Soil Formation
Lesson Plan: NRES B2-4
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Anticipated Problems
1. What are five different factors that affect soil
formation?
2. What are some different types of parent material
that affect soils?
3. What is topography, and how does it affect the
development of soil?
2
Anticipated Problems
4. How do living organisms affect the development
of soil?
5. What is weathering? How does weathering and
time affect soil?
6. How does climate affect soil development?
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Terms
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alluvium
bedrock
chemical weathering
climate
glacial till
glacier
loess
native vegetation
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organic matter
outwash
parent material
physical weathering
prairie soil
timber soil
topography
Factors that Affect
Soil Formation
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Parent material—the type of rock material from
which the soil is formed
Topography—the slope characteristics of the soil
Living organisms—the organisms, including plant
material, that live within the soil
Factors that Affect
Soil Formation
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Time or weathering—the age
of the soil or the climate
where the soil is found
Climate—the temperature and
moisture characteristics of the
area where the soil was
formed
Weathering
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Refers to mechanical
forces caused by
temperature changes
such as heating,
cooling, freezing, and
thawing
Weathering causes
soils to develop,
mature, and age
Weathering
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Water moving through soil
leaches away; many soluble
portions
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Soil becomes acidic because of
limestone loss
Soil looses fertility in acidic
soils= expensive
In permeable soils (permits
movement) fine particles
move downward from
surface soil to subsoil
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Causes higher amount of clay
in subsoil.
Weathering
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This is a continuous process, as soil is destroyed
more is formed
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Balance is the problem
Major Weathering Forces
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Temperature
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Water Action
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Some minerals dissolve when
exposed to water
Plant roots
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Heating and cooling causes rocks
to crack into smaller pieces
Force exerted by growing plants
can become quite great
When crack forms in rocks, soil
soon forms in cracks = plant
growth
Living Organisms
Topography/Slope
Major weathering forces cont..
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Ice Expansion
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Water freezes in ice=
expansion
Mechanical grinding
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Wind, glaciers
Climate and Location
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Greatly affects the rate of weathering by the
temperature.
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The warmer the temperature the more organisms there
are.
This increases the chemical reaction time in breaking down
materials.
Slope and location of a field also affect erosion and
drainage.
High Temperatures along with tree cover and
moisture on leaf residue = acidic soils
Water Action/Rainfall
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Areas of High rainfall are usually leached and
somewhat acidic.
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Leaching – process in which contents are removed from
the soil
To much water prevents or describes microbial
growth and may exclude air by water logging.
Water and temperature have the effect of swelling
and contracting of soil particles.
Living Organisms
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Causes the decay of organic material
Microbes are microscopic plants and animals, which
help in the decaying process.
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Release CO2 and other essential compounds
Insects, worms, and help in this process by mixing
the various soil materials.
Factors causing Factors to
Vary
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Type of vegetation
Mechanical compaction
Weather
Crop production practices
Tillage practices
Geographical Location
Types of Parent Material
Affecting Soil
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Parent material is formed by the disintegration
and decomposition of rock.
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Classified according to the way it was moved
and scattered
Types of Parent Material
Affecting Soil
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A glacier is a large, long-lasting river of ice that
is formed on land and moves in response to
gravity.
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Two major glaciers, the Illinoisan and the
Wisconsinan, had the greatest influence on presentday soils in Illinois.
Types of Parent Material
Affecting Soil
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Soils of glacial origin are classified:
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Loess
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Outwash
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Glacial till
Soils of Glacial Origin
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Loess originated from the blowing of the soil
after the glaciers melted and dried.
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The single most desirable soil parent material.
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Well-balanced mineral content
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Medium texture
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Excellent water-holding capacity
Soils of Glacial Origin
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Outwash originated when the glaciers melted.
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Melted waters carried the gravelly materials away to
be deposited below the glacial ridges.
Sandy outwash was carried farther downstream
The finer materials—silt and clay—were deposited in
lakebeds or slow-moving streams.
Soils of Glacial Origin
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Glacial till is parent material that was physically
moved and deposited by the glacier itself.
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Often contains a variety of sizes of soil particles that
have not been layered from the effects of wind or
water (as have the other two types of glacial soils)
Pebbles and various sizes of boulders are common in
till.
Types of Parent Material
Affecting Soil
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Alluvium is generally a water-borne material
deposited on bottomlands.
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Results of recent sediments deposited by streams
as they flood
Courtesy USDA
Types of Parent Material
Affecting Soil
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Marine Deposits - Deposits left from ancient
ocean floors
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Residual Deposits - Form in Place
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Colluvial Deposits - Deposited by gravity
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Lacustrine Deposited - Deposited by lakes
Types of Parent Material
Affecting Soil
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Bedrock is solid rock usually under the soil or
other loose material.
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Often buried by loess, outwash, glacial till, or alluvium
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3 Categories of Rock
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Ingneaous
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Metamorphic
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Sedimentary
Types of Parent Material
Affecting Soil
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Organic matter is decayed or partially decayed
plants and animals, roots, and living organisms.
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Organic soil is found where formerly shallow ponds
supported swamp vegetation.
The wet conditions slowed decay of the dead plants
so that organic matter could accumulate.
