What is a soil? Advanced Soil Genesis “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 SWES 541 mineralogical properties and their Spring 2006 biological characteristics” (Birkeland, Instructor: Craig Rasmussen 1999; Joffe, 1949).

Useful terms in Pedology: Useful terms in Pedology: • Pedon (rhymes with “head on”) • Profile: basic 2-D unit for observing the 3-D representation of the smallest volume of vertical arrangement of soil components material that accurately represents the characteristics of each soil horizon 2 A • Horizon: soil material • At least 1 m lateral area - extends to “not soil” with properties formed

B largely by soil forming A processes B C

C

Useful terms in Pedology: O • The pedon should describe any cyclical variation that occurs over a distance less than 10 m laterally A A < 10 m

E B A

Bss A B C B Pedon size

C A

BwR Bw

1 Concepts of Soil Genesis

• Polypedon • Principle of Uniformitarianism –Group of – Geologic principle stating that processes occurring pedons that today also occurred in the past comprise a • Simultaneous soil forming processes soil landscape – 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 Morphology • 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 Morphology - Color Soil Morphology • 3 components of Munsell –Hue– dominant spectral color – related to the • Color – Munsell System wavelength – Quantitative system that • Usually yellow and red hues (Y, YR, R) measures visual • Blues and greens for waterlogged or “gleyed” soils (B, differences in color BG, G) characteristics – 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

2 Soil Morphology - Texture 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 - Structure 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 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

3 Diagnostic Horizons Diagnostic Horizons • Surface diagnostic horizons – “epipedons” • 8 epipedons – Must show evidence of pedogenesis

• Epipedons are not the same as O or A horizons, they Histic can include illuvial B horizons Organic Folistic Melanic Both/either organic or mineral Mollic A1 Epipedon Umbric A2 Mineral Ochric Bt Subsurface Anthropic Diagnostic Plaggen R

Histic epipedon Mollic

Melanic epipedon Umbric

Soil formed from andesite parent material

• Ochric epipedon – Does not meet the requirements of other pedons •Color • Organic carbon content •Depth Argillic Horizon with strong prismatic structure

4 O A Ochric Epipedon E

Bh Cambic Horizon Spodic Horizon Bhs

Calcic Horizon BC

Petrocalcic

Duripan

Gelisols Gelisols Histosols • Greek gelid – “very cold” Spodosols • Soils with permafrost (within 2 m of surface) and Andisols gelic materials Oxisols • Gelic materials Vertisols – Mineral or organic soil materials that show evidence of Aridisols cryoturbation Keying out soil orders: Ultisols • Active layer Start at the top – Gelisols, Mollisols – Seasonal thaw layer if soils do not meet the Alfisols properties of a Gelisol, – Freeze/thaw on an annual basis move on to the next order Inceptisols – Histosols, etc. Entisols Entisols – soils without subsurface diagnostic horizon – “all other soils”

5 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

Gelisols Histosols • Greek histos – “tissue” Fibristel located near center of polygonal • Soils composed mainly of organic soil materials patterned ground (OSM) - do not have permafrost • OSM – saturated > 30 days and contain at least 12- 18% OC depending on clay content

Orthel

6 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 Mollisol • Temperate grassland of mid-latitudes • Grassland and prairie soils • Transition from drier desert regions and moister • Deep, dark, friable, fertile surface horizons forest regions – Mollic epipedons • 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

7 Pachic Argicryoll

Argiustolls South Dakota and Texas

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 – argillic horizon

Pachic Ultic Haploxeroll – 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

8 Rhodoxeralf – northern California Formed in basalt Oak woodland Udic Paleustalf Typic Hapludalf – northern Michigan Old landscapes in Texas and Oklahoma Formed in glacial till Oak forests Hardwood forest

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 Kanhapludult One of the most common soils in Southeast US Derived from felsic igneous and metamorphic rocks Bt layers may have ~70% clay Dominated by kaolinite and HIV

Fine-loamy, kaolinitic, thermic Typic Kandiudult Coastal plain of Southeast US Fine, parasesquic, thermic Andic Palexerult Formed from loamy marine sediments Formed in andesitic parent materials Conifer forest of Sierra Nevada of California Kaolinite and gibbsite in surface horizons Halloysite in subsurface horizons

9 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)

Vertisol • “vert” – inverted • Dark, clayey soils that shrink and swell upon drying and wetting • Distributed on every continent except Antarctica

Very-fine, kaolinitic, isohyperthermic Rhodic Eurustox Hawaii – formed in mafic materials (basalt)

10 Udic Haplustert Formed in valley fill derived from limestone and pyroclastic flows Lacks distinct horizonation because of argillipedoturbation

Surface cracking – due to swelling/shrinking 2:1 clays

Slickensides – pressure Fine, smectitic, frigid Xeric Epiaquert faces formed by Flat inter-plateaus basin with silty lacustrine shrink/swell processes parent material Silts weather to 2:1 clays

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

11 Inceptisol • Incipient soil formation • Some diagnostic features in addition to an ochric epipedon or albic horizon

Humic Eutrocryept Formed in glacial till

Entisol • Little to no soil development • Root domains are present – can support plant growth

Typic Haploxerept Formed in residual basalt

Typic Udifluvent Xeric Torripsamment Alluvial parent material Formed in eolian sand

12 Typic Xeropsamment Formed in residual granite Complicated landscape-soil relationships

Possible Soil Development Pathways/Sequences

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