soils

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54 Terms

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regolith

loose material that covers bedrock (soil)

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sediment

loose fragments of rock/minerals from bedrock, precipitated crystals for water, shells

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weathering

physical + chemical breakdown of rock

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physical weathering

breaks intact rock into unconnected clasts

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jointing

rocks heat w/ burial, as they coll they contract + form cracks

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frost wedging

water freezes in cracks + pushes rock apart

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salt wedging

salt crystals grow in open pore spaces + push grains apart

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root wedging

roots grow in joints + push them apart

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abrasion

water, wind, glacial ice scratch at rock

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temp-induced exfolitation

peeling away of rock layers b/c of stresses induced by outer layers being a diff T

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weathering is climate dependent

↑ precip. ↑ chem. weathering

temp. ↑ phys. weathering

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chem. weathering

chem rxns. alter rocks + minerals @ surface conditions

  • b/c most primary minerals not thermodynamically stable at surface conditions

  • deliver nutrients from rocks to biosphere in soluble forms

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mineral susceptibility to chem weathering

less complicated mineral structures less susceptible to chem. weathering

  • ↑ complexity of structure ↑ chem. weathering

  • reverse of Bowen’s rxn series

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dissolution

water, especially when acidic, can chemically attack and break down the bonds within minerals, causing them to dissolve and move

  • soluble product flows away from rxn site → mass loss

  • carbonates + salts

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hydrolysis

water reacts w/ minerals to form secondary ones

  • breaks down silicate minerals

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oxidation

Fe-bearing minerals weakened by the addition of O2

  • breaks down mafic minerals

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hydration

introduction of water causes minerals to expand

  • produces many secondary minerals (clay)

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cation exchange

common in clays in phylosillicates

  • interlayer cations swap

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how to form CZ

break down of rock via weathering w/ biological enhancement

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in situ CZ

evolves from weathering + organismal colonization

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deposited CZ

weathered products transported + deposited before local CZ evolves

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critical zone (CZ)

near surface enviro. from tree tops to groundwater

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soil

containing living matter + capable of supporting plants

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pedogenesis

soil formation

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soil horizons

O, A, E, B, C, R

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O horizon

humus + organic matter

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A horizon

topsoil

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E horizon

eluviated layer → minerals + organics exit

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B horizon

subsoil → minerals stop leaching

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C horizon

parent material

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soil profile

vertical cross-section of soil formation

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inceptisols

barely developed soils

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spodosols

formed under coniferous forests

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histosols

high in organic matter (peat bogs)

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morlisols

grassland soils

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soil formation

CLORPT

  • climate

  • organisms

  • relief

  • parent material

  • time

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residual material

inorganic parent material developed from weatheirng

  • if climate is cool + dry residual material closely resembles parent material

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laterite

Fe oxide/clay layer rich in acid leached elements

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saprolite

retains most original rock texture but is oxidized in warm/humid enviros.

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peat-organic deposit

most extensive in cool climates where precip. > evap. + drainage is poor

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colluvial

  • gravity weathered

  • poorly sorted

  • well drained

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alluvial

  • water sorted

  • nutrient-rich deposits of eroded minerals

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glacial

  • till

  • poorly sorted

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aeolian

  • wind

  • well sorted

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relief

  • soils at bottom of slopes have better developed horizons

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climate

  • precip. + T

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precipitation (climate)

  • essential for chem. weathering

  • water promotes differentiation of diff soil horizons

  • water fuels percolation + leaching

  • increase weathering, increase CZ depth

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effective precip.

portion of total precip. available for plants

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temp. (climate)

increases rate of rxn

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soil biota + formation

  • macro + micro organisms enhance soil health

  • veg. cover reduces erosion rates

  • organic acids released from plants + decay of plant litter promotes accumulation of secondary minerals in B horizon

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development of a soil profile

positive feedback w/ accumulation

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development of a soil profile (phase 1)

  • no layering

  • early colonization by pioneers

  • organics begin to accumulate

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development of a soil profile (phase 2)

  • activity of small soil organisms

  • soluble ions move down → clay formation

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development of a soil profile (phase 3)

  • more/deeper organic matter → distinctive A horizon

  • B horizon deeper + differentiation into sub-horizons