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Soil-Forming FactorsESS 210

Chapter 2pages 31–74

What should you know?

• Weathering processes - physical and chemical

• The five soil forming factors• Types of soil parent materials• Types of rocks and minerals• Impacts of parent material, climate,

organisms, topography, and time on soil formation

Minerals

• Homogeneous, inorganic compounds, with definite chemical formula

• Primary minerals– Formed as molten lava cools and solidifies– Not chemically altered by weathering

processes• Secondary minerals

– Recrystallization and/or alteration products of primary minerals

Primary Minerals• Light colored aluminosilicate minerals

– Quartz [SiO2]: most common, weather very slowly, sand size

– Feldspars: sand size, weather to soil clays• K-feldspars – KAlSi3O8• Plagioclase feldspars:

– Albite – NaAlSi3O8– Anorthite – CaAl2Si2O8

– Muscovite mica – KAl3Si3O10(OH)2• A parent of soil clay minerals: weathers to soil clay

minerals• Thin, translucent sheets (isinglass)

Primary Minerals• Dark colored, ferro-magnesium minerals

– Biotite mica – KAl(Mg,Fe)3Si3O10(OH)2• Thin dark sheets• Weathers to soil clay minerals

– Hornblende – NaCa2Mg5Fe2AlSi7O22(OH)– Diopside – CaMgSi2O6

• Hornblende and diopside weather to soil clay minerals

– Olivine – (Mg,Fe,Mn)2SiO4

• Ferro-magnesium minerals weather more rapidly than aluminosilicate minerals

Secondary minerals

• Al and Fe (metal) oxides and hydroxides (sesquioxides)– Goethite – FeOOH– Hematite – Fe2O3

– Gibbsite – Al(OH)3

– Very stable soil minerals – dominate in OLD soils• Aluminosilicate clay minerals – several types,

common, and chemically complex• Salts: calcite [CaCO3], gypsum [CaSO4•2H2O]

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Rocks

• Mixtures of minerals– Randomly dispersed, individual mineral

crystals; heterogeneous solid• Texture refers to the size of mineral

crystals in rock: fine, intermediate, coarse• Minerals present and rock texture

determine weathering rate

Rock Cycle

Liquid Magma

Igneous Metamorphic

Sedimentary

Heat &Pressure

Heat &Pressure

Heat &Pressure

WeatheringWeathering

Cooling &Crystallization

Igneous Rocks• Formed when molten lava cools• Primary minerals• Coarse textured: granite

– Primarily quartz, feldspars, some dark minerals

– very slow weathering• Fine to intermediate texture: basalt

– hornblende, augite, biotite, and other dark minerals

– relatively rapid weathering

Igneous Rocks

Granite Basalt

Sedimentary and Metamorphic Rocks

• Sedimentary: deposition and re-cementation of weathering products from other rocks– Sandstone, shale, limestone…

• Metamorphic: igneous or sedimentary rocks transformed by high heat and/or pressure

MarbleLimestoneQuartziteSandstoneSlateShaleGneiss, schistGranite

Sedimentary Rocks

Sandstone Limestone

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Metamorphic Rocks

Gneiss Slate

Weathering

• The (1) physical disintegration of rock to form smaller rocks or individual mineral particles and the (2) chemicaldecomposition of minerals to form dissolved substances and new minerals

• Weathering categories– Physical– Chemical

Physical WeatheringA disintegration process that decreases particle

size and increase particle surface area. Occurs through the affect of:

• Temperature– Differential heating or cooling of rocks → exfoliation– Freeze-thaw: water expands upon freezing, exerting

tremendous force• Abrasion by water and water-borne sediments,

windblown particles, and ice in glaciers• Organisms

– Plant roots– Soil animals– Humans

Chemical Weathering• Alters the composition of minerals• Conversion of primary minerals into

secondary minerals, and secondary into other secondary minerals

• Most rapid with warm temperatures, high precipitation, and small particle size

• There are geochemical and biochemical agents of change

• Water is required

Chemical Weathering Processes• Solutioning (dissolution): mineral dissolves in soil

solution; common to soluble salts– CaSO4•2H2O (gypsum) → Ca2+ + SO4

2- + 2H2O– CaCO3 (calcite) → Ca2+ + CO3

2-

• Hydrolysis: water acts upon a substance to create a new substance– Involves both H2O and H+ as reactants– Often results in release of nutrients from minerals and

the formation of sesquioxides– KAlSi3O8 (K-feldspar) + 7 H2O + H+

→ K+ + Al(OH)3 (gibbsite) + 3 H4SiO40

• Hydration: addition of water to a mineral structure– 5 Fe2O3 (hematite) + 9H2O → Fe10O15•9H2O (ferrihydrite)

