GEOL101 LAB 10
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San Diego State University *
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Course
101
Subject
Geology
Date
Feb 20, 2024
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10
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GEOL101 Dynamics of the Earth – Fall 2023
Name:
Lily Razzano
Laboratory 10: Soils and Weathering – Things fall
apart
Section:
Geol101
Learning Outcomes:
● Describe the three general soil formation processes that promote the development of
soils
● List “idealized” vertical of soil horizons and describe the general processes that
influence their physical appearance
● Describe various local factors that influence soil order development thickness in a
given area
● Identify some major soil orders based on physical characteristics and local
environment
Background
Soils are common features of the Earth’s surface. You encounter them daily, but probably give
them little notice or just think of them as the “dirt” in which people grows vegetables or flowers.
Yet, without soils, most if not all of the terrestrial animal life of our planet would not exist! The
United States Department of Agriculture National Resources Conservation Service (NRCS)
defines a soil as follows:
The unconsolidated mineral or organic material on the immediate surface of the
Earth that serves as a natural medium for the growth of land plants.
The upper limit of soil is the boundary between soil and air, shallow water, live
plants, or plant materials that have not begun to decompose. Areas are not
considered to have soil if the surface is permanently covered by water too deep
(typically more than 2.5 meters) for the growth of rooted plants.
The lower boundary separating soil from non-soil is difficult to define. Commonly,
soil grades at its lower boundary to hard rock or to earthy materials virtually
devoid of animals, roots, or other marks of biological activity. For purposes of
classification, the lower boundary of soil is arbitrarily set at 200 cm (2 m).
General Soil Formation Processes
There are three main processes that take place at the Earth’s surface, or very near it, that
contribute to soil formation. These processes are weathering, water, and organic activity
Weathering processes
include chemical and physical processes; for example, chemical
reactions occur when minerals come in contact with air and water. Some reactions involve
volume expansion, which in turn produces internal stresses that physically break rock into
smaller and smaller bits and pieces. In short, weathering contributes to the formation or
production of new material as loose debris, mineral grains, and ions in solution.
Water
is a relatively ubiquitous solvent on planet Earth. As rain falls on the Earth, some runs off
into streams and gutters while some infiltrates cracks and holes produced by weathering and
biological activity. As water percolates downward, it carries with it dissolved ions derived from
chemical weathering and clay particles derived from physical weathering. The region within the
soil where this activity takes place is referred to as the
zone of leaching
. Further down the
removed clay particles may be deposited and the dissolved ions may precipitate as new
minerals, a region termed the zone of accumulation.
Organisms
within soils introduce organic matter, break this organic matter down into inorganic
nutrients, and mix and redistribute soil particles. For example, earthworms and microbes
physically mix and break up soil, consume dead organic matter, and release inorganic nutrients
back into soils. Any gardener will tell you their compost bin would not be effective without the
help of worms and microbes. The accumulation of dead organic material is called humus . . .
not to be confused with hummus!
Soil Horizons
As a result of the above three processes, bedrock and regolith (i.e., loose unconsolidated
sediments that overlie bedrock) are often converted into soil over time. The composition and
character of the soil evolves into something very different than its starting composition. Soil
forming processes act differently at different depths to produce a “soil profile” comprising a
number of “soil horizons.” An idealized soil profile with labeled soil horizons is shown to the
right and the soil horizons are briefly defined below:
O = Organic layer, almost no mineral matter; dark in color
A = Mixed mineral and organic layer, darker in color than lower layers
E = Rich in quartz due to clay removal; zone of leaching; light in color
B = Rich in clays and precipitates; zone of accumulation
C = Fractured/fragmented rock; no accumulation from above; aka regolith
R = Unweathered/unaltered rock parent material (R not shown)
Question 1.
Examine the soil profile below exposed in a grassland. List each horizon in
order from the top horizon to the bottom horizon based on the given descriptions. The
scale in the image is in cm.
Horizon O - The top most layer, where it is composed mostly of organic matter or rich in
organic matter such as decomposing leaves and lacks mineral content.
