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Today
• Models of soil formation
• Soil taxonomy– Horizon descriptions
(1)Parent materials + 4 otherfactors of soil formation
• Climate• Topography• Time• Biota
Can be interrelated and interact
Climate
• Moisture– Quantity– Intensity– distribution
• Temperature
Role of water
• Essential for chemical weathering• Transport materials through horizons• Determine type of plant cover
Rainfall: Intensity andduration
Effective precipitation• Water has to go
into the soil– Infiltration - not
overland flow– Rainfall
exceedsevaporation
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Temperature
• Rate ofbiologicalactivityRate ofchemicalreactions
Organic Matter Accumulation
Time
Structural Development
CaCO3Accumulation
CaSO4Accumulation
Unweathered Loess
Soil Development Time Line
Effect of Time
TopographySame parent material, same
climate
Toposequence
Excessivelydrained
Well drained
Poorly drained
Very poorlydrained
Biota - plantsType of plant :
Grass• Dense root system• Above ground
foliage will dieannually
• Grows wheremoisture < trees
• Thick A horizon
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Soil Profile
A - mollic horizon
Tree - deciduous• Leaves high in cations (Ca, Mg, Fe..)• Annual leaf fall
• Well developed A horizon–Decomposed OM–High nutrients
Deciduous -
• Larger A horizon• More cations• Less acidic• More clay
translocation
Tree - coniferous• Leaves low in cations
– Acidic
• Little leaf fall
• Thin A horizon–Oi - slightly decomposed
• E horizon–eluviation
Coniferous - Acidic
Thin A
E - eluviation washing out
B - illuviationWashing in
Animals
• Pedoturbation– Animal burrows effectively mix soil from
different horizons
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Animals
• Earthworms– Soil mixing– Soil structure– Soil aeration
Animals
• Humans–Agricultural
practices–Accelerate soil
loss–Accelerate soil
formation
Start with a pile of rocks
• Physical and chemical weathering– Breakdown particles– Change mineral structure
Plant starts growing
• Organic matter is added to soilsurface
• Root exudates further break downminerals
First stage ofsoil formation• Increased organic
matter in A horizon
When a plant starts growing
• Organic matter from plants is anutrient source for other organisms
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Next stage will involvedevelopment of a B horizon
• Cations that have been solubilizedby plant exudates, microbialactivity,and acidification will startmoving lower into the soil
• Secondary clays have beenformed in A and are also movingdown the profile
Beginning of aB horizon
• Dark A horizon– Organic matter– Clays
• Something below theA that looks differentfrom the parentmaterial
• Parent material
Next stage will depend onclimate and parent materials• Rainfall and temperature will
determine nature of vegetation• Resistance of parent material to
weathering will determine rate andtype of clay formation, acidity
• Rainfall (effective precipitation) willdetermine extent of translocation
Temperature andPrecipitation
Arctic Temperate Tropical
Arid region
Humid region
United States
BedrockDep
th
60 m Equator
Progression in soildevelopment
• High rainfall• Moderate rainfall
• Low rainfall
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Organic Matter Accumulation
Time
Structural Development
CaCO3Accumulation
CaSO4Accumulation
Unweathered Loess
Soil Development Time Line
Arid environment
• Less organic matter accumulation insurface
• Less translocation of clays, cations• Cations never leave the profile• Accumulation of salts (cations)
Aridenvironment
• Organicaccumulationless pronounced
• White in B =salts, carbonates
• Clays in B
General overview
Humid, temperate climate
A B
Humid, temperate climate
C D
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Soil Taxonomy
• Language and terminology used toidentify, characterize, and classifydifferent types of soils
• Using this, you can understandorigins of soil as well as productivityof soils
