Chapter 6: Soil Formation and Morphology

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Chapter 6 Soil Formation and Morphology 61
Important Facts To Know - Pedology

• The processes and products of weathering.
• Soil-forming factors, how they influence the
  formation of soil profiles, and the relationship
  to landscape characteristics and landform
  development.
• Definitions, morphology and characteristics of
  diagnostic soil horizons.
• Processes and implications of soil degradation.
• Mapping and delineation of soil taxonomic units.

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Homework Chapter 6
Questions 4, 8, 10, 11, 14 _at_ 2pts 10
pts Due 28 October 2009
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62 Weathering of Soil Minerals

• Definition - The physical disintegration and
  chemical decomposition or dissolution of earth
  materials at or near the earth's surface.
• Soils are formed from rock, loose unconsolidated
  materials (may be transported), or organic
  residues.
• Two processes of soil formation
• Weathering
• Horizon development.

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Weathering of soil minerals Physical and
chemical breakdown of rock materials currently
unable to support plant life into smaller size
materials able to support plant growth. During
chemical weathering, primary minerals are
dissolved and often form secondary minerals
(Table 6-1).
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Concept Rocks and physical weathering

• Granite coarse-textured (slow cooling).
• Interfaces among minerals are points of
  weakness.
• Physical weathering of granite creates
• coarse-textured soil.

Thin section of granite. Granite is composed of
the primary minerals potassium feldspars,
plagioclase feldspars (Na and Ca), biotite,
hornblende, and quartz.
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• Basalt is ultramafic rock that cooled quickly.
• Individual minerals small in size.
• Physical weathering creates finer-texture soil.

Thin section of basalt. Mineralogy is dominated
by plagioclase feldspars (laths) with finer
matrix of pyroxenes, iron minerals and olivine.
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Concept Categories of physical weathering

• Thermal expansion/contraction.
• Abrasion
• Freeze-thaw (hydro-fracture)
• Unloading
• Haloclasty

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Physical rock weathering during wildfire
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Concept Chemical weathering and thermodynamics

• Consider the igneous rocks granite and basalt
• Primary minerals formed under high T and P.
• Same minerals are thermodynamically unstable in
  the weathering environment at the earths
  surface.
• Weathering will produce secondary minerals that
  are thermodynamically stable at the earths
  surface
• These secondary minerals include clay minerals
  (kaolinite, smectite, vermiculite), iron
  compounds (geothite, maghemite, ferrihydrite),
  etc.
• Type of secondary minerals depends on primary
  minerals and weathering environment.

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Concept Bowens reaction series
Instability of primary minerals at the earths
surface is proportional to T at which minerals
formed.
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Concept Types of chemical weathering

• Congruent-minerals totally dissolve.
• CaCO3(s) H2O(1) CO2(g) Ca2(aq)
  2HCO3-(aq)
• SiO2(s) 2H2O(1) H4SiO4(aq)
• Incongruent-minerals are transformed into
  another mineral 
• 2KAlSi3O8(s) 2CO2(g) 11H2O(1)
• 2K(aq) 2HCO3-(aq) 4H4SiO4(aq)
  Al2Si2O5(OH)4(s)

Weathering of K-spar in soil to clay mineral
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Chemical weathering of soil minerals processes

• Hydrolysis (H and OH- from water)
• 2KAlSi3O8 2H 9 H2O -gt Al2Si2O5(OH)4
  4H4SiO4 2 K2
• Hydration (taking on water)
• Carbonation (carbonic acid)
• H2O CO2 -gt H2CO3
• Oxidation and reduction (losing or gaining of
  electrons).
• 4Fe2 3O2 --gt 2Fe2O3

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Concept - The plant as a weathering agent

• Influence the degree and rates of mineral
  weathering.
• Generation of weathering agents.
• Siderphores, Chelates (organic acids)
• Produce biogenic minerals (phytoliths).
• Plants are C, N, P, K (grasses high in Si)
• Counteract leaching
• Enrich soil surface in C, N, P, K

