9. Soil-Forming Factors
• Biological Factor (cont.)
– Earthworms most important
• Innumerable tunnels, mixed by
water
• Crumbly surface
• Digestive actions increase
porosity and resist raindrops,
lessening erosion
• Casts excreted by earthworms
• Rearrange material in the soil
• Nitrification promoted
– Microorganisms—enhance
biological decay
9© 2011 Pearson Education, Inc.
Figure 12-7
- 10. Soil-Forming Factors
• Time Factor
– Soil processes very slow
– Length of time needed
depends on parent material
and environmental
characteristics
– Soil degradation or erosion
can occur in only a few years
– From human perception of
time, nonrenewable resource
10© 2011 Pearson Education, Inc.
Figure 12-8
- 11. Soil Components
• Inorganic materials
– Bulk of most soil is mineral
matter in the form of small
but macroscopic particles
– Half the volume of an
average soil is sand or silt
– Smallest particles in soil are
clay
– Clay has colloidal sized
particles
– Clay attracts cations
11© 2011 Pearson Education, Inc.
Figure 12-9
- 12. Soil Components
• Organic matter
– Small percent of soil volume
– Living organisms, dead organisms, decomposing or
completely decomposed organisms
– Rearrange and aerate soil and yield nutrients through
waste products
– Litter—leaves, twigs, other dead plant parts on soil
surface
– Humus—black gelatinous organic matter which
results from decomposed residues
12© 2011 Pearson Education, Inc.
- 13. Soil Components
• Soil air
– Half volume of average
soil is pore spaces
– Spaces provide interstices
among soil particles
– Pores half filled with air,
half with water
13© 2011 Pearson Education, Inc.
Figure 12-11
- 14. Soil Components
• Soil water
– Rainfall, snowfall,
groundwater through
capillary action
– Four types
– Gravitational water:
infiltration from above,
falls down via gravity
– Capillary water: moisture
which is held on soil
particle surfaces by
surface tension
14© 2011 Pearson Education, Inc.
Figure 12-13
- 15. Soil Components
• Soil water (cont.)
– Hygroscopic water: thin film of
moisture bound to soil particles
by adhesion
– Combined water: held in
chemical combination with soil
minerals
– Field capacity and wilting point
– Leaching: water carrying
nutrients down in soil
– Eluviation and illuviation
15© 2011 Pearson Education, Inc.
Figure 12-14
- 16. Soil Components
• Soil-water budget
– Amount of water added by
percolation and snowmelt
diminished by
evapotranspiration
– Soil-water balance
– In general, cool temperatures
mean less evapotranspiration
and water surplus, opposite for
warm temperatures
16© 2011 Pearson Education, Inc.
Figure 12-15
- 17. Soil Properties
• Color
– Soil color occasionally
describes nature and
capabilities of soil
– Stains on particle
surfaces
– Black/dark brown has
large humus content
– Red/yellow indicate high
iron oxide content
– Gray/blue indicate poor
drainage
17© 2011 Pearson Education, Inc.
Figure 12-16
- 18. Soil Properties
• Texture
– Feel for particle sizes
– Separates in size
classification groups
– Texture triangle
(Figure 12-18)
• Structure
– Particles aggregate into
peds
– Soil structure types
– Porosity and permeability
18© 2011 Pearson Education, Inc.
Figure 12-18
- 19. Soil Chemistry
• Colloids
– Particles smaller than
one micrometer in
diameter
– Inorganic consist of clay
– Organic consist of humus
– Both are chemically
active
– Have large water storage
capacity
– Attract ions
19© 2011 Pearson Education, Inc.
Figure 12-17
- 20. Soil Chemistry
• Cation exchange
– Attach to negatively charged
colloids
– Collodial complex
– Cation exchange capacity
(CEC)
– Clay and humus have high
CEC activity
• Alkalinity
– Acid versus base
– Alkaline solutions
20© 2011 Pearson Education, Inc.
Figure 12-20
- 21. Soil Profiles
• Soil development
expressed in two
dimensions—time and
depth
• Four processes that
deepen and age soil
– Addition
– Loss
– Translocation
– Transformation
21© 2011 Pearson Education, Inc.
Figure 12-21
- 22. Soil Profiles
• Horizons—distinct layers
with different
characteristics
• Vertical profile from
Earth’s surface through
soil layers—soil profile
• Six primary horizons
• Solum
• Variations on the profile
– Fossil horizon
– Hardpan
22© 2011 Pearson Education, Inc.
