Field Biology Packet
Soil # 3
Soil Evolution
Pre Reading Strategy: What do you think soil evolution is before you read the information?
During Reading Strategy: Look ahead to your activities; identify what you’ll need to know so
that you can underline the important information in the reading.
BACKGROUND INFORMATION
When asked about the origins of soil, we might think about what can be seen in soil to help us
find a link to its origin. There are particles of different sizes and often tiny insects and maybe
even worms. Air, moisture, some tiny mineral slivers, leaves, small twigs, and tiny pebbles may
also be evident. Some small animals and plants may be decomposing in the soil; and if you were
to examine a watery soil sample under a microscope, you might even see bacteria within the
sample. Some soils have a distinctive color – for example, brown or rust-colored.
If you dig a hole deep into the soil, eventually you will not be able to dig any deeper because you
will reach solid rock – bedrock. The bedrock is the parent material from which soil is made. In
some places, bedrock is at the surface of the earth with no soil covering it; this is called an outcropping. Weathering of bedrock begins as cracks form in the bedrock and small pieces of rock
break away.
Weathering breaks parent material into smaller and smaller pieces. Weathering happens in several
ways. Mechanical and chemical weathering are two general classifications of weathering.
Mechanical weathering occurs when water stands in a crack in the rock. The temperature, either
because of the season or because of night and day, changes drastically, and the water freezes. As
water freezes it expands and the rock breaks even more. This method is called freeze-thaw.
Maryland’s humid and warm climate is conducive to weathering.
Chemical weathering can be a result of plant growth. An example is when plants are able to grow
in cracks in rocks because the wind has blown soil particles into the crack. Some plants then form
acids which cause a chemical change in the pH of the rock. Because the pH changes, the rock’s
cementing material is altered and it begins to break up. This method is called plant acidity.
The best clue to soil origin is color. How is color a clue?
The color of the parent material, the bedrock, is the basis for the color of the soil in the area.
Maryland’s humid and warm climate is conducive to weathering. Soils are absent or thin in polar
regions because of few climatic changes.
Color is also affected by three other factors – the type and amount of organic matter, amount and
intensity of weathering, and drainage conditions.
Field Biology Packet
Soil # 3
OBJECTIVE
The student will complete a diagram of a soil profile and list and define weathering methods in
order to explain the evolution of soil from bedrock.
MATERIALS
soil profile diagram and data table
PowerPoint
PowerPoint Guide questions
colored pencils
Weathering Outline
POWERPOINT NOTES
QUESTIONS/MAIN
IDEAS/KEY TERMS:
SOIL EVOLUTION
NOTES
Horizon O
 Name::_______________________
 Colors: Black, dark brown
 Structure and composition: Loose, crumbly, well ____________

Horizon A




Horizon B




up ____________________________(twigs, leaves)
Processes occurring: __________________________
Name: _______________________
Colors: Dark brown to yellow
Structure and composition: Generally loose, crumbly, well broken
up because _____________ ______ __________ and seeds
germinate here; earthworms and bugs present; minerals present
Processes occurring: ___________________ and
_____________________________
Name: _________________________
Colors: Brown, ____________ to orange in color
Structure and composition: Zone of larger ______________, may
be dense crumbly, but mostly ____________________; clay and
oxidized materials and organic matter accumulates from Horizon A
Processes occurring: ________________________________
__________________________________________________
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Soil # 3
Horizon E




Name: ____________________
Colors: Minerals are moved out (_____________________) so the
soil is light in color because the soil silica remains
Structure and composition: Depends the mass of the horizons
above E; gets more __________________ as soil evolution
develops
Processes occurring: Transition between and and B (eluviation)
where the _______________________ is predominant
Horizon C




Name: ________________ _________________ material
Colors: Depends upon the color of the parent rock
Structure and composition: Dense
Processes occurring: _________________________________
___________________________________________________
Horizon R or D
R is usually used




Name: Parent _________________________________
Colors: Generally _____________________________________
Structure and composition: ____________________________
Processes occurring: _________________________________
___________________________________________________
Weathering

The Chemical or Mechanical break down of bedrock inyo
___________ and ____________________ pieces.