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Lichens are symbiosis of algae and fungus.
Produce oxalic and weak phenolic acids (lichen
acids) that increase kinetics of weathering.
Different plant communities engender different
weathering environments. The pathways and end
products of weathering will differ.
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63 Soil Formation Building a Matrix for Living
Organisms
Soil formation characterizes the physical and
chemical changes in weathered materials. These
include Additions of organic matter Losses of
materials due to leaching Translocation of clays
and dissolved materials from one horizon to
another Transformations within horizons.
Collectively they result in soil formation and
the development of pedogenic soil horizons
(layers)
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Photo of soil profile
Inceptisol Mollisol
Photos courtesy of USDA NRCS, National Survey
Center http//www.statlab.iastate.edu/soils/photog
al/orders/soiord.htm
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Photo of soil profile
Oxisol Spodosol
Photos courtesy of USDA NRCS, National Survey
Center http//www.statlab.iastate.edu/soils/photog
al/orders/soiord.htm
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Pedogenic (soil forming) processes (not in
text) Think of what would happen if you took a
sand, silt, and clay parent material fresh from a
borrow pit and put it out and left it for 10,000
years in 1) boreal forest, 2) temperate
forest, 3) grassland, 4) desert, 5)
tropical forest. What kinds of processes would
affect the development of these materials into
soils?
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• There would be
• 1) Additions (organic matter, particles, dust,
  chemicals) from
• Water (rain and irrigation)
• Sediment from wind (aeolian deposition resulting
  in loess deposits) and water (alluvium)
• Organic matter (most important addition)

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• 2) Losses from
• Leaching of chemicals organic matter, and ions
• Erosion
• Gaseous (organic matter, N, S especially during
  fire and in in flooded soils for N and S)

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• 3) Tranformations (within horizons) as a result
  of
• Dissolution and precipitation
• Organic matter decay and stabilization
• 4) Translocation such as
• A. Movement of clays, organic matter,
  dissolved
• ions (Fe, Al) from one horizon to another

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64 Soil-Forming Factors
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• 1) Parent material
• Especially important in early soil development,
  less so in older soils.
• Influences nutrients (other than N) by both
  total content and rate of weathering.
• Influences initial texture (particle size
  distribution).
• Examples granitic rock, glacial till,
  lacustrine clay, mixed colluvium, limestone on
  mineral or organic landforms (e.g., mesa, butte,
  plateau, plain, terraces, etc).

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Concept Time and soil polygenesis

• Soil from southern Idaho
• Polygenesis multiple cycles of soil formation
  over extended periods of time.
• In the arid west, many soil features are relict
  of the past when climate and vegetation was
  different.

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• 2. Climate Influences chemical weathering
  rainfall rate affects dissolution and leaching
• Accumulation of carbonates and salts in arid
  soils (calcareous or alkaline soils).
• A result of low water availability
• For example dissolved calcium and sulfate
  precipitate to form calcium sulfate (gypsum)
• Ca2 SO42- ? CaSO4

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• Acidic soils from high leaching rates.
• -Dissolution (carbonates, easily weatherable
  minerals)
• -Carbonic acid production (acidifying)
• Carbon dioxide water forms carbonic acid
• CO2 H2O ----gt H2 CO3
• Carbonic acid dissociates to hydrogen and
  bicarbonate
• H2 CO3 ? H HCO3-

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• Influences physical weathering
• 1. Erosion
• 2. Colluviation (downslope movement)
• Important after fire Gondola example
• Stronger in warm regions than in cold regions.
• 3. Not in book
• temperature fluctuations
• frost cracking

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• 3. Biota Influences the rate and nature of
  organic inputs.
• High inputs in tropical forests
• Low in deserts.
• Organic matter accumulation rates vary with
  climate

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• Islands of fertility in deserts caused by
  far-separated plants (photo)
• Tree roots breaking rocks
• Burrowing animals (esp. earthworms)
• Micro-organisms (extremely important)
• Lichens symbiotic relationship between algae and
  fungi, first to colonize rocks

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Concept Vegetation and soil properties
Plant communities are unique in how they cycle
elements (biogeochemical cycling). Contrast
biogeochemical cycling between a cheatgrass
invaded system (above) and an Allenrolfea/Atriplex
system (below). Given sufficient time, a
specific plant community with its unique balance
of biogeochemical cycling, will imprint this
balance on the soil. Invasive plants, such as
cheatgrass can greatly alter biogeochemical
cycling. Can plant invasions alter soil genesis
and what are possible outcomes?
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4. Topography (Relief)

• Aspect and temperature
• Drainage, precipitation, redox potential.
• Erosion, alluvial, colluvial activity (Figures
  6-20, 6-21) Slide Mountain example

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Concept Things are not always what they seem
Talus slope on the Buckskin Range. One would
expect such steep soils to be relatively shallow
and undeveloped.
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5. Time

• Soil development may occur in less than 200 years
  in humid climates, but can continue for
  thousands.
• Rate of development depends upon intensity of
  climate and biota factors and weatherability of
  parent materials.

