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Advanced Soil GenesisAdvanced Soil Genesis
SWES 541SWES 541Spring 2006Spring 2006
Instructor: Craig RasmussenInstructor: Craig Rasmussen
“a natural body consisting of layers (horizons) of mineral and/or organic constituents of variable thickness, which differ from the parent material in their morphological, physical, chemical, and mineralogical properties and their biological characteristics” (Birkeland, 1999; Joffe, 1949).
What is a soil?
Useful terms in Pedology:• Profile: basic 2-D unit for observing the
vertical arrangement of soil components
A
B
C
• Horizon: soil material with properties formed largely by soil forming processes
Useful terms in Pedology:• Pedon (rhymes with “head on”)
3-D representation of the smallest volume of material that accurately represents the characteristics of each soil horizon
• At least 1 m2 lateral area - extends to “not soil”
A
B
C
O
A
E
B
A
B
C
Useful terms in Pedology:• The pedon should describe any cyclical variation that
occurs over a distance less than 10 m laterally
A
B
C
Pedon size
A
RBw Bw
A
Bss
< 10 m
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• Polypedon– Group of
pedons that comprise a soil landscape
Concepts of Soil Genesis
• Principle of Uniformitarianism– Geologic principle stating that processes occurring
today also occurred in the past• Simultaneous soil forming processes
– Many processes occurring at once• Pedogenic regimes
– Distinctive combination of climate, geology, and landforms produce distinctive soils
– Soils are a function of climate, organisms, relief, parent material, and time
Concepts of Soil Genesis• Soil succession
– Present day soils represent a continuum of changing soil properties and soil forming process
• Climate change and soil age– Most soils are no older than the Pleistocene epoch and
have experienced climate and vegetation regimes that differ from today
• Soils are clay factories– Weathering of primary minerals and formation of clay
minerals is hallmark of landscape stability and soil formation
• Complexity– Soil formation is result of many interacting factors that
occur over space and time
Soil MorphologySoil Morphology
Soil Morphology
• Color – Munsell System– Quantitative system that
measures visual differences in color characteristics
Soil Morphology - Color• 3 components of Munsell
– Hue – dominant spectral color – related to the wavelength
• Usually yellow and red hues (Y, YR, R)• Blues and greens for waterlogged or “gleyed” soils (B,
BG, G)– Value – darkness or lightness – function of the
amount of reflected light• High value – light colored• Low value – dark colored
– Chroma – purity of color – dilution by gray• High chroma – relatively pure color• Low chroma – less pure – diluted by gray
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Soil Morphology - Texture• Texture – proportions of sand, silt, and clay
– organic matter does not affect soil texture – but it does affect structure [next section]
• Measured on the < 2 mm fraction - the “fine earth fraction”– Sand 2.0 mm – 53 µm– Silt 53 µm – 2 µm– Clay < 2 µm
Soil Morphology - Texture
Soil Morphology - Structure• Structure
– Aggregation or physical organization of soil sand, silt, and clay particles into larger structures
– Structures have repeatable planes of weakness between individual aggregates
• Planes of weakness persist through time – at least one wetting and drying cycle
– Naturally occurring structures are called “peds” –chunks left after plowing are called “clods”
Soil Morphology - Structure
Soil TaxonomySoil Taxonomy Diagnostic Horizons• Morphology
– Infer soil forming processes– Used in classification
• Diagnostic horizons– Organize information and characteristics into
names– short-hand “soil language”– Base on quantitative data and measurable
properties• Set limits on some soil properties to allow
differentiation between different soil types
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Diagnostic Horizons• Surface diagnostic horizons – “epipedons”
– Must show evidence of pedogenesis• Epipedons are not the same as O or A horizons, they
can include illuvial B horizons
A1
A2
Bt
R
Epipedon
Subsurface Diagnostic
Diagnostic Horizons• 8 epipedons
Histic
Folistic
Melanic
Organic
Mollic
Umbric
Ochric
Anthropic
Plaggen
Mineral
Both/either organic or mineral
Histic epipedon
Soil formed from andesite parent material
Melanic epipedon
Mollic
Umbric
• Ochric epipedon– Does not meet the
requirements of other pedons• Color• Organic carbon content• Depth Argillic Horizon
with strong prismatic structure
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O
A
E
Bh
Bhs
BC
Spodic HorizonCambic Horizon
Calcic Horizon
Ochric Epipedon
Petrocalcic
Duripan
GelisolsHistosols
SpodosolsAndisols
Oxisols
VertisolsAridisols
UltisolsMollisols
AlfisolsInceptisols
Entisols
Keying out soil orders:
Start at the top – Gelisols, if soils do not meet the properties of a Gelisol, move on to the next order – Histosols, etc.
