Document 7232917
Download
Report
Transcript Document 7232917
Unit 3: Soil Formation &
Morphology
Chapter 6
Objectives
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
When rock disintegrates into loose aggregates in
which water, air, and roots may grow, it becomes
soil
Can take many thousand years for this process to
happen naturally
Weathering of Soil Minerals
Processes of Weathering
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
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
Products of Weathering
Some ions remain in the soil
Most soluble ions get leached away
Weathering of Soil Minerals
Least soluble ions may resolidify
Some persist and precipitate in soil solution
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)
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
To become a soil: minerals & organic
residues must be transformed into sand, silt,
clay, humus
Soil Formation: Building a Matrix
for Living Organisms
Additions – anything that might be added to
a soil to ehlp it form
From: water, N from bacterial fixation, energy
from sunlight, sediment, etc.
Losses – materials lost through plant
removal, leaching, etc.
Eroded particles, nutrients removing from
harvest/grazing, etc.
Soil Formation: Building a Matrix
for Living Organisms
Transformations – minerals that change or
dissolve to other forms due to:
Chemical reactions
Biological reactions
Decomposition of organic matter
Translocations – movements of water &
organisms w/in the soil
Clays carried to deeper layers in the soil
Soluble salts
Soil Formation: Building a Matrix
for Living Organisms
Soil Profile
Vertical cross section of soil
Each will tend to be different depending on how
the soil is formed and the environment
Soil Horizons
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:
Parent Material
Climate
Biota
Topography
Time
Soil-Forming Factors
Parent Materials & Soil Formation
Surface materials from which soils form
Unconsolidated materials once elsewhere and
laid down by water, wind, glaciers
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
Climate & Soil Formation
Increasingly a factor in soil formation over
time
Precipitation
Temperature
Ex. Of effects of climate on soil formation
Lime
Acidic soils – intense weathering & leaching of
cations
Erosion
Soil-Forming Factors
Decomposition – speed related to temp of climate
Weathering - rates depend on temp
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
Soil-Forming Factors
Biota (Organisms) & Soil Formation
Living platns & animals & their ogranic
residues
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
Ex. Of important animals?
Soil-Forming Factors
Soil microbes also help by producing weak acids
that help dissovle minerals faster than just water
Topography & Soil Formation
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
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
Time & Soil Formation
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
Helps if it’s acidic, slightly sandy
Development may take just a few to several
hundred years, depending on the
environemnt
Soil-Forming Factors
Interactions of Soil Forming Factors
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
Soil-Forming Factors
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
Soil-Forming Factors
Cold temps
Ongoing accumulation of soil material by
decomposition
Severe erosion
Mixing
Ancient Soils: Paleosols
Normal soils – develop in existing climateic
environment
Soil-Forming Factors
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
Why is this important?
Can help us date & reconstruct history
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
Mesas, plateaus, plains, etc.
May include other characteristics
Soil depth, surface slope, soil texture
Soil Depth
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
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
Slope
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
Soil Formed from Rocks
Most soils form in place (extremely slowly)
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
May include/form smaller mesas, or tiny buttes
Landforms & Soil Development
Bluffs & scarps – steep cuts by erosion
undercutting a top layer
Playa – barren basin where water and
sediment collects temporarily
Materials Deposited from Water
Alluvium – sediment deposited from flowing
water
Floodplains – landforms built by these
deposits
Landforms & Soil Development
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
Deltas – form at mouths of rivers from
waterborne sediment
Materials Depostied from Winds
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)
Landforms & Soil Development
Materials Deposited from Ice
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
Landforms & Soil Development
Sediments Moved by Gravity
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
Morphology of Soil Horizons
Horizons – layers of soil forming parallel to
the soil surface
Most differences in horizons are obvious
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
Morphology of Soil Horizons
Translocations of colloids to deeper depths
Losses of colloids from percolating waters
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
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
Changes often result inadvertently due to its
use
Can reduce cropping yield, if ignored or
undetected
Degradation of Soils
Degradation of Soil
Degraded soils – soil has become less
suitable for its planned use
“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
Degradation by lost humus & organic matter
content
Degradation by salt acccumulation
Degradation by erosion
Loss of soil
Reduced permeability
Deposition of poor soil material
Degradation by compaction
Degradation by contamination
Degradation of Soils
Complete Destruction of Soils
Most commonly occurs in shallow soils over
rocks
Burning of organic soils
Erosion of mountain slopes
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
High levels of salts due to inability to drain an
area
Assignment