Unit 3: Soil Formation &
Morphology
Chapter 6
Objectives
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Knowledge of the processes of weathering
of soil materials
Understanding of how soil profiles form
Investigate 5 soil forming factors & their
affects on soil development
Why soils degrade
Weathering of Soil Minerals
Weathering – disintegration of primary
minerals & reformation of some of those
dissolved materials into new secondary
minerals
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When rock disintegrates into loose aggregates in
which water, air, and roots may grow, it becomes
soil
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Can take many thousand years for this process to
happen naturally
Weathering of Soil Minerals
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Processes of Weathering
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Causes of weathering
Physical – grinding or breaking of minerals into
smaller particle sizes
 Chemical – dissolution or chemical alterations of
minerals
 Water is the material that fosters much of these
processes
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Hydrolysis – dissolution of minerals
Hydration – softening of minerals that helps with
internal swelling pressure
Weathering of Soil Minerals
Carbonation – from carbon dioxide dissolved in
water that acidifies water & helps it’s solvent
action
 Oxidation-reduction – O2 combination with
minerals to form iron oxides helping produce a soil
solution
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Products of Weathering
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Some ions remain in the soil
Most soluble ions get leached away
Weathering of Soil Minerals
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Least soluble ions may resolidify
Some persist and precipitate in soil solution
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Held w/ soil water
May be important to know original parent
minerals, but much soil is formed with new
deposits from other materials (e.g. flooding,
eroded soil)
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Alluvium – sediments deposited from moving
waters
Soil Formation: Building a Matrix
for Living Organisms
Soil formation – formation of soil horizons
& other changes to the soil
 From Rock & Mineral to Soil
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To become a soil: minerals & organic
residues must be transformed into sand, silt,
clay, humus
Soil Formation: Building a Matrix
for Living Organisms
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Additions – anything that might be added to
a soil to ehlp it form
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From: water, N from bacterial fixation, energy
from sunlight, sediment, etc.
Losses – materials lost through plant
removal, leaching, etc.
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Eroded particles, nutrients removing from
harvest/grazing, etc.
Soil Formation: Building a Matrix
for Living Organisms
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Transformations – minerals that change or
dissolve to other forms due to:
Chemical reactions
 Biological reactions
 Decomposition of organic matter
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Translocations – movements of water &
organisms w/in the soil
Clays carried to deeper layers in the soil
 Soluble salts
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Soil Formation: Building a Matrix
for Living Organisms
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Soil Profile
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Vertical cross section of soil
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Each will tend to be different depending on how
the soil is formed and the environment
Soil Horizons
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O, A, E, B, C, R
Soil-Forming Factors
>15,000 soils ID’d worldwide
Varoius minerals/substances/climates
combine to form soils
Five soil forming factros:
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Parent Material
Climate
Biota
Topography
Time
Soil-Forming Factors
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Parent Materials & Soil Formation
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Surface materials from which soils form
Unconsolidated materials once elsewhere and
laid down by water, wind, glaciers
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May be any mixture of soil types & particle sizes
Often the dominant determining factor in the
property of soils (dependig on location)
Soil-Forming Factors
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Climate & Soil Formation
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Increasingly a factor in soil formation over
time
Precipitation
 Temperature
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Ex. Of effects of climate on soil formation
Lime
 Acidic soils – intense weathering & leaching of
cations
 Erosion
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Soil-Forming Factors
Decomposition – speed related to temp of climate
 Weathering - rates depend on temp
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Development – when the soil has detecable
horizons
Also affects soil formation due to the plants
that grow in the region
Addition/losses of organic matter
 Additions/losses of soil moisture
 Protection from wind/water erosion
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Soil-Forming Factors
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Biota (Organisms) & Soil Formation
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Living platns & animals & their ogranic
residues
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Ex. Differences between soils in forest vs. grasses
right next to each other
Decompositoin of plant materials may be slow
and on the surface, or organic matter may be
deep into the soil (1’)
Animals – help mix soil profile, decompose
organic matter, aerate soils
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Ex. Of important animals?
Soil-Forming Factors
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Soil microbes also help by producing weak acids
that help dissovle minerals faster than just water
Topography & Soil Formation
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Primary influence from modificaiton of temp &
wtaer
Steep Hillsides – little water washes through
horizon, increased erosion
Gentle Hillsides – more vertical movement of
water, more/thicker vegetation, more organic
matter
Soil-Forming Factors
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Landlocked depressions – receive
runoff/erosion from higher areas – higher
vegetative producton, slower organic matter
decomposition (lower oxygen levels)
Organic soils – form in areas where soils are
waterlogged much of the year (peat or muck)
Soil-Forming Factors
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Time & Soil Formation
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Length of time required to develop genetic
horizons w/ help of climate, parent material,
biota, topography
Time to develpo soils tends to be shortest in
warm, humid, forested climates
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Helps if it’s acidic, slightly sandy
Development may take just a few to several
hundred years, depending on the
environemnt
Soil-Forming Factors
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Interactions of Soil Forming Factors
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5 soil-forming processes must work in
combination
Deficiencies/unfavorable conditions greatly
affect the ability of the soil to develop
How might the following soils develop?
