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Earth Science: 5.2
SOIL
Soil
Key Concepts for Today . . . .

Today students will learn and identify
 What are the major components of soil
 What are the most important factors in soil
formation
 How soil varies with depth
 The three common types of soil
 How human activities affect the rate of soil
erosion
Soil

Soil, an important product of weathering,
covers most land surfaces. Along with air
and water, soil is one of our most
important resources.

All life depends on a dozen or so
elements that comes from the Earth’s
crust.

Once weathering and other processes
create soil, plants absorb the elements
and make them available to animals,
including humans.
Characteristics of Soil

Soil has four major components
 Mineral matter (broken down
rock)
 Organic matter (humus- the
decayed remains of
organisms)
 Water
 Air
Soil Composition
Good soil
composition

The proportions of these
components vary in different
soils.
45% mineral matter
5% organic matter

In a good quality surface soil,
mineral matter and organic
matter generally make up half
(50%) the mixture.
25% water
25% air
m ineral
m atter
25%

The other half consists of pore
spaces where air and water
circulate generally in equal
amounts (25% each)
45%
organic
m atter
w ater
air
25%
5%
Soil Composition

The percentage of organic matter in
soils vary greatly

Bog soils are composed almost entirely
of organic matter, decayed plant and
leaf material

Desert soils may contain only a small
amount of organic matter

In most soils, organic matter or humus
is an essential component. It is an
important source of plant nutrients and
increases the soil’s ability to hold
water.
Soil Composition

The water and air components of
soil are also vital for plant growth.

Soil water provides the moisture for
chemical reactions that sustain life

Soil water also provides nutrients in
a form that plants can use.

Air is the source of carbon dioxide
plants use to produce sugar during
photosynthesis
Soil Texture

Most soils contain particles of
different sizes

Soil texture refers to the
proportions of different particle
sizes.

To classify soil textures, the
US Department of Agriculture
has established categories
based on the percentages of
clay, silt, and sand in soil.
Soil Texture

For example: loam falls in the
area between 50% to 25%
sand, 25% to 50% silt, and
28% to 40% clay as shown by
the chart

In soils called loam, neither
clay, sand or silt is dominant.
Soil Texture

Texture strongly influences a
soils ability to support plant
life.

Sandy soils drain very quickly
and do not hold water while
clay-rich soils drain slowly.
Soil Texture

Plant roots often have difficulty
penetrating soils that contain a
high percentage of clay and
silt.

Loam soils are usually the best
for plant growth. They retain
water better and store more
nutrients than do soils
composed mainly of clay or
sand.
Soil Structure

Soil particles usually form clumps
that give soils a particular
structure.

Soil structure determines how
easily a soil can be cultivated and
how susceptible it is to erosion.

Soil structure also affects the
ease with which water can
penetrate the soil. This in turn
influences how much nutrients
get to the plant roots.
Soil Formation

Soil Formation: soil forms through the
complex interaction of several factors.

The most important factors in soil
formation are
 Parent material
 Time
 Climate
 Organisms
 slope
Soil Formation

Parent Material:

The source of mineral matter in soil
is known as parent material

bedrock is the native consolidated
rock underlying the planet

Soil that forms on bedrock is called
residual soil

Soil that forms on unconsolidated
deposits (areas of rock/minerals
built up on bedrock) is called
transported soil
Soil Formation: Parent Material

The nature of the parent
material influences soil in two
ways:
 First, because unconsolidated
deposits are already partly
weathered, they provide more
surface area for chemical
weathering. Therefore, transported
soil usually develops more rapidly
than residual soil develops
 Second, the chemical makeup of the
parent material affects the soil’s
fertility affecting the types of plants
that will grow.
Soil Formation:

The longer a soil has
been forming, the thicker
it becomes.

The parent material
largely determines the
characteristics of young
soils.

As weathering continues,
the influence of the
parent material can be
overshadowed by other
factors.
Time
Soil Formation: Climate

Climate has the greatest affect on soil
formation.

Variations in temperature and
precipitation influence the rate, depth,
and type of weathering.

For example; a hot, wet, climate may
produce a thick layer of chemically
weathered soil where a cold climate may
produce a thin layer of mechanically
weathered soil.

The amount of precipitation also affects
soil fertility, influencing the rate at which
nutrients are removed from the soil
Soil Formation: Organisms

The types of organisms and how many
there are in a soil have a major impact on
it’s physical and chemical properties.

Plants are the main source of organic
matter in soil. Animals and
microorganisms also contribute.
Earthworms , for example, mix soil as
they burrow and feed on the organic
matter it contains.

Microorganisms ( including fungi,
bacteria, and protozoans ) play an active
role in decomposing dead plants and
animals. Because organic matter
releases nutrients when it decays, it
contributes to soil fertility.
Soil Formation: Slope

The slope of land can vary greatly in a short distance. This variation
can result in many types of soils. Many of these differences are
related to the amount of erosion and the water content of the soil.

On steep slopes, erosion is accelerated, so soil is thin or
nonexistent.

On flat areas, erosion is minimal, so soils are typically thicker. The
soil tends to be more waterlogged resulting in darker, thicker soil with
more organic matter.
Soil Profile

The processes that form soil
work from the surface
downward

Soil varies in composition,
texture, structure, and color at
different depths.

These variations divide soil into
zones called soil horizons.
Soil Profile: A Horizon

The A horizon is commonly
known as the topsoil.

