INTRODUCTION TO SOILS
LABORATORY EXERCISE #1
OBJECTIVES

To understand the definition of: soil, soil profile, soil horizon, topsoil, and subsoil.

To be able to separate different soil horizons based on color.

Understand how soil properties are determined by interaction of the five soil forming
factors.
INTRODUCTION
One of the objectives of this course is to teach you what soil is and to help you develop a
vocabulary of basic terms used to describe, use, and manage soils as they exist outdoors. The
purpose is to enable you to distinguish between soils and soil features by use of color
variation.
Today's lab exercise is designed to get you started in examining soils and describing what you
see. The first step is to define a soil.
Soil is the collection of natural bodies on the earth's surface containing living matter and
supporting, or capable of supporting plants. Its upper limit is Atmosphere (air) or water, and
at its lateral margins it grades to deep water or barren areas of rock or ice. Its lower limit is
normally considered to be the lower limit of the common rooting (root zone) of the native
perennial plants, a boundary that is shallow in the deserts and tundra and deep in the humid
tropics.
Soils are highly variable in their biological, physical and chemical properties. Therefore,
examination of the soil is necessary to determine the kinds and ranges of these properties.
The properties are then used to classify the soil. Soil classification is focused on a three
dimensional volume or unit (i.e., length by width by depth). In the field the three dimensional
unit that represents the entire soil body is called a pedon. A pedon is the smallest volume of
soil that shows all the of characteristic properties of a particular soil. The topographical
surface area of a pedon is quite small, often between 10 and 100ft2. The depth extends down to
the bottom of the plant root zone. It is not practical to classify all of the soils of the world on
a pedon by pedon basis. However it is practical to group pedons that are similar in properties
into larger groups called polypedons. These polypedons are essentially soil series. A soil series
includes soils that have developed from similar materials by similar processes resulting in
similar appearances and properties. The characteristic properties of a soil series are unique.
Because of soil variability, there are many possible combinations of properties that make a
unique soil. In fact there are approximately 17,000 soil series in the United States alone.
Soil color can suggest soil properties that may influence plants. Dark soil near the surface
usually indicates a high organic matter content. This soil is often easy to cultivate and has a
high nutrient content. A soil of uniform color from top to bottom is often indicative of an
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unweathered, young soil. Young soils usually have properties other than color which are
uniform and they provide an excellent root environment. Yellow and red colors in the soil
well below the surface are typical of older, weathered soils. These soils often have a
decreased permeability for roots, water and air. Gray zones or gray and rust colored blotches
indicate poor drainage and a lack of good aeration.
SOIL FORMING FACTORS
Studies of soils throughout the world indicate that soils are dynamic natural systems. Their
physical chemical and biological properties are determined by the combined effect of climate
and biotic activities, as modified by topography, acting on parent material over periods of
time.
PARENT MATERIAL
Parent materials are defined as the materials underlying the soil (C horizon) and from which
the soil developed (in most cases). Soil characteristics such as water holding capacity and soil
fertility are determined in part by the soil parent material. Soils may develop from a number of
different parent materials. Examples would be things like:
1.
2.
3.
4.
5.
Residual Minerals and Rocks
Glacial Deposits
Loess Deposits
Alluvial and Marine Deposits
Organic Deposits
Residual rocks and minerals weather in place to form soils. Minerals are naturally occurring
inorganic substances with characteristic chemical composition and physical properties. There
are primary and secondary minerals. Primary minerals are formed when molten rock cools
and solidifies. Secondary minerals are formed by the weathering of primary minerals. Rocks
are combinations of two or more minerals. There are three major groups of rocks: igneous,
sedimentary and metamorphic. Igneous rocks form when molten rock cools and solidifies. This
is the most abundant group of rocks within the earth's crust. Sedimentary rocks result from the
erosion and deposition of sediments. Metamorphic rocks form from igneous and sedimentary
rocks exposed to extreme heat and/or pressure.
As minerals and rocks weather, they form sand, silt, and clay particles. These particles are
susceptible to transport by wind, water and ice and are referred to as transported parent
material. The three modes of transport are responsible for glacial, loess, alluvial, and marine
deposits. Glacial deposits, formed as massive ice sheets, moving across North America
approximately one million years ago, "bulldozed" rocks, minerals and soil in front of them. As
the ice sheets melted, the exposed parent material began to weather and soil was formed. Loess
deposits formed as high speed winds picked up predominantly silt-sized particles and carried
them across open areas. As the wind speed slowed the particles fell to the ground covering
the native soil and parent material. This new parent material has since been weathered to
form the productive soils of the Midwest. Much like wind, water can transport material.
Alluvial deposits refer to sediments carried by and deposited in fresh water. Marine
sediments refer to sediments carried by fresh water but deposited in salt water. Marine
sediments can build up over long periods of time until eventually they are quite deep. As the
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earth's crust changes the marine deposits can be elevated above sea level and subjected to
weathering. The results are the productive soils common to the Atlantic coastal plains. The last
of the parent materials we will discuss are organic deposits. Organic deposits originate from
plants that have died or shed their leaves. Organic deposits are common to areas such as
swamps and marshes since the chemical and biological process that decompose the organic
matter are greatly limited by the saturated conditions.
