Soil Evolution
Field Biology
CCTI
Mrs. Conway
Directions
• Read through the following slides while
completing your lab packet.
• Be sure to answer all question in the
packet.
• Use this button to move forward
• Use this button to move backward
Where Does Soil Come From?
• Read the Background Information on your
handout.
• 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
(organic)
• Dissolved minerals/ elements
• Soil air in porous spaces (aeration)
• Fungus and bacteria
• Bugs
What are the two types of
weathering?
• 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
weathering.
Chemical
Weathering
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
apart
Chemical Weathering
• Hydration: the incorporation of H+ and
OH- ions; this addition causes an increase
in volume, adding stress within the rock
Mechanical
Weathering
Two factors are very important for
mechanical weathering, wind and
water.
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
Profile?
The soil horizons make up the soil
profile.
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
Material)
R (Parent Bedrock)
O Horizon
• Colors: Black, dark brown
• Structure and Composition: Loose,
crumbly, well broken up litter (twigs,
leaves)
• 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
present
• Processes occurring: Leaching and nutrient
center
• 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
bedrock
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
material.
Congratulations!
You have just completed
the Weathering and Soil
Formation/Evolution
Tutorial!