SOIL PROFILE DESCRIPTION
Soil – 206 Soil Ecosystem Lab
Objectives
After completion of this lab a student should be able to:
1. Define the terms soil profile, horizon, texture, structure and concentration.
2. Describe how textural class is determined using the feel method.
3. Understand how to use the soil textural triangle.
4. Identify and describe four structure types and indicate their probable location in a soil profile.
5. List 5 soil colors and give a possible cause for each color.
Introduction
The word soil, in a general sense refers to all of the unconsolidated material occupying the earth’s
surface. Soil is a mixture of varying proportions of inorganic mineral and rock particles, living and organic
matter, and voids or pores which contain variable amounts of air and water. It develops at the interface
between the atmosphere and lithosphere (bedrock), forming a blanket ranging in thickness from a few
centimeters to two meters or more.
Soil is the medium from which most plants derive mineral nutrients and water. Soil also provides physical
support for both plants and animals including humans and the structures they build. As you proceed
through these lab exercises, keep in mind that a soil is not an inert, unchanging material. Rather, at any
one time, a soil may be undergoing many simultaneous physical, chemical, and biological changes.
A distinction may be made between the soil (in the general sense) and an individual soil body. An
individual soil body, called a polypedon by soil scientists, is a three dimensional body with definite
recognizable boundaries. Its upper boundary is the earth’s surface, and its lower boundary is the lower
limit of biological activity and weathering. A polypedon is bounded laterally by other soils with properties
different from those of the polypedon being considered. Thus, the general term “soil” is actually a
collective term for a large number of individual soils, each having its own distinguishing characteristics.
Refer to p. 198 in Gardiner and Miller, 10th edition, for further discussion of the pedon/polypedon
concept.
The concept of the polypedon comes from the need to study and communicate information about soils in
a systematic manner. To establish a polypedon, soil scientists first determine the kind and range of soil
properties that characterize each soil. Second, each soil is assigned a name, such as Palouse silt loam,
or Jory clay. This system permits division of “the soil” (again in the general sense) into many separate
and individual units. In a manner similar to plant identification, a specific soil may be referred to by a
common name or by a taxonomic name. For example, the taxonomic name (genus and species) for a
tomato plant is Lycopersicon esculentum. Similarly, the taxonomic class (at the family level) for the
Palouse soil loam is Fine-silty, mixed, mesic Pacheco Ultic Haploxeroll.
As just implied, an individual soil body occupies a certain definite section of the landscape. Soils vary
from one another in their properties and each has a unique internal organization. A soil profile is a
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single vertical cross section of soil extending from the surface into the underlying unweathered parent
material. The soil profile is composed of horizons (horizontal layers of soil) which may be characterized
by physical, chemical, and biological properties. Horizons may be divided into major categories
corresponding to the surface soil (O and A horizons), the “subsurface” soil (E and B horizons), and
the substrate (C and R horizons).
Three of the most basic physical properties used to describe soil profiles are texture, structure, and
color. The following exercises are designed to give you some experience in describing these properties.
Soil Texture
Soil texture is defined as the relative percentage of sand, silt and clay in a soil sample. Hence, soil
texture is concerned with the size of individual mineral particles. A major division in the size of soil
particles is made at a particle diameter greater than 2 mm. Soil particles with a diameter greater than 2
mm belong to the coarse fraction while soil particles less than 2 mm belong in fine earth fraction. The
fine earth fraction itself is divided into three main size classes, according to the U.S Department of
Agriculture:
Classification of Soil Particles by Maximum Diameter
US Department of Agriculture
0.002mm
0.05mm
Clay
Silt
0.10mm
0.25mm
0.50mm
1.0mm
2.0mm
Very Fine
Fine
Medium
Coarse
Very Coarse
Sand
>2.0mm
Gravel
It is unlikely that a soil will consist of mineral particles of a single size category. Normally a soil will
contain some combination of sand, silt and clay in addition to other organic and inorganic constituents.
Soils having similar proportions of sand, silt, and clay are grouped into one of the twelve textural classes.
The textural triangle is designed so that any combination of sand, silt and clay can be placed in a
textural class and assigned a name.
Texture is probably the single most important physical property variable determining such fundamental
soil properties as fertility, water-holding capacity and susceptibility to erosion as well as its influences on
drainage, aeration, plant available water, ease of tillage, and the chemical and physical condition of the
soil. Differences in many of these properties among soils can be attributed to the strong dependence of
texture on soil mineralogy. Quartz, feldspars and micas dominate the sand and coarse silt fractions,
while the much more reactive oxides and clay minerals dominate the clay and fine silt fractions. The
importance of texture as a fundamental soil property is further emphasized by its relative permanence.
Soil texture can change only over very long periods of time through erosion, mineral weathering or
translocation of particles through the soil profile.