Horizon A - It contains mostly minerals from parent material with organic material
incorporated.
Horizon E - commonly known as zone of leaching, contains light coloured mineral particles, a
zone of eluviation and leaching.
Horizon B - commonly known as zone of accumulation, where accumulation of clay particles is
transported from above.
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Question 2.
Now examine the soil profile below from a tropical rainforest environment.
List each horizon in order from the top horizon to the bottom horizon based on the given
descriptions Note the boy for scale.
Question 3.
Comparing the two soil profiles above from grassland and rainforest
environments. How are they similar? How are they different? Which soil horizons are
absent from each location? Hypothesize why a horizon may be missing or did not
form? In your hypothesis, your are encouraged to include a role for water/rainfall and
weathering.
Both regions have different soil profiles. The grassland has comparatively rich organic matter
but will be more acidic since it is grass. In the rainforest region,organic matter will be washed
out due to leaching and the O horizon will be missing.
Soil Forming Factors
Farmers and farming may be the words you associate with soils; however, ranchers, foresters,
and geologists are also interested in soils and how they form. All of these professions
understand that soils are not the same from one location to the next, and that they may differ in
composition, thickness, and texture. Crops, trees, and grass may grow well in one soil and not
grow well or at all in another soil. These differences reflect a variety of soil forming factors..
Climate
: Higher rainfall and warmer temperatures accelerate chemical weathering
and soil formation.
Bedrock
(R horizon): Soils can form on any rock type (aka the R horizon) and the
local rock type will strongly influence soil composition. For example, a soil forming on
basalt would be higher in iron than a soil forming on a quartz-rich (and therefore
iron-poor) sandstone. Unsurprisingly, all other things being equal, soils will tend to
form faster on unconsolidated material (e.g., volcanic ash) than hard bedrock (e.g.,
granite)
Slope
: Also unsurprisingly, all things being equal, more gently slopes will tend to
promote and support more soil formation.
Time
: Soils can form in one to tens of years in protected warm, moist environments
whereas they can take thousands of years to form in exposed cold, dry
environments..
Vegetation
: Plants add and extract different amounts or organic matter and nutrients
to and from soils. Some plants have shallow roots while others have deep roots, and
deeper roots tend to help keep soils intact by limiting erosion.
Question 4.
Which soil forming factor(s) likely played the biggest role in producing the
soil profile in Question 2 based on the information you were given (i.e., it formed in a
tropical rainforest and is composed of the horizons portrayed in the given image)?
The soil forming factors that play the biggest role in producing the soil profile of a rainforest is the
climate and physical weathering. The amount of rainfall in an area is crucial on whether the soil will face
a lot of chemical reactions, especially the high amounts of rainfall in a tropical rainforest.
Question 5.
Your answer to Question 4 may or may not have include climate and
vegetation, but let’s assume it did. Now explain in three or four sentences why you think
climate and vegetation played such a big role in the production of the profile. If your
answer to Question 4 mentioned more factors then climate and vegetation include those
in your answer as well.
In the production of a profile of a tropical rainforest, climate change affects the rainforest's
ability to contribute to evapotranspiration, which helps form clouds. If evapotranspiration is
reduced, there will be less cloud cover, meaning less rainfall and less rainfall may then
continue to contribute to worsening drought conditions. Since there is a lot of sunlight, there is
a lot of energy in the rainforest, this energy is stored in plant vegetation, which is eaten by
animals. The abundance of energy supports an abundance of plant and animal species
Soil Order Classification
Soil scientists worldwide have struggled to develop a classification scheme that everyone
agrees upon. In the United States, we use the U.S. Comprehensive Soil Classification System,
shown below, which defines twelve “soil orders” based on physical characteristics and
formation environment. For this laboratory, we will only focus on the three bolded soils, which
represent soil end-members (i.e., young versus old soils) and the soil in which the majority of
our food is grown.
