Download Lecture 19, April 5, 2017 - EPSc 413 Introduction to Soil Science

Lecture 19: Soil Formation
Major Soil Forming Processes
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
Pedogenesis
• Pedogenesis: Soil forming processes (additions, losses,
translocations, transformations) acting to create and
transform soils
• The soil existing in a location at any given time reflects a
balance between horizon creation and destruction:
– Horizonation: The formation and/or differentiation of parent
material into discrete soil horizons
– Haploidization: The destruction or blending of existing soil
horizons
Horizonation
New horizons are forming:
Change in soil type
Static
Pedogenesis
Haploidization
Horizons are stable:
No change in soil type
Horizons are disappearing:
Change in soil type
Soil Forming Processes that Transform
or Translocate Soil Material
• Pedoturbation: Mechanical mixing of soil
• Depletion and Accumulation of Soil Material:
– Eluvation: Movement of material out of a soil horizon
– Illuvation: Movement of material into a soil horizon,
usually from horizons above
• Clay-specific Processes:
– Lessivage: Eluvation of clays from A and E horizons and
illuvation of clays into a B horizon, forming a Bt
– Argillation: Formation of clays in place through
weathering
See: Soils: Genesis and Geomorphology by Schaetzl and Anderson, 2005, Cambridge University Press
Specific Soil Forming Processes
• Podzolation: Processes associated with spodosol
formation; Migration of both OM and Al (±Fe) to the
B horizon, leaving an E horizon
• Andisolization: Process of ash weathering, binding of
OM, and formation of short-ranged order minerals
• Calcification: Formation of secondary carbonates
• Salinization: Accumulation of soluble salts
– Solonization: Illuvation of salts into B, forming Btn
• Melanization: Darkening of A by addition of OM
• Gleization: Process of gleying; iron reduction
See: Soils: Genesis and Geomorphology by Schaetzl and Anderson, 2005, Cambridge University Press
Key Concepts in Soil Forming Processes
• Pedogenesis is the formation and evolution of
soil through the action of soil forming
processes
– This reflects a balance between horizonation and
haploidization
• Specific soil forming processes involve
primarily the translocation or transformation
of material in a soil
– Individual processes may promote horizon
formation or destruction
Controls on Soil Formation
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
Interaction of Soil Forming Factors
Parent Material
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 alteration 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”
Origin of Parent Material
Geographic Variation of Parent Material
Colored regions contain substantial residual parent material
Residual Parent Material
• Soil forms in place
by weathering of
local bedrock
• Parent rock can be
found at depth
• Soil chemistry and
mineralogy strongly
controlled by
composition of
original bedrock
Saprolite
• Residual parent
material that retains
some semblance of the
original rock structure
• Typically only seen for
granitic igneous and
metamorphic rocks
• Often found in areas of
intense weathering
Colluvial Parent Material
• Transported downslope by erosion (e.g., a
landslide); formed from local parent material
Alluvial Parent Material on Floodplains
Ap
C
• Floodplain deposits are often layered
• Also often contains coarse material in former
channels
Outwash
Till
Glacial-Derived
Parent Material
Soils formed on Aeolian Deposits
Loess
• Windblown silt deposits, typically found in areas near
glaciers or draining glacial meltwater
• Becomes productive farmland after soil development
• Highly susceptible to erosion
From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Organic Deposits
• Accumulation of organic
matter in wetlands leads
to organic soil formation
• Forms in bogs, fens,
swamps, and marshes
– Often called Peat
Key Concepts in Parent Material
• Parent material shows geographic variations
across a country
• Multiple origins for parent material
– Residual (formed in place from rock weathering)
versus transported
• Common transported parent materials include
colluviums, alluvium, glacial material, and
aeolian deposits (sand, dust, and loess)
Climate
Biomes of N.