Chemical Weathering ProcessesHydrolysis is an important weathering process• Presence of H+ (acidity) accelerates weathering• Sources of protons

– CO2 in rainfall produces carbonic acid: CO2 + H2O →H2CO3 → H+ + HCO3

– (rainfall is naturally acidic; pH ~ 5.6)

– Plant roots and soil organisms respire and produce carbonic acid

– Soil organic matter is a proton source– Other acidic substances in rainfall: SOx/NOx + H2O →

H2SO4/HNO3– Fertilizers (e.g., NH4

+)

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Chemical Weathering Processes• Oxidation/reduction (redox) reactions (the

second most important weathering process)– Addition or loss of electrons (e–) from atom in a

mineral– Oxidation = loss of e–; reduction = gain of e–

– Electron-rich elements are termed reduced (e.g., Fe2+); electron-poor elements are termed oxidized(e.g., Fe3+)

– O2 is most common oxidizing agent– Elements in primary minerals commonly exist in a

reduced state– Oxidation and reduction occur together; they are

coupled

Redox Reactions• Oxidation of Fe2+ by O2 (O2 is the oxidant, it will

be reduced during the redox process)• Oxidation half-reaction:

Fe2+ → Fe3+ + e–

• Reduction half-reaction:¼O2 + e– + H+ → ½H2O

• Complete redox reaction: Fe2+ + ¼O2 + H+ → Fe3+ + ½H2O

Complexation Reactions• Microorganisms and plant roots exude

organic acid anions, e.g., citrate, oxalate, and malate

• These organic acids bond with (chelate) metals, e.g., Al3+ and Fe3+, to form soluble complexes

• The metal-organic complex is stable and much more soluble than the metal ion alone

Complexation ReactionsExample: Al3+ complexation by ketogluconate

Al(OH)3 (gibbsite) + 3H+ → Al3+ + 3H2OAl3+ + C5O5H9COO– → C5O5H8COOAl+ + H+

Soil Formation Processes• Soil is an open system• Additions - movement into profile

– Organic matter– Rainfall– Sediments– Chemicals: natural and anthropogenic

• Losses - movement out of profile– Evapotranspiration– Erosion– Leaching of water and chemicals– Gaseous losses of nutrients– Removal by vegetation

Soil Formation Processes• Translocations: movement within the soil

profile– Eluvial processes– Illuvial processes

• Transformations: a change in form– Physical weathering– Chemical weathering– Microbial degradation

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Five Soil Forming Factors

• Soil is a dynamic natural body formed by the combined effects of climate and biota, as moderated by topography, acting on parent materials over time.

• Soil = ƒ(climate, biota, topography, parent materials, time)

Factor One: Parent Material• Parent material impacts

– Soil textural class– Innate soil fertility– Types of clay minerals– Soil pH

• Classes of parent materials based on placement– Residual– Transported (six types of transported

materials)

Residual Parent Materials• Soils develop from underlying bedrock

– Igneous, sedimentary, metamorphic• Type of rock strongly influences type of

soil– Limestone → clayey soils– Sandstone → coarse, acidic soils– Granite → coarse, acidic soils– Slate, shale → clayey soils

Transported Parent Materials

• Colluvial debris• Alluvial deposits• Marine sediments• Lacustrine sediments• Eolian deposits• Glacial deposits

Colluvial Debris

• Poorly sorted fragments on steep slopes or at the foot of slopes, carried by gravity