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Interactions of soil forming factors Low
rainfall low leaching --gt calcareous, salt
buildup, high pH low production --gt low
SOM High rainfall high leaching --gt loss of
CaCO3, base cations, acidification more horizon
buidups high production ---gt higher SOM, less
patchy.
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Factors that may retard soil development Low
rainfall Low RH High CaCO3 content (soil
materials less mobile) Sandy parent material
(often ends up as Entisol) High clay (poor
aeration, slow water movement). Resistant parent
material (quartzite) Steep slopes - lots of
erosional renewal
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High water tables (slow leaching, decomp). Cold
temps (low biota, slow chemical
reactions) Constant deposition (like steep
slopes, alluvial, aeolian
deposits) Severe wind or water erosion (similar
to 10) Mixing by animals (but can also create
mollisols) Toxics - serpentine, etc.
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Paleosols (Ancient soils) Soils formed under
previous climate, vegetation. Mostly during the
Quarternary (since beginning of last ice
age) Relict soils exposed paleosols, quite
different from those currently exitsing. Example
near Ultisols in the middle of aridisols near
Pyramid lake Fossil soils Relict soils that are
buried and preserved.
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• 14C dating 14C/12C ratio in atmosphere is
  constant until bomb testing in the 60s.
• 14C decays to 14N, emitting beta.
• 1/2 life of 14C is 5568 yr

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6.5 Landforms and Soil Development
Different parent materials are often found on a
distinctive landform. Review Fig 6-9. Most
soils form on rocks in place (residual soils).
(I am not sure I agree with this, especially in
hilly country.)
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• Erosion tends to level off parent rock and leave
  a layer of soil behind.
• This landform is called a pediment.
• Water then can cut these up, leaving behind
  plateaus (large flat areas) or mesas (smaller
  flat areas) or buttes (even smaller areas).
• These are cut up with bluffs and scarps on the
  sides.

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• Materials deposited by water are called alluvium.
  
• Floodplains low flat deposits (floods)
• River Terraces Somewhat less flat areas
  deposited by rivers at flood stage or in previous
  channels.
• Alluvial Fans Not flat material carried down
  mountain canyons land left during flood events.
• Bajadas Coalesced alluvial fans at base of
  mountains

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• Peneplains Nearly level areas near streams from
  low flooding.
• Playa Dried up lake bed (lacustrine deposit)
• Marine Sediments make up limestone and dolomite
  deposits, beaches, deltas

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• Materials deposited by winds are Aeolian
  deposits
• Fine silts, sands, clays.
• A lot of this during the last glacial period
  forming
• loess soils.
• Loess soils in the midwest are some of the best
• for farming because of optimal particle size
  (silt)
• Also much aeolian activity in desert systems
• today.
• You have seen this in Nevada good examples
• near Honey Lake and Washoe Lake, for
• example.

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• Materials deposited by ice (Ice sheet during
  Pleistocene)
• Glacial till general name for glacial deposits.
• Terminal moraine ice melted as fast as it
  formed,
• leaving a sort of dike
• Lateral moraine On the sides of glacier,
  melting
• and deposition
• Ground moraine ice melted faster than it formed,
  
• leaving a flatter deposit from material
  within the
• ice.

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Map of Glacial Extent in North America http//geol
ogy.wcupa.edu/ajohnson/ess_101/powerpoint/glaciati
on/
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• Outwash plains near edges of glaciers, water
• borne.
• Eskers tunnels beneath the ice, filled up with
• sand and other deposits.
• Kettle block of ice left, sediment washed around
  it, then ice melted leaving a hole.
• Erratics large boulders left behind.
• Basal till book fails to mention this. It is
  what was under the ice and very compacted
  (cement-like). Holds water.

