HOMEWORK
• Text Book: Read pages 264-266. Do
questions 1-5 on page 266.
• Review Book: Read pages 83-85. Do
questions 17-23 on page 85.
• Write out questions and answers.
HOW DO SOILS
FORM?
How Do Soils Form?
• Soil is a mixture of weathered rock,
microorganisms, and organic remains
that usually covers the bedrock.
• The composition of the soil depends on
the rocks from which they weathered and
the local climate.
How is soil created from
rock?
• Physical weathering breaks solid rock into small
particles.
• Chemical weathering changes the minerals,
often increasing the clay content.
• Plants and animals add organic materials in the
form of waste products and the remains of dead
organisms.
• Therefore, soil is the result of mechanical and
chemical weathering and biological activity over
long periods of time.
How do we classify where a
soil is formed?
• The material from which a soil is formed
is called a its Parent Material.
• Based on the soil’s parent material, soil
can be classified as either a residual or
transported soil.
Soil Composition
• The parent bedrock determines what
kinds of minerals a soil contains.
• The parent rock and climatic conditions
of an area determine the length of time it
takes for soil to form.
Describing the Soil Types
• Because climatic conditions are the main
influence on soil development, soils are
often classified based on the climates in
which they form.
• The four major types of soil are polar,
temperate, desert, and tropical.
Soil Types
Polar Soils
– Polar soils form at high latitudes and high
elevations in places such as Greenland,
Canada, and Antarctica.
– These soils have good drainage but no distinct
horizons because they are very shallow,
sometimes only a few centimeters deep.
Soil Types
Temperate Soils
– Temperate soils vary greatly and are able to
support such diverse environments as forests,
grasslands, and prairies.
– The specific amount of rainfall in an area
determines the type of vegetation that will
grow in temperate soils.
– Grasslands, which have an abundance of
humus, are characterized by rich, fertile, soils.
Soil Types
Desert Soils
– Deserts receive low levels of precipitation.
– Desert soils often have a high level of
accumulated salts and can support only a
limited amount of vegetation.
– Desert soils have little or no organic matter
and a very thin A horizon, but they often have
abundant nutrients.
Soil Types
Tropical Soils
– Tropical areas experience high temperatures
and heavy rainfall, leading to the development of
intensely weathered and often infertile soil.
– The intense weathering combined with a high
degree of bacterial activity leave tropical soils
with very little humus and very few nutrients.
– These soils experience much leaching of soluble
materials, such as calcite and silica, but they
have high concentrations of iron and aluminum.
Soil Types
Soil Profiles
• A soil profile is the vertical sequence of
soil layers.
• A soil horizon is a distinct layer, or zone,
within a soil profile.
• There are three major soil horizons: A, B,
and C.
Soil Profiles
• A soil profile is a vertical cross section of
the soil that displays the horizons.
• Soil horizons are layers within the profile
that result from soil forming processes.
• The horizons can differ in texture,
structure, color, development, and parent
material.
• The horizons are named based on their
textural, color, and structural properties.
Soil Horizons
– Horizon A contains high concentrations of
organic matter and humus.
– Horizon B contains subsoils that are enriched
with clay minerals.
– Horizon C, below horizon B and directly above
solid bedrock, contains weathered parent
material.
Soil Horizons
• In mature soils, three distinct zones or soil
horizons can be seen in the soil profile.
• The O horizon has a very high
accumulation of organic matter above the
mineral portion of the soil.
• The A horizon, called topsoil, has a high
accumulation of organic matter and tends to
be very dark in color because it usually
contains Humus.
• Humus is organic matter that forms from
decayed plants and animals.
Soil Horizons
• The B horizon begins with the subsoil. The
subsoil is usually red or brown from the ironoxides that formed in the A horizon and
were washed down into the B horizon. It is a
zone of clay accumulation.
• The C horizon is composed of weathered
unconsolidated parent material.
Typical New York State
Section Assessment
1. Match the following terms with their
definitions.
A. the vertical sequence of
soil layers
C
___
residual soil
A transported soil B. soil located above its
___
parent material
D
___
soil profile
C. a distinct layer, or zone, within
B
___
soil horizon
the vertical
sequence of soil layers
D. soil that has been moved
to a location away from its
parent bedrock
Section Assessment
2. What differences would you expect to
find between soil profiles taken from a
slope and a valley floor?
Soils on slopes tend to be thin, course,
and infertile, whereas soils formed in
valleys tend to be thick and fertile.