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ENVIRONMENTAL
SCIENCE
A Study of Interrelationships
Chapter
13
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14th Edition
Soil and Its Uses
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Outline
• 13.1 The Study of Soil as a
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Science
13.2 Geologic Processes
13.3 Soil and Land
13.4 Soil Formation
13.5 Soil Properties
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Outline
• 13.6 Soil Profile
• 13.7 Soil Erosion
• 13.8 Soil Conservation Practices
• 13.9 Conventional Versus Conservation Tillage
• 13.10 Protecting Soil on Nonfarm Land
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13.1 The Study of Soil as a Science
• Soil science is the study of soil as a natural resource on
the surface of the earth including soil formation,
classification and mapping; physical, chemical, biological,
and fertility properties of soils; and these properties in
relation to the use and management of soils.
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13.2 Geologic Processes
• The crust of the Earth is an extremely thin, less-dense
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solid covering over the mantle.
The mantle makes up the majority of the Earth, and
surrounds a small core of iron.
The outermost portion of the mantle is solid.
The crust and solid outer mantle are collectively known as
the lithosphere.
The asthenosphere is a thin layer below the outer mantle
capable of plastic flow.
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Structure of the Earth
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13.2 Geologic Processes
• Plate tectonics is the concept that the outer surface of
the Earth is made of large plates of crust and outer mantle
that are slowly moving over the surface of the liquid outer
mantle.
• Heat from the Earth causes the slow movement.
• Plates are pulling apart in some areas, and colliding in others.
• These building processes are counteracted by processes tending
to make elevated surfaces lower.
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Tectonic Plates
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Geological Processes
• Weathering processes are important in reducing the size
of particles that can then be dislodged by moving water
and air.
• Mechanical weathering results from physical forces that
reduce the size of rock particles without changing the
chemical nature of the rock.
• Freezing and thawing cycles
• Actions of plants and animals
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13.2 Geologic Processes
• Chemical weathering involves the chemical alteration of
rock in such a manner that it is more likely to fragment or
be dissolved.
• Rock fragments exposed to the atmosphere may oxidize or
hydrolyze.
• The process of loosening and redistributing particles is
called erosion.
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13.2 Geologic Processes
Physical Fragmentation by
Freezing and Thawing
An Eroded Landscape
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13.3 Soil and Land
• Land is the portion of
world not covered by
water.
• Soil is a mixture of
minerals, organic material,
living organisms, air, and
water that together support
growth of plant life.
• Good agricultural soil:
• 45% Mineral
• 25% Air
• 25% Water
• 5% Organic Matter
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13.4 Soil Formation
• Soil forming factors include the following:
Parent Material
Climate
Time
Topography
Biological
Factors
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13.4 Soil Formation
• Soil formation begins with fragmentation of parent
material.
• Parent material consists of ancient layers of rock, or more recent
deposits from lava flows or glacial activity.
• The first organisms to gain a foothold in modified parent
material also contribute to soil formation.
• Lichens form pioneer communities.
• Decomposition of dead lichens further alters underlying rock.
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13.4 Soil Formation
• Humus is the organic material resulting from the decay of
plant and animal remains.
• It mixes with top layers of mineral particles, and supplies needed
nutrients to plants.
• It creates a crumbly soil that allows adequate water absorption and
drainage.
• Burrowing animals such as earthworms bring nutrients up
from deeper soil layers, improving soil fertility.
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13.5 Soil Properties
• Soil texture is determined by the size of mineral particles
within the soil.
• Too many large particles (sand, gravel) lead to extreme leaching.
• Too many small particles (clay) lead to poor drainage.
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Soil Texture
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13.5 Soil Properties
• Soil structure refers to the way various soil particles
clump together.
• An ideal soil for agricultural use is loam, which combines the good
aeration and drainage properties of large particles with the nutrient
retention and water-holding ability of clay particles.
• In good soils, one-half to two-thirds of spaces contain air after
excess water has drained.
• A good soil is friable, which means that it crumbles easily.
• Protozoa, nematodes, earthworms, insects, algae, bacteria, and
fungi are typical inhabitants of soil.
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Soil Organisms
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13.6 Soil Profile
• The soil profile is a series of horizontal layers of different
chemical composition, physical properties, particle size,
and amount of organic matter.
• Each recognizable layer of the profile is known as a
horizon.
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Soil Profile
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13.6 Soil Profile
• Over 15,000 separate soil types have been classified in
North America.
• Most cultivated land can be classified as either grassland
or forest soil.
• Grassland soils usually have a deep topsoil layer. A lack of leaching
results in a thin layer of subsoil.
• In forest soils, which are typically high rainfall areas, the topsoil
layer is relatively thin, but topsoil leachate forms a subsoil that
supports substantial root growth.
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Major Soil Types
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13.6 Soil Profile
• O horizon is made of litter, undecomposed or partially
decomposed organic material.
• A horizon is the topsoil, or the uppermost layer. It
contains most of the soil nutrients and living organisms.
• E horizon is formed from leaching darker materials.
• Usually very nutrient poor.
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13.6 Soil Profile
• B horizon is the subsoil. It contains less organic matter
and fewer organisms, but accumulates nutrients leached
from topsoil. It is poorly developed in dry areas.
• C horizon is weathered parent material.
• R horizon is bedrock.
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13.6 Soil Profile
• Two features of
tropical rainforests
have great influence
over the nature of the
soil:
• High temperatures lead
to rapid decomposition
of organic matter, with
little litter.
• High rainfall leads to
excessive leaching of
nutrients.