Chemical weathering incorporates a ______________
_____________ in the material

_______________ weathering simply breaks the material into
smaller and smaller pieces.
Chemical
Weathering


Oxidation – combines ___________ with another substance in the
rock; this usually changes the _______________ of the rock

Carbonation – happens in rocks that contain calcium carbonate
(limestone), _________________________________
____________________________________________________

Acide – formed by ________________ ____ __________________
compounds will cause rocks to fall apart

Hydration – the incorporation of H+ and OH- ions; this addition
causes an increase in______________________ adding stress
within the rock
Field Biology Packet
Soil # 3
Mechanical
Weathering

Root action –
_____________________________________________________
_____________________________________________________

Thermal expansion – rocks heat up in the _______________ and as
the temperature drops at _________________________, the rocks
can flake easily (exfoliated)
OR
Summary

Freeze/thaw will also deposit water in ______________, if it
freezes, the ice pushes outward and caused piece of rock to break
off; lie a _______________________

Hydraulic action - Water (usually from powerful waves) rushes into
cracks in rocks; a tiny bit of air is ___________________ at the
crevice of the crack; when the water __________________, the air
is forcefully released taking away fragments or rock with itl

Organic activity – _________________________ attach to rocks
and this causes physical as well as chemical breakdown of the rock
(plant roots exert ___________________ pressure on rock cracks
and plants for a an ____________________ on the rocks that further
breakdown the rock).

Gravity – Large rocks often fall from __________________ and as
they hit bottom they break _________________ and
______________ pieces

What are two ways that rock and soil are broken down?

What differentiates soil layers?
Field Biology Packet
Soil # 3
NOTES
Horizon
O
A
B
E
C
R
PROCEDURE
SOIL PROFILE DATA TABLE
Name
Colors
Structure and
Composition
Processes occurring
Field Biology Packet
Soil # 3
Field Biology Packet
Soil # 3
Directions: On the previous page: Label each of these layers with your knowledge of soil
horizons.
Weathering
Directions: on this page, identify the weathering methods.
Chemical
Mechanical
1.
1.
2.
2.
3.
3.
4.
4.
5.
Field Biology Packet
Soil # 3
ANALYSIS
WEATHERING AND SOIL FORMATION/EVOLUTION
Match column A with column B. Write a number on each line of column B.
Column A
1. topsoil
Column B
_____ layer of soil below the topsoil that is less
suited for growing crops
2. bedrock
_____ best soil for growing crops
3. organic
_____ parent material from which soil is made
4. subsoil
_____ plant or animal matter
5. physical weathering
_____ carbonation or oxidation
6. aeration
_____ thermal changes – extreme heat to cold
temperatures
7. chemical weathering
_____ incorporation of atmospheric air into the soil
8. horizon O
_____ horizon that accumulates minerals from the
topsoil
9. horizon A
_____ horizon below the twig and leaf layer
10. horizon B
_____ horizon that is often termed the
decomposition layer
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Soil # 3
Fill in the blanks using the WORD BANK that follows.
1. The six components of soil are: ______________________,
__________________________, ___________________,
_______________________________, __________________________ and
________________________.
2. _______________________ means that minerals are moved OUT of this layer. The A
horizon loses minerals to the B horizon by this process.
3. ___________________________ is another name for translocation.
4. Rarely are vegetation roots found in horizon _____.
5. The A horizon is made up of dark colored soil called __________________.
6. The soil horizons make up the soil ____________________.
7. The two factors that are very active in mechanical weathering are _______________ and
_________________.
8. Horizons _____ and _____ are the first and second layers of the soil profile.
WORD BANK
O
B
water
leaching
soil water
A
wind
humus
translocation
soil air in porous spaces
profile
rocks, rock particles
decaying plant and animal matter
dissolved minerals/elements
fungus, bacteria, bugs, etc.
Field Biology Packet
Soil # 3