• Small geographical areas• Usually rocky and stony, no layering• Physical weathering processes dominate

relative to chemical weathering processes• Well-drained but unstable

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Alluvial Deposits

• Floodplains– During flooding, water spreads and slows,

and fine sediment is deposited.– Horizontal and vertical stratification– Terraces are old floodplains above the current

floodplain– Usually very fertile soils and important for

agriculture, forestry, wildlife– Poor choice for homes and other urban

development

Alluvial Deposits

• Alluvial fans– Usually gravelly/stony in mountainous

regions, can have finer material as well.– Stream leaves narrow upland channel,

descends to broad valley below

Alluvial Deposits

• Delta deposits– The continuation/terminus of the floodplain– Rivers carry much clay/fine silt to lake or

ocean– Very slow water = deposition of fine particles– Very clayey, swampy, poorly drained– Example: Mississippi River delta in Louisiana

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Marine and Lacustrine Sediments

• Marine - Coastal Plains– Ocean sediments build up over time– Exposed by changes in elevation of earth’s crust– Materials are gravely, sandy, clayey depending on

area– Atlantic and Gulf Coastal areas, ~ 10% of US

• Lacustrine– Lake sediments build up over time– Clayey soils formed as lakes dried– Major areas of lacustrine soils in glaciated areas

Eolian Deposits• Loess deposits

– Common in central United States– Wind carried silts (coarse clays to fine sands) from

glaciated areas– Cover other soils or parent materials– Western one-third of Tennessee is loessial– Very thick (8+ m) at Mississippi River to non-existent

at Tennessee River– Blankets much of Iowa, thick at the Missouri River,

thin on eastern side• Others - sand dunes (sand-size), aerosolic dust

(clay-size), volcanic ash (allophanic soils)

Glacial Till• As glacier advances, grinds up rock and carries

it• Till is unsorted, unconsolidated material• Deposited as glacier melts and recedes• Till deposits called moraines

– Ground moraine - material deposited in relatively uniform layer during retreat

– Terminal or end moraine - material left pushed up in ridge at southern-most edge of advancing glacier

– Recessional moraine – terminal moraines from more than one advance

Ground moraines

Terminal moraines

Glacial Outwash

• As glaciers melt, glacial rivers and streams form and carry sediments– Coarse materials drop first– Fine materials carried furthest

• Deposits are sorted

Factor Two: Climate

Influences soil formation three ways:1. Precipitation2. Temperature3. Native Vegetation

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Climate: Precipitation

• As rainfall increases, chemical and physical weathering rates increase

• Profile depth increases• Nutrient status changes

– Loss of base cations Ca2+, Mg2+, K+, Na+

– Al3+, Fe3+, Mn2+, H+ increase• Soil acidity increases

Soil Moisture Regimes

• Aquic: saturated with reducing conditions most of the year

• Udic: soil moisture control section is dry for < 90 cumulative days per year

• Ustic: is dry for > 90 cumulative days per year

• Aridic: dry in all parts for > half the year• Xeric: moist winters, dry summers

(Mediterranean, California)

Soil Moisture Regimes

• Aquic = wet = tile needed for row crops• Udic = enough precipitation for “corn”• Ustic = enough precipitation for “wheat”• Aridic = cacti without irrigation• Xeric = precipitation when not needed for

production of most crops → winter

Climate: Temperature

• Chemical and biological reaction rates double for every 10 ºC increase

• Climates with extreme T, physical weathering (e.g., freeze-thaw) more significant than chemical weathering

• Evapotranspiration increases with increasing T

Soil Temperature Regimes• Cryic – mean annual T < 8 ºC• Frigid – mean annual T < 8 ºC; difference

between mean summer and mean winter T is > 6 ºC

• Mesic – mean annual T > 8 ºC and < 15 ºC; difference between mean summer and mean winter T is > 6 ºC

• Thermic – mean annual T > 15 ºC and < 22 ºC; difference between mean summer and mean winter T is > 6 ºC

• Hyperthermic – mean annual T > 22 ºC; difference between mean summer and mean winter T is > 6 ºC

Climate: Type of vegetation

• Humid = forest• Sub-humid, semi-arid = grasslands• Arid = shrubs, brush, succulents

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Factor Three: Biota

• Plants, animals, microorganisms• Important for MANY processes in soil formation• Chemical weathering

– Organic acid anions, carbonic acid, oxidation-reduction

• Organic matter accumulation (humification)– Water holding, nutrient holding

• Aggregation– Polysaccharides, gelatinous materials

Biota

• Nutrient cycling– Base recycling– Ca, Mg, K

• Nitrogen addition– Microbial N-fixation– N2 → NH4

+

• Profile mixing– bioturbation– earthworms, insects, etc.

