Transcript Everything you*ve ever wanted to know about Soil.*

EVERYTHING YOU’VE EVER
WANTED TO KNOW ABOUT
SOIL.*
* But, were afraid to ask.
External Processes
Weathering – disintegration and
decomposition of rock at or near Earth’s
surface.
 Mass Wasting – transfer of rock material
downslope under the influence of gravity.
 Erosion – Incorporation and transportation of
material by a mobile agent, usually water,
wind, or ice.
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Weathering
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All materials are susceptible to weathering.
Consider the difference between fresh concrete and
concrete exposed to the elements for a period of
time. (root heaves, cracks from temperature
changes, wearing away from traffic, effects of
chemicals)
There are two types of weathering – mechanical
and chemical.
Mechanical Weathering
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Rocks are broken into smaller and smaller pieces, each
retaining the characteristics of the original material.
When the rocks break into smaller pieces, the rate at which
they weather increases. Why?
Surface area/Volume.
Frost Wedging
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Any cracks in rocks can be made bigger when water infiltrates
and expands upon freezing.
When this occurs and rocks fall to the bottom of a rise, we
called that a talus slope.
Unloading
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Rocks like granite will break off in large pieces in a process
called sheeting.
This process can be likened to removing the layers of an onion.
This end result of this process is an exfoliation dome.
Thermal Expansion
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Different materials have different expansion rates
due to heat gain and loss.
Rocks which contain different materials will fracture
with drastic temperature changes.
This is also present when different materials are
used in construction. (Expansion joints)
The relationship between rebar and concrete.
Biological Activity
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Roots penetrate cracks and exert hydraulic pressure on
surrounding rock, expanding the crack.
Burrowing animals can increase the rate at which rock
disintegrate by gnawing and clawing.
Decomposition of organic material also releases acids.
Chemical Weathering
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Primarily involves water.
If oxygen is dissolved in water, oxidation will occur.
Carbon oxides, nitrogen oxides, and sulfur oxides
(released from the combustion of carbonaceous
material) will react with water vapor to form acids.
Spheroidal weathering refers to the physical wearing
away of stone edges and forms rounded stones.
Weathering has accelerated since the 1800s. Why?
Weathering
Weathering
Factors that affect the rate of Weathering.
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The chemical composition of the rock material. Granite,
composed primarily of silicates, wears away far more
quickly than marble, which is composed primarily of
carbonates.
Climate (not weather) greatly affects the rate of
weathering. Cleopatra’s Needle was relocated to
England and the rate of weathering was much faster
than similar obelisks from the same era that remained in
Egypt.
The optimum climate for weathering would be a warm
and moisture-rich environment.
Soil
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Soil is the interface between the biotic and abiotic.
Soil is composed primarily of regolith (layer of
rocks and mineral fragments produced by
weathering), organic material, water, and air.
Pore spaces hold air and water, allowing plants and
microorganisms to survive.
If soil becomes compacted, pore space is limited
and fertility of the soil suffers.
Soil (continued)
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Productive surface soil is composed of 50%
inorganic minerals and 50% humus (decayed
remains of animals and plants).
Soil texture is descriptive of the percent composition
of different particle sizes.
Loam is optimal soil.
Soil Profile
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Soil Horizons – distinct, identifiable, and measureable
layers.
O Horizon – primarily organic material (leaf litter, twigs,
branches)
 A Horizon – mixture of organic and minerals.
 E Horizon – zone of eluviation (“washing out”).
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Leaching describes nutrients and minerals being carried deep in
soil by water.
B Horizon – sub-soil. High in minerals and nutrients.
 C Horizon – partially altered parent material
 Parent material – bedrock (residual soils) or unconsolidated
deposits (transported soils).
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Soil Types
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Pedalfer – combination of “ped” (Pedon, Greek,
meaning soil), “Al” (aluminum) and “Fe” (iron).
 Typical
of the clays in mid-latitude regions where iron
oxides and aluminum laden clays are found.
 A-Horizon is leached of minerals that accumulate in the
B-Horizon.
 This soil is very common in the mid-Atlantic region.
 This soil tends to be acidic.
Soil Types (continued)
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Pedocal – combination of “ped” (Pedon, Greek,
meaning soil) and “cal” (from calcium carbonate).
 Typical
of the drier soils in the Western United States.
 Weather is less intense, so clay build-up is not typically
seen.
Soil Types (continued)
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Laterite – the result of intense weathering in hot,
moist environments.
 Because
of the rapid weathering and bacterial action,
laterite soil is devoid of humus and contains virtually no
nutrients or minerals.
 This soil is typical of the tropical region of the Earth,
particularly, the rain-forests.
 This soil is the poorest soil for farming.
Soil Erosion
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Water, wind, and ice are eroding agents.
Water droplets from rain strike the ground with
surprising force and loosen the top layer.
Loose soil is transported downhill in rills and gullies.
Soil Erosion (continued)
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Wind erosion occurs when soil becomes desicated and can be
carried aloft.
Typically occurs when areas that receive minimal rainfall are
intensively farming – removing all moisture from pore spaces.
Soil Erosion
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Ice erosion occurs as glaciers advance and retreat
over land masses, grinding away the soil beneath
the ice.