Two types: peat and muck
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Technicians process soil samples for
organic matter analysis. (courtesy
USDA)
What is Topography?
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Topography is the slope characteristics of a soil.
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Degree or steepness
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Length
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Shape
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Direction of a slope
What is Topography?
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These factors influence the
amount of rainwater runoff or
the amount that enters the
soil or collects in small
depressions on the soil
surface.
What is Topography?
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Soils on steep slopes have higher amounts of
runoff and erosion than those on level
topography.
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Soils in depressions and on nearly level topography
are likely to have poor or very poor natural
drainage.
Soils on moderately sloping to steep topography
are usually well drained.
Topography
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Lay of the land
Items in topography
include
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Slope
Drainage
Plants around
Topography
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Slope and drainage affect soil formation both
directly and indirectly.
Steep slopes cause soil runoff of water.
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Breaks up soil.
Adds deposits to lower levels
Exposes lower layer of soil (subsoil).
Percolation – process in which water passes
through the soil.
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Soils on level land = poor drainage
Soils on sloped land = better drainage
Topography
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Drainage affects water table
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When soil is saturated little or no air can penetrate.
Lack of air reduces action of microbes and other soil
forming activities.
Poor air and water movement causes marbling.
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Marbling – discoloration of the soil.
Poor drainage and excessive water retards plant
growth.
Slope
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Amount of rise and fall
of the land in a set 100
feet.
Rise/Run
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Given as a percentage.
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There is a 12 foot rise
in 75 feet.
12/75 = 16% slope
Types of Organisms
Affecting Soil
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Organisms that live in soil—plants, animals, and
microbes—actively affect soil formation.
The greatest effect on the development of soil is
from plants that once grew in it. These plants are
referred to as native vegetation.
Types of Organisms
Affecting Soil
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Native vegetation determines the kind and amount
of organic matter in the soil. Two types in Illinois:
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Tall prairie grass
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Deciduous-hardwood forests
Types of Organisms
Affecting Soil
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Soils in the areas of this vegetation are called:
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Prairie soil
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Timber soil
Types of Organisms
Affecting Soil
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Prairie soil has a dark and deep surface layer.
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Roots from the prairie grass filled the top of the soil
to a depth of 1 to 2 feet or more.
Partial decay of these roots over a
long period gave the soil high
organic-matter content
Types of Organisms
Affecting Soil
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Timber soil tends to have a thin, moderately
dark layer.
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Due to organic matter accumulating on the surface,
where decay occurs more rapidly.
When tilled, this dark material is mixed with the soil
below to produce a lighter color.
Types of Organisms
Affecting Soil
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Other living matter that influences the
development of soil includes various kinds of
animal life.
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Earthworms, crawfish, ground
squirrels and other burrowing
animals, and various insects that
incorporate organic matter into
the soil.
How Weathering & Time
Affects Soil
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Weathering plays a major role in the formation of
soil. Weathering is the decomposition of rocks.
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Physical weathering
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Chemical weathering
How Weathering & Time
Affects Soil
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Physical weathering—the effects of climatic
factors, such as temperature, water, and wind.
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Freezing and thawing is a major contributor
How Weathering & Time
Affects Soil
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Chemical weathering—the effects of factors
that change the elemental makeup of rock and
break it down.
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Rainwater is mildly acidic and can
slowly dissolve many soil minerals.
Some minerals react with oxygen in
the atmosphere.
Oxidation further acts to decompose rock.
How Weathering & Time
Affects Soil
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Weathering causes soil to
develop, mature, and age.
How Weathering & Time
Affects Soil
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Develop—Soil develops rapidly.
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Plant nutrients are released, and organic matter
accumulates.
Soils will develop faster in humid regions than in
arid regions.
How Weathering & Time
Affects Soil
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Mature—Mature soil is at peak productivity, with a
high amount of organic matter.
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Water begins leaching away nutrients
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Plant growth starts to decline.
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This results in less organic matter.
How Weathering & Time
Affects Soil
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Age—Minerals continue to break down, and clay is
leached into the subsoil.
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The soil becomes lighter in color
from less organic matter.
Courtesy USDA
Climate Affects Soil
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Elements of climate influence soil formation.
Climate is the weather in a region over a long
period.
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Three elements of climate related to soil
development are temperature, rainfall, and wind.
Climate Affects Soil
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These factors are either directly or indirectly
responsible for the breakdown of rocks and
minerals, the release of plant nutrients, and many
other processes affecting the development of soils.
Climate Affects Soil
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The climate in Illinois is said to be of the
continental type.
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Characterized by hot summers and cold winters.
Enhances the weathering process through freezing
and thawing.
Climate Affects Soil
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Rainfall during the growing season is similar
across the state; however, southern Illinois
receives more rainfall during the winter and early
spring.
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Soils in humid regions are subject to more leaching
than soils in dry regions.
Rainfall wears the rock away a little at a time.
Climate Affects Soil
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Wind also wears the rock away. Particles carried
by the wind strike the rock and wear it down.
Review
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How are the soils of glacial origin classified?
What is topography and the factors that describe
it?
What various kinds of animal life might influence
the development of soil?
Name the two types of weathering that play a
major role in the formation of soil.