It’s upper part consists mostly of
organic matter, including loose
leaves and partly decomposed
plant structures.

It is teeming with insects, fungi,
and microorganisms.

The lower part of A is a mixture
of mineral matter and organic
matter
Soil Profile: B Horizon

The B horizon, or subsoil,
contains fine clay particles
washed out of the A horizon by
water that filters through the
pore spaces.

The B horizon is the lower limit
of most plant roots and
burrowing animals.
Soil Profile: C Horizon

Between the B horizon and the
unaltered parent material is the
C horizon, which contains
partially weathered parent
material.

While the A and B horizons
barely resemble the parent
material, the C horizon does.
Soil Types

Recall that climate is the
most important factor in
soil formation. Climate
also has a major affect
on the type of soil that
forms.

Three common types of
soil are
 Pedalfer
 Pedocal
 Laterite
Pedalfer Soil

Pedalfers usually form in temperate
areas that receive more than 63 cm
of rain each year.

This soil type is present in much of
the Eastern US, most often in
forested areas.

The B horizon in pedalfers contains
large amounts of iron oxides and
aluminum-rich clays giving it a
brown-red color.
Pedocal Soil

Pedocols are found in the dryer
western US in areas that have
grasses and brush vegetation.

Because chemical weathering
slower in dry climates, pedocols
generally contains less clay than
pedalfers.

Pedocols contain abundant
amounts of calcite or calcium
carbonate and are typically a light
grey-brown in color.
Laterite Soil

Laterites form in hot, wet, tropical
areas. Chemical weathering is intense
in these areas.

Laterites go deeper than soils that
develop in other areas over the same
amount of time.

The large amounts of water in these
climates filters out the calcite and silica
in these soils washing them out when it
rains.

The iron oxide that is left behind gives
the soil a distinctive red color.

Laterite contains almost no organic
materials and thus few nutrients.
Laterite is one of the poorest soils
there is for farming.
Laterite Soil

When dried, laterite becomes hard
and virtually waterproof. It is cut and
shaped into bricks and used for
construction in other parts of the
world
Temple of Angkor Wat, Cambodia
Soil Erosion

Because soils are necessary for the
growth of rooted plants, they are the
foundation of our own food supply.

The loss of topsoil in an area can
make an area no longer able to
sustain crops and grow food.

Destruction of topsoil through poor
farming practices is an important
environmental issue that affects
farmers all over Earth
Soil Erosion: Water and Soil Erosion

How Water Erodes Soil:
 Every time it rains,
raindrops strike the soil
with amazing force. Each
drop acts like a minute
bomb, blasting soil particles
off the surface. Water
flowing across the ground’s
surface in thin sheets than
carries these particles
away as sediments.
 Because thin sheets of
water move the soil
particles away, this process
is called sheet erosion.
Soil Erosion: Water and Soil Erosion

How Water Erodes Soil:

After flowing as a thin
sheet of water, the water
forms tiny streams called
rills.

As more water enters the
rills, they erode further
turning into deeper gullies,
eventually moving down
slope to streams.

These streams carry the
sediments away until the
water eventually slows
enough to deposit them.
Rates of Erosion:

In the past, soil eroded more
slowly than it does today
because the land was covered
with trees, grasses, plants and
forests. The roots of all these
hold the soil together and
prevent erosion.

Any human activities that
remove plants or vegetation;
such as logging, farming, and
construction, greatly accelerate
erosion.
Rates of Erosion:

Without plants, soil is more
easily carried away by wind
and water.

Water generally erodes soil
much faster than wind does.

During a strong drought
however, strong winds can
remove large quantities of soil
when all the plant material has
died and the soil is revealed.
Rates of Erosion:

The rate of soil erosion
depends on soil characteristics
and on factors such as climate,
slope, and type of vegetation.

In many regions, including
about one third of the world’s
farmlands, soil is eroding faster
than it is being formed.

This results in lower
productivity, poorer crop
quality, which threatens the
world’s food supplies
Sediment Deposition

Another problem caused by
excessive soil erosion is the
deposition of sediment.

Rivers that accumulate
sediments must be dredged to
remain open for shipping.

As sediments settle in reservoirs
they become less useful for
storing water, controlling floods
or generating electricity
Sediment Deposition

Some sediments are contaminated
with agricultural pesticides. These
wash down streams to rivers, lakes
and eventually the ocean,
contaminating the water supplies and
killing fish and plants.

Soil also contains soil nutrients. When
these nutrients wash into lakes, they
stimulate the growth of algae and
plants in the lake leading to algae
blooms crowding out other life forms in
the lake, killing the lake’s ecosystem.
Controlling Erosion

Although we can not eliminate
erosion, we can slow it down using
soil conservation measures and
erosion control techniques.

These measures include
 Planting rows of trees called windbreaks
 Plowing along the contours of hills to stop
sheet runoff
 Rotating crops
 Using erosion barriers to prevent
sediments from moving during
construction
Computer Lab:

Review the PowerPoint lecture and use resources
off the internet to write a one page report in Word
answering one of the following questions:
 How does climate affects soil formation? Explain with
three detailed examples.
 How can activity such as road construction affect soil
erosion? Explain and give three effective erosion control
techniques that prevent erosion in this situation.
 Which activity would cause more sediment to be
deposited in a river that flows through a gently sloping
valley: cultivating the valley or cultivating the hills that
surround the valley? Explain your answer fully.