Soil Orders of the U.S. Comprehensive Soil Classification System
Alfisol
Rich in subsurface clay accumulation and abundant nutrients; forms in humid
forests.
Andisol
Forms in volcanic ash
Aridisol
Low in organic matter with carbonate horizons; forms in arid environments
Entisol
No obvious horizons; relatively recently formed
Gelisol
Underlain with permanently frozen ground
Histosol
Very rich in organic debris; forms in swamps and marshes
Inceptisol
Moist with poorly developed horizons;
relatively recently formed
Mollisol
Soft, black, and rich in nutrients; forms in subhumid to subarid grasslands
Oxisol
Very weathered; Al-and Fe-oxide rich, nutrient poor; forms in tropical regions
Spodisol
Acidic, nutrient poor, ashy, Al- and Fe-oxide rich; forms in humid forests
Ultisol
Very mature, strongly weathered soils, nutrient poor
Vertisol
Clay-rich so can swelling when wetted; shrinking and cracking common when dry
Entisols
are common in arid regions, have no distinguishable horizons, and are essentially
unaltered from their starting composition, which can vary from loose sediment to solid bedrock.
In contrast,
Ultisol
are red clay rich soils produced by long periods of intense weathering in a
warm and wet climate, such as in the southeastern United States.
Mollisols
are the foundation
for much of the agriculture across the United States, and are characterized by a thick, dark
surface horizon produced through progressive addition of organic material. Mollisols are
particularly common across Illinois, Iowa, Kansas, Montana, Nebraska, North Dakota,
Oklahoma, and South Dakota.
Question 6.
Based on the above descriptions of entisol, ultisol, and mollisol, which best
fits the image in Question 1 of the grassland profile?
Mollisol
Question 7.
Which best fits the image in Question 2 of the tropical rainforest profile?
Ultisols
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The map below shows the global distribution of soil orders based on the U.S. Soil Classification
System. Notice entisols are displayed in aqua blue, ultisols in yellow, and mollisols in green.
Question 8.1:
The locations of entisols are shown in an aqua blue color in the soil orders
map. What one or two other soil type(s) are often found adjacent to entisols?
Aridisols and Entisols are together on the map, meaning they can coexist. This makes sense
because both Aridisols and Entisols share certain traits:
-Neither has well-defined layers, suggesting they're relatively new.
-Arid and semi-arid regions, where Aridisols are common, often have less developed soils.
-These soils are easily moved due to their instability, affected by wind direction.
-Geographically, they're usually found in arid places like the Sahara andCalifornian desert,
Question 8.2:
Look at the
Soil Orders of the U.S. Comprehensive Soil Classification
System
table above for the descriptions of the soil(s) in your answer to Question 8
above. Does it make sense that they would all be found alongside entisols? Why or why
not, and in what climate region are these soils typically found?
Yes, it does make sense that they would be found because both are found within a desert solid,
however in really dry regions of the sahara and australian outback we see entisols. Entisols are
like sand dunes which are too dry for major soil development.
Question 8.3:
Using the given latitudes in the map above, circle the combination of
latitudes below that best describes where the majority of entisols seem to occur.
A. 30°N and 60°N
C. 15°N and 45°N
B. 15°N and 15°S
D. 15°S and 45°S
Option C is CORRECT: The map shows a significant portion of Entisols between 15°N to 45°N
latitudes.
Question 9.1:
Ultisols are shown in yellow in the soil order map above. What are the one
or two other soil type(s) that can be found adjacent to ultisols?
Ultisols are shown in yellow in the soil order map above. What are the one or two other soil
type(s) that can be found adjacent to ultisols?
We can see oxisol being adjacent to ultisol.
Question 9.2:
Look at the
Soil Orders of the U.S. Comprehensive Soil Classification
System
table above for the descriptions of the soil(s) in your answer to Question 9
above. Does it make sense that they would all be found alongside ultisols? Why or why
not and in what climate region are these soils typically found?
Yes, it makes sense that they would be alongside each other as oxisol can be found in hot,
tropical climates and ultisol can be found in humid temperate and tropical areas of the world.
