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Introduction to Soils
Laboratory Exercise #1
By: Carlin Conk
Definition of Soils
Soil is the collection of natural bodies on earth’s
surface containing living matter and supporting, or
capable of supporting plants. Its upper limit is the
atmosphere (air) or water, and at its lateral margins
it grades to deep water or barren areas of rock and
ice. Its lower limit is normally considered to be the
lower limit of the common rooting zone (root zone) of
the native perennial plants, a boundary that is
shallow in the deserts and tundra and deep in the
humid tropics.
Soil Profile
A soil profile is the
vertical display of
soil horizons.
Soil Pedon
Pedon is a 3-D unit that
represents the entire soil
body and is the smallest
volume of soil that
shows all of the
characteristic properties
of a particular soil.
Typically represents 10100 ft2 of surface area.
A group of pedons is
calles polypedons.
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
There are more than 20,000 soil series in the United
Soil Series Example
Soil Color
Soil color can suggest soil properties that may
influence plants.
Dark soil near the surface may indicate a high
organic matter content, which may lead to
easier cultivation and higher nutrient content.
Soil Color in Young & Old Soils
A young, unweathered soil may have a soil of
uniform color.
In older, weathered soils, yellow and red colors
may dominate well below the surface of the
In North Carolina, we typically have bright red
subsoils, would this indicate young or old
soils? Weathered or unweathered?
Soil Forming Factors
CL - Climate
OR - Organisms
P - Parent Material
T - Time
T – Topography
=CLORPTT (an easier way to remember the
soil forming factors)
Climate refers to rainfall and temperature.
The main effects are: weathering, the
production of organic matter , and the
decomposition of organic matter.
Climate of North Carolina
In North Carolina, rainfall is fairly similar across the
Piedmont and Coastal Plain. However, the rainfall is
more variable in the mountains of North Carolina.
Coastal Plain
Organisms or Biotic Activity
Vegetation is the main biotic factor.
The type of vegetation affects the soil color
and organic matter content, especially in the A
(or top) horizon.
Grasslands vs. Forests
Trees contribute less organic matter to the soil each
year when compared to grasslands.
The top horizons, or A horizons, in forests are
usually thinner than A horizons in grasslands.
OM content
Parent Material
Parent materials are defined as the materials
underlying the soil and from which the soil
There are many different parent materials, including:
Residual minerals and rocks (bedrock)
Glacial deposits
Loess deposits
Alluvial and marine deposits
Organic deposits
Residual Rocks and Minerals
Residual minerals weather in place to form soils.
Glacial Deposits
Glacial deposits formed as
massive ice sheets, moving
across North America,
approximately one million
years ago.
As the glaciers expanded,
they "bulldozed" rocks,
minerals and soil in front of
As the ice sheets melted, the
exposed parent material
began to weather and soil
was formed.
Glacial Soils
Boulders and Rocks
Loess Deposits
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
Loess Deposits Example
Alluvial Deposits
Alluvial deposits
refer to sediments
carried by and
deposited in fresh
Flooding events
carry rocks and
minerals, which
are deposited in
Marine Deposits
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
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
Organic Soils of North Carolina
Time as a Soil Forming Factor
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.
“Old” Soils
"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
Young soils have
weakly developed
horizons and may often
lack E and B horizons.
This soil profile lacks
any horizon
Absolute Age
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.
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.
Soils on a Steep Slope
As slopes increase, soils
become shallower and
have thinner and fewer
This is because the
steeper the slope the
greater the runoff and
the greater the erosion.
Review of Soil Forming Factors
Parent Material
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.
Soil Horizon & Layer Designation
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 – O Horizon
0 Horizon: Organic horizons
are dominated by organic
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.
O Horizon
Master Horizon – A Horizon
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
Properties of the A horizon
may reflect plowing,
pasturing or similar
Master Horizon – E Horizon
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
Color is lighter than the
overlying A horizon.
Master Horizon – B Horizon
B Horizon: Mineral
horizon that includes
layers in which
illuviation or
accumulation of
materials has taken
Clay, iron and
aluminum oxides from
the overlying E horizon
have accumulated here.
Master Horizon – C & R Horizons
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
Transitional Horizons
Transitional horizons
are layers of soil
between two master
Subordinate Distinctions
Subordinate distinctions
are specific features
within master horizons
that are designated by
lowercase letters.
There are many
subordinate distinctions
such as:
p – plow layer
t – accumulation of clay
b – buried layer