Entisols – soils without subsurface diagnostic horizon – “all other soils”
Gelisols• Greek gelid – “very cold”• Soils with permafrost (within 2 m of surface) and
gelic materials• Gelic materials
– Mineral or organic soil materials that show evidence of cryoturbation
• Active layer– Seasonal thaw layer– Freeze/thaw on an annual basis
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Patterned ground formed through freeze/thaw processes
Ice wedges and frost heaving
Forms microrelief
Outer rim – raised frost heaved ridge
Center of polygons –collapsed depressions
GelisolsFibristel located near center of polygonal patterned ground
Orthel
Histosols• Greek histos – “tissue”• Soils composed mainly of organic soil materials
(OSM) - do not have permafrost• OSM – saturated > 30 days and contain at least 12-
18% OC depending on clay content
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Spodosols• Acid forest soils• Subsurface accumulation of organo-metal
complexes
Andisols• Formed in volcanic ash• Dominated by SRO minerals
– Allophane, imogolite, ferrihydrite
Mollisol• “mollis” – latin for soft• Grassland and prairie soils• Deep, dark, friable, fertile surface horizons
– Mollic epipedons
Mollisol• Temperate grassland of mid-latitudes• Transition from drier desert regions and moister
forest regions• Commonly mixed with Entisols, Aridisols, and
Alfisols• Wide range of landscape ages• Holocene – post glaciation• Mollisols with argillic horizon – polygenetic – past
climate change• Forest-grassland ecotone
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ArgiustollsSouth Dakota and Texas
Pachic Argicryoll
Pachic Ultic Haploxeroll
Alfisols• Central concept –stable landscape positions and
subsurface zone of clay accumulation• Morphologically well developed
– Structure, horizonation, clay films• Five prerequisites
– Accumulation of layer lattice clays in subsurface – argillichorizon
– Relatively high BS%, >35% in lower part of argillic horizon– Contrasting soil horizons– Favorable soil moisture regimes– Relatively little accumulation of OM in mineral soil
horizons
Fragixeralf – northern Idaho
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Udic PaleustalfOld landscapes in Texas and OklahomaOak forests
Typic Hapludalf – northern MichiganFormed in glacial tillHardwood forest
Rhodoxeralf – northern CaliforniaFormed in basaltOak woodland
Ultisols• Strongly leached, acid, forest soil with relatively low
fertility with subsurface accumulation of clay • Few base cations in subsurface, BS% <35 in the
argillic horizon
Fine, kaolinitic, thermic Typic KanhapludultOne of the most common soils in Southeast USDerived from felsic igneous and metamorphic rocksBt layers may have ~70% clayDominated by kaolinite and HIV
Fine-loamy, kaolinitic, thermic Typic KandiudultCoastal plain of Southeast USFormed from loamy marine sediments
Fine, parasesquic, thermic Andic PalexerultFormed in andesitic parent materialsConifer forest of Sierra Nevada of CaliforniaKaolinite and gibbsite in surface horizonsHalloysite in subsurface horizons
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Oxisol• “Ox” - Oxide dominated• < 10% weatherable minerals in the 50-200 µm sand
fraction– Feldspare, micas, olivine, pyroxene, amphibole,
carbonates• Low CEC• Low activity clays
– Kaolinite, halloysite, sesquioxides (hematite, goethite, gibbsite)
Very-fine, kaolinitic, isohyperthermic RhodicEurustoxHawaii – formed in mafic materials (basalt)
Vertisol• “vert” – inverted• Dark, clayey soils that shrink and swell upon drying
and wetting• Distributed on every continent except Antarctica
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Udic Haplustert
Formed in valley fill derived from limestone and pyroclastic flows
Lacks distinct horizonation because of argillipedoturbation
Fine, smectitic, frigid Xeric Epiaquert
Flat inter-plateaus basin with silty lacustrineparent material
Silts weather to 2:1 clays
Surface cracking – due to swelling/shrinking 2:1 clays
Slickensides – pressure faces formed by shrink/swell processes
Aridisol• Arid systems• Occur in both cool temperate deserts
– Between 35° and 55°N• Warm deserts at lower latitudes
Argid and Cambid landscape
Typic Aquisalid Typic Haplocambid
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Inceptisol• Incipient soil formation• Some diagnostic features in addition to an ochric
epipedon or albic horizon
Humic EutrocryeptFormed in glacial till
Typic HaploxereptFormed in residual basalt
Entisol• Little to no soil development• Root domains are present – can support plant
growth
Typic UdifluventAlluvial parent material
Xeric TorripsammentFormed in eolian sand
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