Download Lecture 3, January 25, 2017 - EPSc 413 Introduction to Soil Science

Lecture 3: Introduction to Soil
Formation and Classification
Soil Formation
One Planet, Different Soils
Andisol
Spodosol
Aridisol
Mollisol
Alfisol
Ultisol
Major Soil Forming Processes
• Transformations:
– Mineral weathering
– Degradation of organic matter
• Translocations:
– Horizontal or vertical movement of
materials
– Key to forming soil horizons
• Additions:
– Organic matter from dead plants
– Wind-blown dust
• Losses:
– Erosion and leaching by water
– Decomposition of organic matter to CO2
Factors Controlling Soil Formation
1. Parent material: Soil precursor
2. Climate: Temperature and precipitation
3. Biota: Native vegetation, microbes, soil
animals, humans
4. Topography: Slope, aspect, landscape
position
5. Time: Duration of exposure of parent
material to weathering
Parent Material Review: Rocks and
Sediments
• Major classes of rocks:
– Igneous: Formed from cooling of molten rock
– Sedimentary: Formed from lithification of sediments upon
burial at Earth’s surface
– Metamorphic: Formed from alternation of igneous or
sedimentary rocks
• Key types of sediment materials:
– Sand & Silt: Typically composed of quartz [SiO2]
– Clay: Dominated by microscopic aluminosilicate minerals
– Loess: Wind-blown silt produced by glaciers; “rock flour”
Weathering: Turning Parent Material
into Soil
Primary and Secondary Minerals
Climate
• Climate determines the nature and intensity of
weathering of parent material
• Temperature and precipitation are the major climate
variables that affect soil formation
Global Atmospheric Circulation
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Variation in Soil Depth with Latitude
From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Influences of Biota
Topography
Time
(Granite in SE United States)
Interaction of Soil Forming Factors
Key Concepts in Soil Formation
• Five Major Soil Forming Factors: Parent
Material, Climate, Biota, Topography, Time
– The combined effects of these factors leads to the
soil found at a specific location
• Four Major Soil Forming Processes:
Transformations, Translocations, Additions,
Losses
Soil Classification
Variation in US Soils
Andisol
Spodosol
Aridisol
Mollisol
Alfisol
Ultisol
Concept of Individual Soils
• Soil varies across a landscape, but soils in specific locations display
specific characteristics
• The smallest unit to display a set of unique characteristics is called a
pedon
• A group of similar, closely associated pedons is called a polypedon
or soil individual
Soil Taxonomy
• A comprehensive classification system for soils
– Hierarchical structure
– System is based on soil properties that can be observed
and measured
• Diagnostic soil horizons
• Clay type and content
• Base saturation
• Provides a universal means of discussing soils
• Well-documented, publicly available method for fully
naming any soil on Earth
Official Soil Classification System
Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/class/taxonomy/
Official Instruction Manual
Available at: https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/survey/class/taxonomy/
Categories in Soil Taxonomy
12 Soil Orders
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Alfisol
Andisol
Aridisol
Entisol
Gelisol
Histosol
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Inceptisol
Mollisol
Oxisol
Spodosol
Ultisol
Vertisol
I AM A SUAVE HOG
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Inceptisol
Alfisol
Mollisol
Andisol
Spodosol
Ultisol
Aridisol
Vertisol
Entisol
Histosol
Oxisol
Gelisol
I GAVE US OMAHA
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Inceptisol
Gelisol
Alfisol
Vertisol
Entisol
Ultisol
Spodosol
Oxisol
Mollisol
Andisol
Histosol
Aridisol
Argillic Horizon
Alfisols
• Moderately leached soils
with a clay-rich B-horizon
• Typically found in forests
Andisols
• Dominated by short-rangeorder minerals
• Form in volcanic ash
Aridisols
• Dry and often saline soils
• Form in arid regions
Histic Epipedon
Entisols
• Little or no evidence of
pedogenesis
• Often sandy or shallow
Gelisols
• Contain permafrost
• Found in arctic and high
alpine regions
Histosols
• Dominantly organic soil
material, no permafrost
• Wetlands, bogs, marshes
Mollic Epipedon
Oxic Horizon
Inceptisols
• Soils with altered horizons
but no accumulations
• Young soils, still developing
Mollisols
• Dark surface layer rich in
base cations
• Found in grasslands
Oxisols
• Highly leached soils with
iron and aluminum oxides
• Found in tropical regions
Albic Horizon
Spodic Horizon
Argillic Horizon
Spodosols
• Accumulation of Fe/Al and
OM below leached horizon
• Found in coniferous forests
Ultisols
• Highly leached soils with a
clay-rich B-horizon
• Found in warm, humid
regions
Argillic Horizon
Vertisols
• High content of swelling
clay; deep cracks when dry
• Shrink-swell behavior
Soil Orders Vary with the Degree of Weathering
Hypothetical Soil Development
Sequence
Entisol
Inceptisol
Alfisol
Ultisol
Oxisol
Increasing Age, Increasing Soil Development, Increasing Weathering
Process of Assigning Soil Orders
Official Definitions More Complex
Lower Levels of Classification
• Suborder: Soils of an order having a unique property
(moisture or temperature regime)
• Great Group: Soils of a suborder having a type of
diagnostic horizon or other feature
• Subgroup: Soils of a great group sharing another
common feature
• Family: Soils of a subgroup sharing common soil
properties
• Series: Soils of a family sharing all major
characteristics, including depth of horizons
Order
Suborder
Great
Group
Subgroup
Family
Series
Soil Series
• All the soil individuals
in the world with a
common suit of soil
profile properties and
horizons are called a
soil series
– Generally unique to a
state; rarely global
– 25,000 soil series
named in US
MENFRO SERIES
The Menfro series consists of very deep, well drained,
moderately permeable soils formed in thick loess deposits on
upland ridgetops, backslopes and benches adjacent to the
Missouri and Mississippi Rivers and their major tributaries.
Slopes range from 2 to 60 percent. Mean annual temperature
is 56 degrees F, and mean annual precipitation is 36 inches.
TAXONOMIC CLASS: Fine-silty, mixed, superactive, mesic Typic
Hapludalfs
Taxonomic Class
• A soil’s taxonomic class is a descriptive term that
covers all of the basic features of the soil
• Example: Fine-silty, mixed, superactive, mesic Typic
Hapludalfs
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Fine silty texture in the upper mineral surface horizon
Mixture of clay types with high cation exchange capacity
Average soil temperature of 8 to 15°C
“Typic” means a standard Hapludalfs
Hapludalfs: A moist (-ud-) Alfisol (-alfs) with no other
special features (hapl-)
Key Concepts in Soil Classification
• A system has been created to objectively
classify soils
– This system relies of characteristics of the soil,
including diagnostic horizons and physical and
chemical properties
• There are 12 major soil orders: Know these!
• There are many lower levels of classification
– The soil series is the most important level for
understanding soils at a specific location