High rainfall & good drainage climate w/ warm
weather
 Poor drainage, lowland area
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Soil-Forming Factors
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Conditions that retard soil development:
Low rainfall
 Low relative humidity
 Sandy parent materials
 High clay content
 Resistant bedrock materials
 Very steep slopes
 Presence of substances toxic to plants
 Hgih water tables
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Soil-Forming Factors
Cold temps
 Ongoing accumulation of soil material by
decomposition
 Severe erosion
 Mixing
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Ancient Soils: Paleosols
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Normal soils – develop in existing climateic
environment
Soil-Forming Factors
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Paleosols – soils formed mostly in previously
exising climates (may now be buried)
Relict – exposed paleosols formed in a
previous time under environmental conditioins
quite different from current conditions
Fossil – paleosols buried in the geological
section of the Earth, but deeper than present
soil development processes
Anthrosols – exhibit deliberate actions of
people
Soil-Forming Factors
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Why is this important?
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Can help us date & reconstruct history
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Ex. Soils that may be buried after a cataclysmic event
http://www.uwsp.edu/geo/faculty/ritter/geog
101/textbook/soil_systems/soil__development
_soil_forming_factors.html
Landforms & Soil Development
Depositions by water, wind, glaciers form
landforms
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Mesas, plateaus, plains, etc.
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May include other characteristics
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Soil depth, surface slope, soil texture
Soil Depth
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Vertical distance from the surface to a
restricting layer where roots cannot penetrate
Landforms & Soil Development
May be restricted by: rock, compacted zone,
water table, permafrost
 Deeper soils are more desirable
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Why?
How deep do you think our soils are?
Term
Depth (in.)
Very shallow
<9.8
Shallow
9.8-19.7
Moderately deep
19.7-39.4
Deep
39.4-59.1
Very deep
>59.1
Landforms & Soil Development
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Slope
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Elevation changes in the landscape
Slope = (vertical distance/horizontal distance)
* 100%
Usually want to measure at least 33’ to avoid
any localized differences that may skew your
slope measurement
Most obvious & important factor in
determining suitability of use for a particular
purpose
Landforms & Soil Development
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Soil Formed from Rocks
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Most soils form in place (extremely slowly)
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May be as little as 1/8 to 1/2” per 100 years, but
can be much quicker in tropical regions with little
erosion
Pediments – erosion-formed gradual slopes
Plateaus – large, flat level areas cut apart by
water gullies
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May include/form smaller mesas, or tiny buttes
Landforms & Soil Development
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Bluffs & scarps – steep cuts by erosion
undercutting a top layer
Playa – barren basin where water and
sediment collects temporarily
Materials Deposited from Water
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Alluvium – sediment deposited from flowing
water
Floodplains – landforms built by these
deposits
Landforms & Soil Development
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Alluvial fans – deposited soils coming from
hgiher areas that “fans” out into the lowland
area
Lacustrine deposits – sediments in
standing water (lakes)
Marine sediments – sediment deposited by
oceans (some actually exists in the Great
Plains, more commonly today in the Gulf of
Mexico)
Landforms & Soil Development
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Deltas – form at mouths of rivers from
waterborne sediment
Materials Depostied from Winds
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Eolian deposits – wind-transproted materials
May be coarse/fine silts, sands, clays
 Wind-deposited materials following the last glacial
period referred to as loess (common in our area)
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Landforms & Soil Development
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Materials Deposited from Ice
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Glacial till – material deposited from glacial
movements
Could create stony hills, ridges, smooth plain
deposition, outwash plains
 Only northern and eastern parts of IL affected by
glacial till
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Landforms & Soil Development
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Sediments Moved by Gravity
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Downslope movements cuased by the effect
of gravity on weathered rock debris
Material moved – colluvium
 Some may move in dry conditions, some may
move under wet conditions
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Morphology of Soil Horizons
Horizons – layers of soil forming parallel to
the soil surface
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Most differences in horizons are obvious
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Color/structure/texture differences
Can have indistinct changes in horizons due to
color/texture as well
Form due to:
Differences in weathering w/ depth
 Amounts of humus accumulated
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Morphology of Soil Horizons
Translocations of colloids to deeper depths
 Losses of colloids from percolating waters
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Identifying horizons
May have one master horizon (A)
 May see clear horizons clearly represented
together (AB)
 Soil scientists have many other breakdowns to
clearly ID horizons & describe its characteristics
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http://www.dmtcalaska.org/exploration/ISU/unit2/images/SOILTAMU.GIF
http://www.public.iastate.edu/~arossi/Copy%20of%20soil_profile2.gif
Degradation of Soils
Soils can be improved/damaged according
to hwo they’re managed
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Changes often result inadvertently due to its
use
Can reduce cropping yield, if ignored or
undetected
Degradation of Soils
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Degradation of Soil
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Degraded soils – soil has become less
suitable for its planned use
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“worn-out” soils
Can often occur due to deficiencies in soil
nutrients because of continual cropping w/out
repalcing by fertilization sufficinetly
Soils used for filtration of wastewater can also
become “full” and cannot filter the chemicals
targeted
Degradation of Soils
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Degradation by lost humus & organic matter
content
Degradation by salt acccumulation
Degradation by erosion
Loss of soil
 Reduced permeability
 Deposition of poor soil material
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Degradation by compaction
Degradation by contamination
Degradation of Soils
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Complete Destruction of Soils
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Most commonly occurs in shallow soils over
rocks
Burning of organic soils
 Erosion of mountain slopes
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Contaminated soils – if they can’t be
reclaimed because of cost or impracticality
Large applications of sewage sludge resulting
in high levels of heavy metals
Degradation of Soils
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High levels of salts due to inability to drain an
area
Assignment