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13.7 Soil Erosion
• Erosion is the wearing away and transportation of soil by
wind, water, or ice.
• Worldwide, erosion removes 25.4 billion metric tons of soil
per year.
• Made worse by deforestation and desertification.
• Poor agricultural practices increase erosion and lead to the
transport of associated fertilizers and pesticides.
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13.7 Soil Erosion
• Most current agricultural practices lose soil faster than it
can be replenished.
• Wind erosion may not be as evident as water erosion, but
is still serious.
• It is most common in dry, treeless areas.
• Great Plains of North America have had four serious bouts of wind
erosion since European settlement in the 1800s.
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Worldwide Soil Erosion
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13.8 Soil Conservation Practices
• When topsoil is lost, fertility is reduced or destroyed, thus
fertilizers must be used to restore fertility.
• This practice raises food costs, and increases sediment load in
waterways.
• Over 20% of U.S. land is suitable for agriculture, but only 2% does
not require some form of soil conservation practice.
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13.8 Soil Conservation Practices
• Agricultural Potential
• Worldwide:
• 11% of land surface is suitable
for crops.
• An additional 24% is in
permanent pasture.
• United States:
• 20% land surface suitable for
crops.
• 25% in permanent pasture.
• African Continent:
• 6% land surface suitable for
crops.
• 29% can be used for pasture.
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Poor and Proper Soil Conservation Practices
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13.8 Soil Conservation Practices
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Soil Quality Management Components
Enhance organic matter
Avoid excessive tillage
Manage pests and nutrients efficiently
Prevent soil compaction
Keep the ground covered
Diversify cropping systems
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Contour Farming
• Contour farming is tilling
at right angles to the slope
of the land. Each ridge
acts as a small dam.
• Useful on gentle slopes.
• One of the simplest methods
for preventing soil erosion.
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Strip Farming
• Strip farming is the
practice of alternating
strips of closely sown
crops to slow water flow,
and increase water
absorption.
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Terracing
• Terracing is the practice of
constructing level areas at
right angles to the slope to
retain water.
• Good for very steep land.
Terraces
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Waterways and Windbreaks
• Waterways are depressions in sloping land where water
collects and flows off the land.
• Channels movement of water.
• Windbreaks are plantings of trees or other plants that
protect bare soil from full force of the wind.
• Windbreaks reduce wind velocity, decreasing the amount of soil
that can be carried.
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Protection of Waterways Prevents Erosion
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Windbreaks
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13.9 Conventional Versus Conservation Tillage
• Plowing has multiple desirable effects:
• Weeds and weed seeds are buried.
• Crop residue is turned under, where it will contribute to soil
structure.
• Leached nutrients brought to surface.
• Cooler, darker soil brought to top and warmed.
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13.9 Conventional Versus Conservation Tillage
• Each trip over the field is an added expense to the farmer,
and at the same time increases the amount of time the
soil is open to erosion via wind or water.
• Reduced tillage is a practice that uses less cultivation to
control weeds and prepare soil, but generally leaves 1530% of soil surface covered with crop residue after
planting.
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13.9 Conventional Versus Conservation Tillage
• Conservation tillage further reduces amount of soil
disturbance and leaves 30% or more of soil surface
covered with crop residue.
• Mulch tillage: Tilling entire surface just prior to planting.
• Strip tillage: Tilling narrow strips that will receive seeds.
• Ridge tillage: Leaves ridges; the crop is planted on the ridge with
residue left between ridges.
• No till farming: Involves special planters that place seeds in slits cut
in the soil.
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13.9 Conventional Versus Conservation Tillage
• Positive Effects of Reduced Tillage:
• Wildlife gain winter food and cover.
• Less runoff results in reduced siltation of waterways.
• Row crops can be planted in sloped areas.
• Fewer trips over the field means lower fuel consumption.
• Two crops may be grown on a field in areas that had been
restricted to a single crop.
• Fewer trips over the soil means less soil compaction.
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13.9 Conventional Versus Conservation Tillage
• Drawbacks of Conservation Tillage
• Plant residue may delay soil warming.
• Crop residue reduces evaporation and upward movement of water
through the soil, which may retard the growth of plants.
• Accumulation of plant residue can harbor plant pests and diseases,
requiring more insecticides and fungicides.
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Comparison of Various Tillage Methods
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13.10 Protecting Soil on Nonfarm Land
• By using appropriate soil conservation practices, much of
the land not usable for crops can be used for grazing,
wood production, wildlife production, or scenic and
recreational purposes.
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13.10 Protecting Soil on Nonfarm Land
Noncrop Use of Land
to Raise Food
Forest and Recreational Use
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Summary
• The surface of the Earth is in constant flux.
• The movement of tectonic plates results in the formation
of new land as old land is worn down by erosive activity.
• Soil is an organized mixture of minerals, organic material,
living organisms, air, and water.
• Organisms affect soil building by burrowing into and
mixing the soil, releasing nutrients, and decomposing.
• The ability of soil to grow crops is determined by the
inorganic matter, organic matter, water, and air spaces in
the soil.
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Summary
• A soil profile typically consists of the:
• O horizon of litter
• A horizon, which is rich in organic matter
• E horizon, from which materials have been leached
• B horizon, which accumulates materials leached from above
• C horizon, which consists of slightly altered parent material.
• Soil erosion is the removal and transportation of soil by
water or wind.
• Proper use of conservation practices such as contour
farming, strip farming, terracing, waterways, windbreaks,
and conservation tillage can reduce soil erosion.
• Land unsuitable for crops may be used for grazing,
lumber, wildlife habitats, or recreation.
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