In the Piedmont and Mountain land regions of North Carolina, parent materials change when
the rock type changes. Coastal Plain soils are formed from sediments, both alluvial and
marine. There are three types of alluvial deposits: Beaches, Floodplain, and Deltas. These
sediments have similar minerals, so parent material differences are related to changes in the
amounts of sand, silt, and clay. The properties of delta soils are different from those of
floodplain soils in part because of differences in the sand, silt, and clay. Properties of parent
materials within the same landform vary if changes in texture occur. For example, a single
floodplain may contain pockets of sands and clays at different locations. These differences
produce changes in soil water holding capacity and fertility. Two different parent materials
deposited side by side (same climate, biotic, topography, and age) will result in two soils
having different properties.
CLIMATE
In soil formation, climate refers to rainfall and temp. The main contribution of temperature
and rainfall is the effects on weathering, OM production, OM Decomposition. In North
Carolina, climate primarily affects the soil organic matter content, which is easily seen by
noting the color of the A horizon. As its organic matter content increases, the A horizon
becomes blacker and thicker.
In North Carolina mean annual rainfall varies across the state, ranging from 50-60 inches along
the coast, and from 44-48 inches in the Piedmont. Mean annual rainfall in the mountains
varies from less than 40 inches to 104 inches in isolated areas! In the southwestern North
Carolina mountains, moist southerly winds are forced upward causing cooling, which
produces the high rainfall.
Average annual temperature along the Southeastern coast is 20 degrees greater than
temperatures at the highest mountain elevations. The higher the annual temperature, the lower
the soil organic matter content. This is because more decomposition of organic matter occurs
as temperature increases.
BIOTIC FACTORS
Vegetation is the main biotic factor. The type of vegetation affects the soil color and organic
matter content, primarily the A horizon. The most important vegetation types are forests and
grasslands. Trees contribute less organic matter to the soil each year than grasses. As a result,
the A horizons in forested areas are usually thin and contain less organic matter than soils
which formed under grasses. When considering vegetation effects, consider the natural
vegetation. Remember that soil formation occurs over thousands of years; therefore, the
vegetation that existed before agricultural crops were planted is most important. However,
years of farming may have drastically changed the appearance of the soil, primarily by soil
erosion, which removes the dark A horizon.
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TOPOGRAPHY
Topography consists of three parts: elevation, slope, and aspect. Slope is the tilt or inclination
of the land. Elevation is the height above mean sea level. Aspect is the direction the slope is
facing. As slopes steepen, soils become shallower and have thinner and fewer horizons. This
is because the steeper the slope the greater the runoff and the greater the erosion. This is only
true when you compare soils at the same point on the slope. For example, two soils may have
the same slope, but if one occurs at the top of the slope and the other at the bottom, then
eroded soil from the upslope soil may be deposited on the downslope soil.
Landscape position affects soil drainage. As elevation decreases soil drainage becomes poorer
and soils stay wet or waterlogged (saturated) for longer periods of time. This is because water
flows downhill, both over the surface and through the subsoil. Water seeping into the soil at
the top of the hill will eventually drain down to the soil in the lower elevations. Soils at lower
elevations stay saturated longer than upslope soils because more water is supplied to these
soils and because the water has no place to drain. Long periods of saturation result in a dark
color in the A horizon. Soils at higher elevations have bright colors in the B horizon, while
soils that occupy in between positions have mottled colors in the B horizons.
TIME
We may regard time as continuous yet still recognize a "time zero" for a given soil. Time zero
is the point in time at which a catastrophic event (flood or earthquake) is completed and a
new cycle of soil development is initiated. Time is important in soil formation because it
determines the degree to which the other soil forming factors express themselves. A soil
scientist does not think of soil age in terms of years or inches but rather in terms of horizon and
profile development. "Old" soils are those that have experienced intense weathering of parent
material in the presence of biotic factors. These soils will have well developed profiles
containing A, E, and B horizons. Young soils have weakly developed horizons and may
often lack E and B horizons. This is the idea of relative stage of development.
Climate can greatly affect the relative stage of development. Under warm, moist conditions a
soil profile will develop strong horizons in a shorter period of time compared to a soil under
dry, cool conditions. If the absolute age of a soil is of interest, a soil scientist may measure the
activity of radioactive carbon. Knowing the activity of the radioactive carbon the scientist
can estimate the age of the parent material that has been subjected to weathering. Using this
technique, scientists can determine the effect of topography on relative soil development.
Soils on level upland positions have parent materials that have weathered for thousands (in
some case millions) of years. These soils have well-developed A, E and B horizons. On slopes
where soil is lost by erosion, fresh parent material is weathered to replace the lost soil. Because
soil formation is a slow process, soils on steep slopes may have only A and C horizons and
are usually young soils. Soils in lowland positions that stay wet may also have only A and C
horizons.
