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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