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• Sediments moved by gravity are called colluvium
• Talus rocky material
• Soil creep slow movement
• Solifluction slow movement (few cm per day)
• Debris flow, mudflow, earthflow
  self-explanatory.
• Avalanche rapid flow

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66 Morphology of Soil Horizons

• Think again of what would happen to our parent
  materials laid out in different climates.
• Materials move downward, forming soil layers that
  are often approximately parallel with the soils
  surface.
• Boundaries can be abrupt (distinct) or indistinct
  
• Boundaries can also be smooth (even thickness) or
  tonguing
• Horizons form by physical and chemical movement,
  sometimes also by animals (Mollisols).

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Oxisol Spodosol
Photos courtesy of USDA NRCS, National Survey
Center http//www.statlab.iastate.edu/soils/photog
al/orders/soiord.htm
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Aridisol Entisol
Photos courtesy of USDA NRCS, National Survey
Center http//www.statlab.iastate.edu/soils/photog
al/orders/soiord.htm
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Inceptisol Mollisol
Photos courtesy of USDA NRCS, National Survey
Center http//www.statlab.iastate.edu/soils/photog
al/orders/soiord.htm
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• Soil Horizons
• Letters are used to designate horizons A,B,C,R.
• Capitals are master horizons
• Small letters are suffixes for characteristics of
  master horizons (e.g., Bt)
• Arabic numerals after master indicate vertical
  subdivisions
• Arabic numerals before master indicate
  discontinuities (i.e., buried horizons)

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Basic soil profile Horizon Characteristics O
Organic (litter) A Mineral soil high in
organic matter and/or E Eluviated
(leached, loss of clay) B Accumulation (Fe, Al
oxides, clay) C Fractured parent material
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Photo of soil profile
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Photo of soil profile
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Photo of soil profile
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67 Diagnostic Horizons

• Used to differentiate and classify
• Epipedon upper profile
• Endopedon lower profile

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Epipedon Surface Horizon Anthropic Affected
by people (e.g., plough layer) Folistic
Infrequently saturated organic horizon. Histic
Frequently saturated organic horizon. Melanic
Thick, humus rich (black). Mollic Rich in
organic matter, dark brown-black, not very
acidic Ochric Light-colored Low in organic
matter Plaggen People-caused high humus.
(Manureing) Umbric Dark, but more acidic than
Mollic
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• Endopedon Subsurface horizon
• Agric Clay and humus rich due to ploughing
• Albic Strongly leached, whitish colored
• Argillic Clay-rich due to migration
• Calcic Calcium carbonate
• Cambic Altered horizon but not one of the above.
  Occupies B horizon position
• Duripan Cemented pan by caused by silica
• Fragipan Physically hard compacted pan Not
• chemically cemented

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• Glossic Degraded clay accumulation (used to be
  argillic, kandic, or natric)
• Gypsic Calcium sulfate
• Kandic Argillic horizon of kaolinite clays
  (lower CEC than Argillic)
• Natric Argillic horizon with high sodium or
  other base
• Orstein Thick, cemented illuvial horizon
• Oxic Highly-weathered, rich in Fe and Al oxides.
  Often reddish

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• Petrocalcic Carbonate-cemented
• Petrogypsic Gypsum cemented
• Placic Reddish, thin cemented pan of Fe, Mg and
  organic matter complexes
• Salic Salty
• Sombric Acidic, humus accumulation, tropical B
  horizon
• Spodic High in organic matter, iron, and
  aluminum oxides due to podzolization

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6.8 Degradation of Soils

• Any contamination, loss, or alteration that makes
  the soil less productive or less usable than it
  was prior to the change.
• Generally associated with anthropogenic impacts
• Loss of soil humus
• Nutrient depletion
• Excessive acidification/salinization
• Susceptibility to erosion/deposition
• Toxic substances

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6.9 Soil Individuals and Mapping Units

• Soil Individual Soil Series (Reno) Type (Silt
  Loam).
• Pedons Smallest unit of soil that can be called
  an individual.
• Polypedons Contiguous soils having the same
  thickness,
• humus, pH. Adjacent pedons which are similar,
  relate to series.
• LRR (Land Resource Regions) MLRAs (Major Land
• Resource Areas) The latter is a geographically
  associated area that has a common pattern of
  soils, climate, water resources, and land uses.
• GIS is critical to mapping, presenting, and
  managing large
• geographic databases.