To develop E and B horizons, clay and chemical ions (cations and anions) must be leached
out of the A horizon and accumulate in the B horizon. This movement of clay and chemical
ions, known as leaching, requires that water move downward through the soil. If soil is
saturated, downward movement of water is restricted, so little clay and chemicals leach into
the B horizon. Thus, many saturated soils have little horizon development and therefore are
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considered young soils. Saturation of soils causes the soil forming processes and weathering
to occur with less intensity than in better drained soils.
SOIL HORIZON AND LAYER DESIGNATION
Horizon designations differ from country to country. In the United States soil horizons are
designated by a code of letters and numbers developed by soil scientists of the National
Cooperative Soil Survey. Master horizons are major layers designated by capital letter such as 0, A,
E, B, C and R. Master horizons are described below. Transitional horizons are layers of soil
between two master horizons. Subordinate distinctions are specific features within master
horizons that are designated by lowercase letters. Transitional horizons and subordinate
distinctions will be further discussed in a later laboratory exercise.
0 Horizon: Organic horizons are dominated by organic material. The 0 horizons contain
organic litter from plants and animals. 0 horizons are usually present on the soil surface except
in the case of peats and mucks where the 0 horizon extends almost to the bottom of the soil.
A Horizon : Mineral horizons that have formed at the soil surface or just below the 0
horizon. The A horizon may contain some organic material mixed with mineral material.
Properties of the A horizon may reflect plowing, pasturing or similar activities.
E Horizon : Mineral horizon in which the major characteristic is loss of clay, iron and
aluminum oxides by eluviation or leaching. An increase in concentration of sand and silt size
particles of resistant minerals occurs as clay is leached to lower depths. Color is lighter than the
overlying A horizon.
B Horizon: Mineral horizon that includes layers in which illuviation or accumulation of
materials has taken place. Clay, iron and aluminum oxides from the overlying E horizon have
accumulated here.
C Horizon: Mineral horizon consisting of unconsolidated, partially weathered material that
is neither soil or rock. The horizon is below the zone of most biological activity. The upper
layer of the C horizon may become part of the B horizon as weathering continues.
R Layer: Underlying consolidated bedrock
DESCRIBING SOILS
As a soil develops on the landscape, distinct layers or bands parallel to the earth's surface
may form. These layers or bands are called soil horizons. Soil horizons, are soil layers that
differ from the overlying and underlying layers in some property, such as color, clay content,
abundance of cracks, etc. Color is one property that is commonly used to separate different
soil horizons. We may now improve our definition of soil profile. A soil profile is a vertical
slice of the soil showing the different horizons and their thickness.
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LABORATORY ACTIVITY 1:
Examine the 4 soil profiles linked below. Use the official series descriptions (also linked)
to complete exercise 1.
1.
2.
Separate each profile into 3 major horizons on the basis of color.
Find the depth and thickness of each of the three horizons in the series
description
3.
Describe the color or colors of each horizon. Use colors such as red, yellow,
brown, black, and gray.
Use the form to record your observations. Note that each soil profile is given a name. This
name is the soil series.
NAME:
Profile #1 Soil Series:
Horizon
1.
2.
3.
Profile #2 Soil Series:
Horizon
1.
2.
3.
Profile #3 Soil Series:
Horizon
1.
2.
3.
Profile #4 Soil Series:
Horizon
1.
2.
3.
LABORATORY ACTIVITY # 1
LAB SECTION:
Cecil
Series Description
Depth
Color
Altavista
Series Description
Depth
Color
Fuquay
Series Description
Depth
Color
Leon
Series Description
Depth
Color
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LABORATORY ACTIVITY 2:
DIFFERENCES IN SOIL FORMING FACTORS
Use the official series description of to compare differences in soil forming factors among
the given series. In the tables below, the soil forming factors are listed in the left column. For each
factor, decide whether differences occur between the listed soil series. If no differences occur, write
"same" in the middle column.
Norfolk vs Cecil
Soil Forming Factor
Parent Material
Differences
marine sediments/ residual rock
Norfolk
SOIL FORMING FACTOR
(photo) vs. Cecil ( Photo)
DIFFERENCES
Parent Material
Climate
Biotic Activity
Topography
Time
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Norfolk (photo) vs Rains (photo)
SOIL FORMING FACTOR
DIFFERENCES
Parent Material
Climate
Biotic Activity
Topography
Time
Norfolk (photo) vs French (photo)
SOIL FORMING FACTOR
DIFFERENCES
Parent Material
Climate
Biotic Activity
Topography
Time
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Norfolk (photo) vs Ponzer
SOIL FORMING FACTOR
DIFFERENCES
Parent Material
Climate
Biotic Activity
Topography
Time
STUDY QUESTIONS
1.
Define soil, pedon, soil series, soil profile, soil horizon, topsoil, subsoil, eluviation and
illuviation.
2.
Is the lower boundary of soil a precise line? If not, why not?
3.
What is the upper limit of a soil profile?
4.
How do soil horizons differ from soil profiles?
5.
What is the difference between topsoil and subsoil?
6.
List 3 characteristics of topsoil.
7.
List 3 characteristics of subsoil.
8.
First, list the five factors of soil formation, and then:
Explain how each of the factors might influence the depth of leaching in a soil profile.
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