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Soil Evolution
Field Biology
Mrs. Conway
• Read through the following slides while
completing your lab packet.
• Be sure to answer all question in the
• Use this button to move forward
• Use this button to move backward
Where Does Soil Come From?
• Read the Background Information on your
• Highlight the definitions the of new
vocabulary words you encounter.
What are the six components
of soil?
• Rocks and rock particles
• Decaying plant and animal matter
• Dissolved minerals/ elements
• Soil air in porous spaces (aeration)
• Fungus and bacteria
• Bugs
What are the two types of
• Chemical weathering can be the result of
plant growth, causing a chemical change.
• Mechanical weathering occurs when
water stands in the cracks of rock.
• Read through the next slides while
completing page 630 in your packet.
• List and describe four types of chemical
weathering and five types of mechanical
What are some examples?
Chemical Weathering
Oxidation: Combines oxygen with another
substance in the rock: this usually
changes the color of the rock
Chemical Weathering
• Carbonation: happens in rocks that
contain calcium carbonate (limestone);
speeds up as the temperature decreases
Chemical Weathering
• Acids: formed by sulfur and nitrogen
compounds and will cause rocks to fall
Chemical Weathering
• Hydration: the incorporation of H+ and
OH- ions; this addition causes an increase
in volume, adding stress within the rock
Two factors are very important for
mechanical weathering, wind and
What are some examples?
Mechanical Weathering
• Root Action: As roots grow they break
rocks apart
Mechanical Weathering
• Thermal expansion: especially in
deserts, rocks heat up in the day
and, as the temperature drops
greatly at night, the rocks can
flake easily (exfoliate).
Freeze/thaw will deposit water in
cracks when this freezes the ice
pushes outward and causes
pieces of the rock to break off, like
a layer of an onion.
Mechanical Weathering
• Hydraulic Action: Water rushes into
cracks in rocks; but a tiny bit of air is
compressed at the crevice of the crack:
when the water recedes the air is forcefully
released taking away fragments of rock
with it.
Mechanical Weathering
• Organic Activity: Mosses and lichens
attach to rocks and this causes physical as
well as chemical breakdown of the rock.
 (plant roots exert extreme pressure on rock cracks and plants form
an acid on the rocks that further breakdown the rock).
Mechanical Weathering
• Gravity: Large rocks often fall from
mountainsides and as they hit bottom they
break into smaller and smaller pieces.
What is a Soil
The soil horizons make up the soil
Now turn to Page 632 in your packet!
Soil Profile: Click on the boxes to move forward!
O (organic)
A (Topsoil)
B (Subsoil)
E (Transition Area)
C (Weathered Parent
R (Parent Bedrock)
O Horizon
• Colors: Black, dark brown
• Structure and Composition: Loose,
crumbly, well broken up litter (twigs,
• Processes occurring: Decomposition
• Some people might not realize that this
layer is actually part of the solid ground
• This layer is known as the twig-leaf layer!
A Horizon
• The A horizon is made of dark colored soil
called humus.
• Colors: Dark brown to yellow
• Structure and Composition: Generally loose,
crumbly, well broken up because roots are
present and seeds germinate here;
earthworms and bugs present; minerals
• Processes occurring: Leaching and nutrient
• Most roots of vegetation are found here! This
is the best soil for growing crops!
B Horizon
• Colors: Brown, reddish to orange in color
• Structure and Composition: Zone of larger
chunks, may be dense, crumbly but mostly
cement-like; clay and oxidized materials
and organic matter accumulates from A
horizon (topsoil)
• Processes occurring: Accumulation
• This layer is less suited for growing crops!
E Horizon
• Colors: Minerals are moved out from A to B
(translocated) so the soil is light in color
because the soil silica remains; this is also
known as leaching.
• Structure and Composition: Depends upon the
mass of the horizons above E; gets more
compact as soil evolution develops
• Processes occurring: Transition between A and
B (eluviation) where the leaching is predominant
C Horizon
• Colors: depends upon the color of the
parent bedrock
• Structure and Composition: Dense
• Processes occurring: Weathering or
disintegration of parent material or parent
R Horizon
Colors: generally solid (gray)
Structure and Composition: Dense
Processes occurring: Weathering
This layer is the parent material from
which soil is made!
How much did you learn?
• Complete the questions located on page
627 and 628.
• Be sure that all answers are complete.
• Use the back or forward arrows to review
You have just completed
the Weathering and Soil