Impact of Native Vegetation

• Grasslands– High OM below surface– Continuous root production, high interception

of rain• Coniferous Forests

– Vegetation low base cations (Ca, Mg, K)– Low recycling– Highly leached, acidic soils

Impact of Native Vegetation

• Deciduous forests– High in basic cations– High base cycling– Slightly to moderately acid

• Forest soils are usually more “developed”with more horizons, etc...

Grassland vs. Forest SoilsGrassland Deciduous Coniferous

Mollisol Alfisol Spodosol

Factor Four: Topography

• Affects amount of water soil “sees” (yellow arrows): concept of “effective precipitation”

• Slope aspect affects soil temperature

Uplandstable Sideslope

active erosionFloodplain

active deposition

Terrace/Fanstable

Footslopeactive deposition

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Landscape Positions• Upland

– Soil developed in residuum or in stable, unconsolidated materials (loess, glacial till)

– Rocks angular (except in till)– Well-developed soils– Highly-dissected

• Footslope– Bottom of slope, colluvial and alluvial deposits– Partly rounded rock, immature/younger soils

Landscape Positions• Terrace (second bottom, bench land)

– Old alluvium, higher elevation than current Floodplain

– Round stones, rocks - indicates water worked– Mature soils, some dissection

• Bottomland (floodplain)– Deposited by present stream action– Rounded stones– Immature soils, little dissection

Topography: Catena or ToposequenceSoils with same parent material, differ primarily in topographic location

Typical pattern of soils and underlying material in theHawthorne-Dellrose-Mimosa general soil map unit (Marshall Co., TN)

Hawthorne-Dellrose-Mimosa

MimosaDellrose

Hawthorne

ABt1Bt2Bt3Bt4

BC

C

R

BA

Bt1

Bt2

2Bt3

AAEBwC

Cr

A

AlfisolUltisol

Inceptisol

Factor Five: Time• Pretty obvious!• Works in concert with other factors• Chronologically old soil may be

developmentally young, e.g., arid region soils which have very little development

• Soil “age” is a relative thing!• “Old” soils = high water throughput

(Ultisols & Oxisols)• “Young” soils = low water throughput

(Aridisols)

Physiography of Tennessee

Modified from "Geography of Tennessee", published by Ginn and Co.

Plateau Slope

CentralBasin

GreatValley

UnakaRange

MississippiRiver

CumberlandPlateau

HighlandRim

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Physiographic RegionsMississippi

Riverfloodplain

Loess CoastalPlain

HighlandRim

CentralBasin

CumberlandPlateau

Valleyand

Ridge

SmokyMountains

Regions and their soils• Unaka Range

– Generally young (developmentally), shallow soils.

– Parent materials are metamorphic and igneous rock

– Inceptisols very common - weak horizonation– Ultisols in valleys, low elevations

• Valley and Ridge region (Knoxville)– Well-developed soils – Ultisols and Alfisols in

limestone, sandstone, shale

Regions and their soils• Cumberland Plateau

– Generally loamy soils– Sandstone is dominant parent material– Ultisols dominant

• Highland Rim– Generally clayey soils, many cherty– Limestone is dominant parent material– Ultisols and Alfisols

• Central Basin– Clayey, often shallow soils– Alfisols, Ultisols, Mollisols, Inceptisols

Regions and their soils• Coastal Plain – Ultisols & Alfisols

– Clayey soils from fine sediments– Loamy soils in coarse sediments– Fine-loamy soils in loess over sediments

• West Tennessee Loess Region - Alfisols– Fine-loamy soils in loess deposits, many

fragipans– Erosion is major risk

• Mississippi River Floodplain– Entisols, Inceptisols, Alfisols, Mollisols– Young, productive soils