Figure 12-22
- 23. Pedogenic Regimes
• Five major soil forming
regimes
• Laterization
– Brick red-colored soil
– Requires significant
annual moisture surplus
– Rapid weathering,
dissolution, and
decomposition
– Little humus but adequate
nutrients
– Latosols
23© 2011 Pearson Education, Inc.
Figure 12-23
- 24. Pedogenic Regimes
• Podzolization
– Gray soil
– Limited vegetation nutrient requirements
– Acidic plant litter
– Effective leaching
– Podsols
• Gleization
– Muddy soil
– Poor drainage, acidic
– Gley soils
24© 2011 Pearson Education, Inc.
- 25. Pedogenic Regimes
• Calcification
– Semiarid climates with little leaching
– Little percolating water
– Different productivity depending on vegetation
• Salinization
– In semiarid areas, capillary action brings up
water
– Intense evaporation leaves behind salts on
surface
– White color to the soil
25© 2011 Pearson Education, Inc.
- 27. Soil Classification
• Soil classification scheme— Soil Taxonomy
– Generic classification, hierarchical system
– Highest level: soil order
– Diagnostic horizons
• Epipedon
• Subsurface
– Second level: Suborders
– Third level: great groups
• Scale of order and mapping of the soil
taxonomy
27© 2011 Pearson Education, Inc.
- 29. Global Distribution of Major Soils
• 12 major types
• Entisols
– Little profile
development
– Due to age or
chemical composition
• Inceptisols
– No diagnostic
horizons
– Lack of maturity
– Common in tundra
and floodplains
29© 2011 Pearson Education, Inc.
Figure 12-27 Figure 12-28
- 30. Global Distribution of Major Soils
• 12 major types
(cont.)
• Andisols
– Develop from
volcanic ash
– Young with dark
upper layers
• Gelisols
– Young soils that
develop slowly due to
cold conditions
– Found in arctic and
subarctic regions
30© 2011 Pearson Education, Inc.
Figure 12-29 Figure 12-30
- 31. Global Distribution of Major Soils
• 12 major types
(cont.)
• Histosols
– Organic, saturated
continuously
• Aridisols
– Soils of dry lands
– Distribution correlated
to desert climate
31© 2011 Pearson Education, Inc.
Figure 12-31
Figure 12-32
- 32. Global Distribution of Major Soils
• 12 major types
(cont.)
• Vertisols
– Little profile
development
– Due to age or
chemical composition
• Mollisols
– High in humus and
cations
– Soft when dry
– Found in midlatitudes
32© 2011 Pearson Education, Inc.
Figure 12-33 Figure 12-34
- 33. Global Distribution of Major Soils
• 12 major types (cont.)
• Alfisols
– Subsurface clay
horizon
– Medium to generous
supply of cations,
nutrients, and water
• Ultisols
– Weathered and
leached of nutrients
– Reddish color
– Alfisols degenerate
into Utilisols
33© 2011 Pearson Education, Inc.
Figure 12-35 Figure 12-36
- 34. Global Distribution of Major Soils
• 12 major types (cont.)
• Spodosols
– Illuvial dark layer
where organic matter
and other elements
accumulate
– Infertile
• Oxisols
– Most thoroughly
weathered of soils
– Vegetation is efficient
in cycling limited
nutrients
34© 2011 Pearson Education, Inc.
Figure 12-37 Figure 12-38
- 36. Summary
• Soil is a thin layer of organic and inorganic
material on top of Earth’s crust
• Regolith is inorganic material under soil
• There are five primary soil forming factors that
give rise to the global distribution of soils
• Soil primarily consists of organic and inorganic
material, soil water, and soil air
• Each soil has unique color, texture, and
structure
36© 2011 Pearson Education, Inc.
- 37. Summary
• Colloid-cation interaction is important in soil
chemistry
• Soil acidity is important in its ability to support
plant life
• Soil development is expressed in terms of
horizontal soil horizons
• There are five primary pedogenic regimes that
are used to understand the soil-forming
processes of different soil types
37© 2011 Pearson Education, Inc.
- 38. Summary
• The Soil Taxonomy allows different soil types to
be classified
• There are 12 primary soil types, each of which
has unique color, texture, and structure
• Ten of the 12 primary soil types are observed in
the contiguous United States
38© 2011 Pearson Education, Inc.