Description of an individualsoil:
• Pedon - smallest3D unit thatdisplayscharacteristic ofan individual soil
Tools to study soildevelopment
• Horizons
–O- organic horizon,consists of OM invarious stages ofdecomposition
–Occurs in forests
O Horizon
• Oi - slightly decomposed– Source of material easily recognized
• Oe - moderately decomposed– some recognizable components
• Oa - highly decomposed– primarily organic, not recognizable
Horizons• A- surface horizon,
dark in color due toOM accumulation,coarser in texturedue totranslocation offiner clays
Horizons• E- forms below
O,A horizons– horizon of loss
(clays and cations)from eluviation
–Generally lighter incolor than A or Bhorizon
–Acidic parentmaterial
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• B- forms belowO,A,E horizons– horizon of
accumulation(clays and cations)from illuviation
–sufficiently alteredso it is no longerrecognizable asparent material
Horizons
• C- At the bottom– insufficiently
changed fromparent material toqualify as part ofthe solum
Words to further describeeach horizon
• These descriptives are used toclassify a soil
• Different tools to delve into particularcharacteristics of the pedon underinvestigation
Epipedon - to characterize Ahorizon
• Mollic - >25 cm thick, >0.6% C,soft when dry, high cation
• Umbric - same as above, butcations (%base saturation)lower
• Melanic - high OM, fluffy, onvolcanic soils
Epipedon - to characterize Ahorizon
• Histic - accumulation of OMoverlaying a mineral soil, wet
• Ochric - OM but not enough toqualify for other designations
Ochric
Histic
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Melanic Mollic Salic• Surface accumulation of soluble
salts, usually occurs in dry climateswhere
moisture in subsoil is wicked to the surface
Beginningsof a B
horizon• Cambic - some
indications ofchange, usually acolor difference
Cambiare to change• Albic - more
pronounced, lightcolor with clay, Feand Al removed
SubsurfaceHorizons
• Argillic (Bt)– High
accumulation ofclays in the Bhorizon
– See them ascoatings orskins
Clay skins
• Argillia (clay) - skins on the surfaceof particles in the B horizon
Natric (Btn)
• Clay skins withexchangeable Na> 15%, columnaror prismatic soilstructure
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Prismatic
• Common in Bhorizons in aridareas (whereyou will havehigh saltaccumulation)
SubsurfaceHorizons
• Kandicaccumulation ofFe and Al oxideclays in the Bhorizon
SubsurfaceHorizons
• Oxic (Bo)- veryhigh Fe and Al,few weatherableminerals, > 30cm deep
SubsurfaceHorizons
• Spodic (Bh, Bs)-illuvial horizon,below E, high inOM, Al and Feoxides
• Found inprimarily inacidic soils
SubsurfaceHorizons
• Calcic (k)-accumulation ofcarbonates, canappear as whitenodules
Cemented horizons• Duripan (qm) - Hardpan, strongly
cemented by silica• Fragipan (x) - Brittle pan, dense• Petrocalcic (km) Cemented calcic
horizon• Petrogypsic (ym) Cemented gypsic
horizon• Placic (sm) Cemented with Fe, Mn,
OM
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Petrocalcic (km)Cemented calcic
horizon
Petrogypsic (ym)Cemented gypsichorizon
Petra - rock in greekCemented horizons:
• Impermeable orslowly permeable bywater
• Barrier to root growth
You are in a forest
• There are pine trees all around• It’s raining• The ground is level• The surface soil has a lot of leaf litter
and feels sandy
• What would you expect the soilto look like?
You are on the prairie• The areas all around you are covered
with grass• It’s sunny and dry• The soil where you are standing has
much less grass and a white crust onthe surface
• What would you expect the soilto look like?
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Moistureregimes
• Aquic• Udic• Ustic• Aridic• Xeric
Temperature
• Classifications based on soil T at50 cm depth
• Consideration differencesbetween summer and winteraveraged
Temperature
• Include terms like:Frigid, Mesic, HyperthermicCold Moderate HotISO (greek meaning same)Isofrigid, Isomesic, Isothermic