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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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Typic XeropsammentFormed in residual granite

Complicated landscape-soil relationships

Possible Soil Development Pathways/Sequences