Both sharing that tropical climate shows why they can be next to each other.
Question 9.3
Using the given latitudes in soil order map, circle the combination of
latitudes below that best describes where the majority of ultisols seem to occur.
A. 30°N and 60°N
C. 15°N and 45°N
B. 15°N and 15°S
D. 15°S and 45°S
This is so that ultisols can form, which needs warm weather and lots of rain to do.
Question 10.1:
Based on the global soil map on the next page, mollisols and alfisols tend
to form adjacent to one another, and alfisol is the state soil of California (really, we have a
state soil!). Does this make sense based on their descriptions in the
Soil Orders of the
U.S. Comprehensive Soil Classification System
table? Why are these soils important to
you as a human?
This does make sense because in mesic or cool climates Alfisols often occur adjacent to
Mollisols (grassland soils). Both the soil have well developed horizons/layers and have thick soil
profile and Both soils are fertile in nature. As a human these soils are important because These
soils support Agriculture when aerated with water. These are rich in humus as well as minerals,
and thus, they support a huge variety of Agricultural and Horticultural crops.
Question 10.2:
Describe where in North America these soils form (i.e., are they only in the
US?). Where are the closest mollisols and alfisols to San Diego? What does that mean
about where we most likely get our food?
In North America, apart from US, these soils also found in Southern parts of Canada and
Northern parts of Mexico. San Diego lies on most of the South Western part of the US, and
these soils are also found there in the North Western part. We most likely get our food within
San Diego itself because these soils support Agriculture and livestocks. It is interesting to know
that San Diego has a Californian type of climate which is very suitable for viticulture and animal
farming. This city has lots of farmers.
Question 11.1:
The global map to the right shows arid regions in tan and light brown.
Circle the latitudes where the majority of arid regions are found.
A. 30°N to 60°N
B. 15°N to 15°S
C. 15°N to 45°N
D. 15°S to 45°S
Question 11.2:
Does your answer to Question 11 match your answer to Question 8.3?
Yes it does match, tan arid regions are at a mid latitude.
Question 12.1:
In general, when people refer to the tropics they mean the area around the
equator. In the map above, wet regions are shown in dark green. Circle the latitudes
below that best describe where the majority of tropical (wet) regions occur on Earth.
A. Between 30°N and 60°N
C. Between 15°N and 45°N
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B. Between 15°N and 15°S
D. Between 15°S and 45°S
The vast majority of tropical (wet) areas are located between the latitudes of 15° N and 15° S
Question 12.2:
Does your answer to Question 12.1 match your answer to Question 9.3?
Yes, the responses I provided in response to questions 9.3 and 12.1 match. I concluded that
the bulk of tropical (wet) locations on Earth are located between the latitudes of 15° N and 15° S
for both inquiries.
Question 13:
Notice that there are wet regions found on Earth other than those found at
the equator. Take a moment to locate these in the global map above based on the
predominance of green. With these locations in mind, take a look at the global soils map.
What soil type (entisol, ultisol, or mollisol) tend to be located in these wet areas?
Ultisols
Question 14:
In three or four sentences, summarize where on Earth entisols, ultisols, and
mollisols tend to form; include words like, arid regions, tan, wet regions, green, and
latitude in your answer.
Entisols are found in the tan areas of the upper parts of Africa and Australia. Ultisols tend to
develop in humid, warm, and green regions, often found in areas with high rainfall and lush
vegetation, like tropical rainforests. Mollisols, known for their rich, dark, and fertile soil, are
typically found in temperate regions at mid-latitudes with moderate moisture levels, where they
support productive agriculture, as seen in the fertile plains of the central United States.
Question 15:
Why should you care about mollisols?
Mollisols are mainly used as cropland. They are recognized as inherently productive and fertile
soils. They are extensively and intensively farmed, with extensive fibrous root systems, which
needs significant inputs of fertilizers and tillage. Mollisols are also important soils in pasture,
range and forage systems.