Soil Worksheet
Part 1: Soil Formation
Soil is a mixture of weathered rock & organic matter that usually covers bedrock (solid rock that
underlies all soil). Both chemical & mechanical processes are involved in the development of
soils.
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Chemical weathering turns hard minerals into soft ones
Mechanical weathering breaks solid rock into smaller pieces
Plant & animals add organic materials in the form of waste products & dead organisms
The decay of organic matter produces acids which accelerate chemical weathering
Burrowing Animals, such as earthworms, insects & rodents, help circulate air and water
through the soil & mix mineral & organic remains.
The material from which soil forms is called its
parent material. Soil that has weathered directly
from the bedrock beneath it and therefore matches
its parent material is called residual soil.
Soil that does not match the bedrock it is over is
called transported soil. It did not weather from the
bedrock beneath it but was brought there by agents
of erosion such as winds, rivers or glaciers. Much
of New England & the Midwest are covered by soil
that was deposited by the movement of glaciers
after the last Ice Age.
A cross section of soil exposed by digging is called the soil profile. The weathering of soil
produces layers known as soil horizons. The topsoil or A horizon is usually rich in dark-colored
organic remains called humus (labeled O horizon below). The subsoil or B horizon contains
minerals that have been transported deeper by
groundwater. Most of the clay in soil has also been
washed down to this layer. The partially weathered
bedrock or C horizon is composed of broken up
bedrock on top of the solid bedrock (parent material).
Soil erosion is the removal of topsoil by the action of
running water or wind. It takes between 100 & 400
years for one centimeter of topsoil to form.
Loss of topsoil can be caused when plant roots are no longer present to hold down soil. Salting
roads can raise the salinity of the soil and kill the plants. Over grazing can kill plants. Winds,
construction, & mining can all effect plant cover.
Means of soil conservation include the following:
 Windbreaks – belts of trees along the edge of fields
 Contour farming – crops are planted in rows
parallel to land
 Terraces – flattening hill slopes to slow the flow of
water & erosion
 Strip Cropping – a crop that leaves bare ground
between rows is alternated with a crop that completely
covers the ground ex. Corn & Alfalfa
 No-till method – plowing, planting and fertilizing
are all done at the same time so there is less change of wind
removing topsoil
Use the worksheet above to answer the following questions.
_____ 1. Which layer in the diagram below contains the most organic material?
a. A
b. B
c. C
d. the bedrock
_____ 2. How is soil created from rock?
a.
b.
c.
d.
Physical weathering without chemical weathering
Chemical weathering without physical weathering
Erosion without weathering
Weathering without erosion
_____ 3. Approximately how many years does one
centimeter of topsoil take to form?
a.
b.
c.
d.
100-400 years
10-40 years
1000-4000 years
10,000-40,000 years
_____ 4. Which of the following is found in the greatest % in soil?
a. Mineral matter
b. Organic matter
c. Water
d. Air
_____ 5. Which layer of a soil profile forms first from the bedrock?
a. A horizon
b. B horizon
c. C horizon
d. Humus
For each of the soil profiles below, label the horizons (A,B or C) and the parent material in each
of the soil profiles using the spaces provided next to each image.
Part 2: What is the Difference between Sand, Silt and Clay
Sand is a naturally occurring granular material composed of finely divided rock and mineral
particles. The composition of sand is highly variable, depending on the
local rock sources and conditions, but the most common constituent of
sand in inland continental settings and non-tropical coastal settings is
silica (silicon dioxide, or SiO2), usually in the form of quartz.
The second most common type of sand is calcium carbonate, for
example aragonite, which has mostly been created, over the past half
billion years, by various forms of life, like coral and shellfish. It is, for
example, the primary form of
sand apparent in areas where
reefs have dominated the
ecosystem for millions of years
like the Caribbean.
Silt is granular material of a size somewhere between sand and clay whose mineral origin is
quartz and feldspar. Silt may occur as a soil or as suspended sediment (also known as suspended
load) in a surface water body. It may also exist as soil deposited at the bottom of a water body.
Silt has a moderate specific area with a typically non-sticky, plastic feel. Silt usually has a floury
feel when dry, and a slippery feel when wet. Silt can be visually observed with a hand lens.
Clay minerals are typically formed over long periods of time by the gradual chemical
weathering of rocks, usually silicate-bearing, by low concentrations of carbonic acid and other
diluted solvents. These solvents, usually acidic, migrate through the weathering rock after
leaching through upper weathered layers. In addition to the weathering process, some clay
minerals are formed by hydrothermal activity. Clay deposits may be formed in place as residual
deposits in soil, but thick deposits usually are formed as the result of a secondary sedimentary
deposition process after they have been eroded and transported from their original location of
formation. Clay deposits are typically associated with very low energy depositional
environments such as large lakes and marine basins. The distinction between silt and clay varies
by discipline. Geologists and soil scientists usually consider the separation to occur at a particle
size of 2 µm (clays being finer than silts), sedimentologists often use 4-5 μm, and colloid
chemists use 1 μm.
Primary clays, also known as kaolins, are located at the site of formation. Secondary clay
deposits have been moved by erosion and water from their primary location
Sources: Wikipedia
Particle Size:
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1. Summarize what you learned about the difference between silt, clay and sand in 3-5
sentences.
Part 3: Soil Triangle
Scientists have created classes which break the distribution of particle sizes (soil textures) into
12 categories. Textural Triangle 3 is one of the tools soil scientists use to visualize and
understand the meaning of soil texture names. This textural triangle is a diagram which shows
how each of these 12 textures are classified based on the percent of sand, silt, and clay in each.
Note: these percentages are based on the USDA definition of sand and silt only.
Follow these steps to determine the textural class name of your soil sample:
1.
2.
3.
4.
The percent sand is read up slanting to the left.
The percent silt is read down slanting to the bottom.
The percent clay is read straight across.
The descriptive name of the soil sample (textural class) is written in the shaded area where
the mark is located. If the mark should fall directly on a line between two descriptions, record
both names. You are looking for where the three lines intersect!!!! Remember, your percents
should add up to 100!
5. So for example: Let’s say you have a sample that is 66.4 % Sand, 12.8% Silt and 20.8%
Clay. To find the name of the soil sample you would trace the following below:
You would get the name of the soil sample to be: Sandy Clay Loam!
Now try it yourself! You can use tracing paper over your triangle so it’s easier to read the
names.
%Sand
%Silt
%Clay
a.
75
10
15
b.
10
83
7
c.
42
37
d.
52
21
e.
35
50
f.
30
g.
5
h.
55
i.
21
70
40
45
10
Texture Name