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Soil-Forming Factors
ESS 210
Chapter 2
pages 31–74
• 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
Primary Minerals
Minerals
• Light colored aluminosilicate minerals
• Homogeneous, inorganic compounds, with
definite chemical formula
• Primary minerals
– Quartz [SiO2]: most common, weather very
slowly, sand size
– Feldspars: sand size, weather to soil clays
• K-feldspars – KAlSi3O8
• Plagioclase feldspars:
– Formed as molten lava cools and solidifies
– Not chemically altered by weathering
processes
• Secondary minerals
– Albite – NaAlSi3O8
– Anorthite – CaAl2Si2O8
– Muscovite mica – KAl3Si3O10(OH)2
• A parent of soil clay minerals: weathers to soil clay
minerals
• Thin, translucent sheets (isinglass)
– Recrystallization and/or alteration products of
primary minerals
Primary Minerals
Secondary 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
• 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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Rock Cycle
Rocks
Liquid Magma
• 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
Igneous Rocks
Heat &
Pressure
Cooling &
Crystallization
Heat &
Pressure
Igneous
Weathering
Metamorphic
Weathering
Heat &
Pressure
Sedimentary
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
Sedimentary and Metamorphic
Rocks
Granite
Basalt
Sedimentary 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
Granite
Shale
Gneiss, schist
Slate
Sandstone
Quartzite
Limestone
Marble
Sandstone
Limestone
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Metamorphic Rocks
Weathering
• The (1) physical disintegration of rock to
form smaller rocks or individual mineral
particles and the (2) chemical
decomposition of minerals to form
dissolved substances and new minerals
• Weathering categories
Gneiss
Slate
Physical Weathering
A 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 Processes
• Solutioning (dissolution): mineral dissolves in soil
solution; common to soluble salts
– CaSO4•2H2O (gypsum) → Ca2+ + SO42- + 2H2O
– CaCO3 (calcite) → Ca2+ + CO32-
• 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
– Physical
– Chemical
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
Hydrolysis 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+)
– 5 Fe2O3 (hematite) + 9H2O → Fe10O15•9H2O (ferrihydrite)
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Chemical Weathering Processes
• Oxidation/reduction (redox) reactions (the
second most important weathering process)
(e–)
– Addition or loss of electrons
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
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
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
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
Example: Al3+ complexation by ketogluconate
Al(OH)3 (gibbsite) + 3H+ → Al3+ + 3H2O
Al3+ + C5O5H9COO– → C5O5H8COOAl+ + H+
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)
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
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)
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
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
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)
Ground moraines
Terminal 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
Glacial Outwash
Factor Two: Climate
• As glaciers melt, glacial rivers and streams
form and carry sediments
Influences soil formation three ways:
1. Precipitation
2. Temperature
3. Native Vegetation
– Coarse materials drop first
– Fine materials carried furthest
• Deposits are sorted
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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 = 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
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
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)
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
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, oxidationreduction
• Organic matter accumulation (humification)
– Water holding, nutrient holding
• Nutrient cycling
– Base recycling
– Ca, Mg, K
• Nitrogen addition
– Microbial N-fixation
– N2 → NH4+
• Profile mixing
• Aggregation
– bioturbation
– earthworms, insects, etc.
– Polysaccharides, gelatinous materials
Impact of Native Vegetation
• Grasslands
Impact of Native Vegetation
• Deciduous forests
– 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
Grassland vs. Forest Soils
Grassland
Biota
Deciduous
Coniferous
– High in basic cations
– High base cycling
– Slightly to moderately acid
• Forest soils are usually more “developed”
with more horizons, etc...
Factor Four: Topography
• Affects amount of water soil “sees” (yellow
arrows): concept of “effective precipitation”
• Slope aspect affects soil temperature
Footslope
Upland
stable
Sideslope
active deposition
active erosion
Mollisol
Alfisol
Spodosol
Floodplain
active deposition
Terrace/Fan
stable
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Landscape Positions
Landscape Positions
• Upland
• Terrace (second bottom, bench land)
– 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
Topography: Catena or Toposequence
Soils with same parent material, differ
primarily in topographic location
– 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
Hawthorne-Dellrose-Mimosa
Inceptisol
A
AE
Bw
C
Ultisol
Alfisol
Typical pattern of
soils and
underlying
material in the
HawthorneDellrose-Mimosa
general soil map
unit (Marshall Co.,
TN)
A
BA
A
Bt1
Bt2
Bt3
Bt4
BC
C
Bt1
Cr
Bt2
2Bt3
R
Hawthorne
Dellrose
Mimosa
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
Central
Basin
Mississippi
River
Great
Valley
Plateau Slope
Highland
Rim
Unaka
Range
Cumberland
Plateau
Modified from "Geography of Tennessee", published by Ginn and Co.
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Physiographic Regions
Mississippi
River
floodplain
Loess
Coastal
Plain
Highland
Rim
Central
Basin
Cumberland
Plateau
Valley
and
Ridge
Smoky
Mountains
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
• 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
• 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
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