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The following table indicates the effect of texture on some soil properties (Soils, Dubbin, 2001).
Property
Textural Class
Water-holding Capacity
Drainage Rate
Water Erosion Susceptibility
Wind Erosion Vulnerability
Cohesion, stickiness, shrink-swell
Inherent Fertility
Ease of Pollutant Leaching
Ease of Compaction
Clay
Silt
Sand
High
Slow (unless cracked)
Moderate
Low
High
High
Low (unless cracked)
High
Moderate
Moderate
High
High
Moderate
Moderate
Moderate
Moderate
Low
Fast
Low
Moderate
Low
High
High
Low
Two methods exist for textural analysis, particle size analysis (mechanical analysis) and the feel
method (hand texturing). One type of mechanical analysis, called the hydrometer method, involves
dispersing a soil sample in water and determining the sedimentation rate of the sand, silt, and clay
particles. This method will be addressed in a later laboratory session.
The second method of textural analysis allows for the determination of the textural class of a soil without
the aid of laboratory equipment. This method, the feel method, is commonly used to estimate soil
texture in a field situation. An experienced person can accurately estimate the sand, silt, and clay content
of a soil sample with this method. A soil containing large quantities of sand will feel gritty when rubbed
between your fingers. Silt has been described as having the feel of flour. A soil high in clay will be
somewhat sticky (depending on the type of clay and the moisture content) and can usually be molded
like modeling clay.
Sample number
1
2
3
% sand
% silt
% clay
Textural class
Soil Structure
Structure refers to the arrangement of sand, silt, clay and organic matter into larger units called
aggregates. Aggregates that form naturally are called peds, whereas those that form artificially, as
during plowing or digging, are known as clods. Peds, which may range form one to several hundred
millimeters in size, develop through soil-forming processes over decades and centuries as the soil
matures. However, human interference can very quickly modify or destroy this structure. Soil structure
greatly influences water infiltration, susceptibility to erosion, and ease of root penetration and seedling
emergence. For these reasons, much effort has been directed towards understanding the factors that
promote and maintain good soil structure. We will examine the chemical (abiotic) and biological (biotic)
characteristics of peds in future labs, so this lab section is focused on the identification and location of
peds.
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The individual soil particles that comprise peds are held together by binding agents including organic
matter, clays, calcium carbonate, and iron oxides and are described in terms of their shape or type. The
following table is a brief introduction to the classification of peds (Dubbin, 2001, pp 20-21).
Type
Size
Horizon
Description
Granular
1 to 10mm
A
Also know as spheroidal, rounded, common in soils with
high organic matter, loosely packed
Platy
1 to 10mm
A and B
Thin, horizontal, plate-like, common in leached horizons or
at depth, maybe from compaction
Blocky
5 to 50mm
B
Prism-like
10 to 200mm
B
Roughly cube-shaped, found in sub-surface, facilitate
good aeration, drainage, root penetration
Vertical oriented columns, found in sub-surface, height
and shape vary, often too dense for root penetration, tops
can be rounded (columnar) and often associated with high
sodium
Soil Color
Color can provide soil scientists and land users with many clues about the genesis and mineralogy of a
soil, provided the observers understand the cause of the various colors and are able to interpret the
colors in terms of soil properties. Information concerning organic matter content, mineralogy, drainage,
and aeration may be discerned from color and this information applied towards the management and
potential uses of a particular soil. It should be stressed however that this information must be used with
educated caution.
The colors of soil are derived largely from organic matter and minerals. Dark brown to black colors at
or near the surface of a soil profile generally indicate an accumulation of organic matter (more
specifically called humus). The oxidation state of iron and manganese also influence soil color. Red,
yellow and reddish-brown colors in soil are often the result of oxidized iron (Fe3+) and manganese (Mn2+).
Thus, yellow and reddish-brown colors may indicate the soil is well drained and well aerated.
When the oxygen availability is limited, such as in saturated soils, iron and manganese usually exists in
their reduced states-- Fe2+ and Mn+. Under these conditions, the soil color will be more subdued shades
of gray and blue. This condition is referred to as gleyed soil color. A grayish-blue coloration in the lower
profile may indicate that the soil is poorly aerated.
Poorly drained soils may also exhibit flecks or spots of orange and yellow. This mottled soil color
indicates a zone of alternate oxidizing and reducing conditions caused by seasonal fluctuations in the
water table. The water table is usually at its highest point during late winter causing saturation of the
lower portion of the profile. Iron and manganese are converted to their soluble reduced forms under
these poorly aerated conditions. During the dry season, the water table recedes and the availability of
oxygen in the profile increase. Iron and manganese are converted to their insoluble oxidized forms under
these well-aerated conditions. The insoluble iron and manganese will precipitate locally as brightly
colored iron and manganese deposits called concentrations. The proximity of concentrations to the
surface of the soil indicates the approximate depth of the seasonal water table.