America
Soil Orders Vary with Climate
Global Atmospheric Circulation
Dry
Wet
Dry
Wet
Dry
Wet
Dry
Variation in Weathering Depth with Latitude
From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Aridisol-Mollisol-Alfisol Climosequence
Key Concepts in Climate
• Climate affects soil formation by controlling
the amount of rainfall and the temperature
– This is reflected in the dominant vegetative cover
– Different soil types dominate different climate
zones, but there are always exceptions
• The deepest weathering occurs in equatorial
and temperate regions because that is where
rainfall is concentrated
• Regions often display clear climosequences
Biota
Islands of Fertility in Arid Environments
Pedoturbation by Animals
Influences of Biota:
Organic Matter Accumulation
OM deposited
by roots in A
horizon
OM deposited
as leaves on
the soil
surface
Influences of Biota:
Cation Cycling
• Conifer needles
are low in Ca2+
• Little cation
recycling occurs
• Organic matter on
soil surface
becomes acidic
with low base
saturation
• Lower uptake of
Ca2+ from deep in
the soil may lead
to slower
weathering
• Deciduous leaves
contain
substantial Ca2+
• This is recycled
into the surface
soil, reducing acid
saturation
• Deciduous trees
effectively leach
Ca2+ from
minerals,
increasing
weathering lower
in the soil profile
Spodosol
Alfisol
Difference in Base
Cation Cycling of
Vegetation Affects
Soil Formation
Mollisol
From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Key Concepts in Biota
• Biota affect soil formation by:
– Localizing OM and soil development
– Mixing the soil
– Inputting OM and organic acids and removing
water via transpiration
• Trees cycle cations through the soil
• Cation availability affected by ability of plants
to cycle base cations
Topography
Topography
Interaction of Topography and Parent
Material
• Residual, colluvial, and alluvial parent
materials occupy distinct landscape positions
Interaction of Topography and Parent Material
Typical Soil Association
in the Appalachian
Mtns., USA
Toposequence in Volcanic Terranes
From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Time
Approximate Time to Form Soil
Features
General formation curves compiled from studies of chronosequences
• The time required to form diagnostic horizons
and their associated soil orders varies from 10
to 1,000,000 years
From: Schaetzl and Anderson (2005) Soils: Genesis and Geomorphology
Soil Development on Granite Material
in a Warm, Humid Climate
Soil Development on Calcareous Loess
in a Warm, Subhumid Climate
Key Concepts in Topography and Time
• Topography acts as a large-scale control that
modifies the other soil forming factors
– Affects rainfall and temperature, which affects the biota
present and the extent of weathering
• Soil development is fundamentally a temporal
process
– The more time the soil forming factors have to act, the
greater the soil development
• Clear time-series progressions of soil development
are seen in specific climates
Weathering: Making Soil from
Parent Material
Weathering: Turning Parent Material
into Soil
Physical and Biogeochemical
Weathering
A combination of physical and
chemical weathering causes
exfoliation
Chemical weathering (mostly
oxidation) produces concentric
bands in this cobble
Relative Importance of Chemical and
Physical Weathering depends on Climate
Mollisols
Alfisols
Ultisols
Oxisols
Aridisols
Physical Weathering
Expansion as water turns to ice
fractured this rock
Sediment-laden water eroded this
sandstone
• Breakdown of rocks into smaller particles without a
change in composition
• Caused by wind, water, and ice
Biogeochemical Weathering
• Chemical breakdown of rock-forming minerals
– Produces solutes and secondary minerals
– Driven by water, oxygen, and biological chemicals
Primary and Secondary Minerals
Biogeochemical Weathering Reactions
• Congruent Dissolution:
Mg2SiO4,forsterite + 4H+ = 2Mg2+ + H4SiO4o
• Incongruent Dissolution:
2NaAlSi3O8,albite + 9H2O + 2H+ =
Al2Si2O5(OH)4,kaolinite + 2Na+ + 4H4SiO4o
• Oxidation-Reduction:
4Fe2+ + 6H2O + O2 ↔ 4FeOOHgoethite + 8H+
• Complexation/Chelation:
2H2C2O4 + NaAlSi3O8,albite + 4H2O =
Al(C2O4)+ + Na+ + C2O42- + 3H4SiO4o
Weathering Pathways
Weathering Changes Composition
Granite Gneiss and the Soil Formed Above It
Weathering Alters Mineralogy
Key Concepts in Weathering
• Weathering occurs through both physical and
biogeochemical processes
– These physically breakdown parent material into smaller
pieces and transforms it into new minerals
• Weathering change the composition and mineralogy
of a system
• Clay mineralogy reflect the extent of weathering and
the climate
• Soil orders vary in the extent of weathering that has
occurred
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Exam 2 Results
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