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Whitish-gray soil colors may be the result of several processes and explaining their presence depends
largely on climatic considerations and the positions of the color in the soil profile. Whitish-gray colors
found near the surface, or overlaying a clay layer, may indicate a zone of extensive leaching in which
only the light-colored silicate minerals remain. This type of leaching frequently produces albic horizons
(Gardiner and Miller, 193). If whitish-gray colors are found in a subsurface horizon of an arid region soil,
the color may be due to accumulation of lime (CaCO3), gypsum (CaSO4), or other salts. These horizons
are commonly found in arid climates where there is insufficient moisture to leach the soluble salts out of
the profile. Lastly, in certain regions, whitish-gray colors may indicate deposits of volcanic ash.
Color is the sensation produced when light of a particular wavelength enters the human eye.
Unfortunately, everyone does not perceive nor describe color in the same manner. The color of light is
most accurately described by measuring its three principal properties, hue, value, and chroma. Hue
refers to the dominant wavelength of the light. Value, also called brilliance, refers to the total quantity of
light. It increases from dark to light colors. Chroma is the relative purity of the dominant wavelength of
light. It increases with decreasing proportions of white light. Refer to p. 45-48 in Gardiner and Miller for
further discussion of soil color.
The Munsell Soil Color System is a standardized color designation that specifies the relative degree of
the three properties of color. The Munsell color notation can be quickly determined by comparison of a
soil sample with a standard set of color plates. The color notation (e.g. 10 YR 6/2) can be translated into
a more conventional color name. For instance, 10 YR 6/2 corresponds to the color name light brownish
gray.
Horizons
The soil forming processes alter the parent material and produce a series of horizons that comprise the
soil profile. The type of horizons, their thickness and sequence, help to define the soil and its properties.
Often the easiest way to view a profile is to examine a road cut, many of which are sufficiently long to
reveal how profiles vary across a landscape.
The accumulation of degradation products of organic matter near the surface of the soil profile
produces a dark-colored horizon (A). In many forest soils, leaf litter and other organic debris accumulate
at the mineral surface, forming an organic (O) horizon. If leaching and weathering have been intense, a
light-colored eluvial (E) horizon may be present below the A horizon. During this leaching, clays, oxides
and carbonate minerals that have been washed from the upper part of the profile accumulate in an
underlying (illuvial) layer known as the B horizon. The A, E and B horizons together comprise the
solum. Below the B horizon is a layer, the C horizon, which is unconsolidated but has been relatively
unaffected by soil-forming processes. The solum and C horizon are collectively called the regolith,
which is defined as the unconsolidated mantle above bedrock (R). Because weathering begins at the
surface, the upper horizons have been altered the most, while the horizons at the bottom of the profile
are most like that of the unaltered parent material.
The letters O, A, E, B and C designate the master horizons. These upper case letters give an indication
of only very general horizon properties. If a more detailed description is needed, lower case letters may
be included to designate subordinate distinctions within the master horizons. For example, Bt indicates
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a B horizon in which clays have accumulated while Bk indicates an accumulation of carbonates in the B
horizon. Numerals preceding the master horizon designation indicate different geologic parent materials
(discontinuities) while a lower case ‘b’ indicates a buried horizon. Transition horizons are indicated
through the use of both horizon designations with the horizon having the more dominate features
appearing first, as an EB horizon would indicate a transition that is more like an E than a B horizon.
Even though soil classification is an involved and lengthy process the above information will be enough
so you can begin to identify the principle features and characteristics of your two assigned soils. The
worksheet and study questions will facilitate this experience.
Table 1.1: Additional Clues to the Feel of Textural Classes
SAND, LOAMY SAND
-almost all sand
-individual grains easily seen and felt
-moist soil forms a cast that crumbles when squeezed
SANDY LOAM
-sand dominates noticeably
-moist soil forms a cast that can be gently handled
LOAM
-can feel all three soil separates but none dominates
-moist soil forms a cast that be freely handled
-cast may be squeezed to form short, broken ribbons
SILT LOAM
-dry soil has both a smooth and gritty feel
-forms a stable cast when moist
-short, broken ribbons (<2.5 cm) may be formed
SANDY CLAY LOAM
-feels very gritty yet moist soil will form a cast
-medium ribbons (2.5 to 5.0 cm) may be formed
CLAY LOAM
-moderate grittiness
-medium ribbons (2.5 to5.0 cm) may be formed
SILTY CLAY LOAM
-feels smooth, little grittiness
-medium ribbons (2.5 to 5.0 cm) may be formed
CLAY, SILTY CLAY, AND SANDY CLAY
-often sticky, however, stickiness varies with clay type
-long ribbons (>5.0cm) may be formed
-cast is often very tough to work between thumb and finger
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