Download Soil and Its Uses

ENV 301: Environmental Science
A Study of Interrelationships
Discussion notes: Scott M. Graves
Text: Enger • Smith
Ninth Edition
Chapter 14
Soil and its Uses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Soil and Its Uses
Chapter 14
Chapter Outline:
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Geologic Processes
– Weathering
Soil Formation
Soil Properties
– Profile
– Horizons
Soil Erosion
Soil Conservation
– Tillage Practices
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Geologic Processes
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Crust— Extremely thin, less-dense soil.
Mantle—Makes up majority of earth.
– Surrounds core of iron.
– Inner portion is solid.
– Outer portion capable of flow.
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Structure of the Earth
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Geologic Processes
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Plate Tectonics—Large plates of crust and
outer mantle (lithosphere) slowly moving
over liquid surface of mantle.
– Heat from earth core causes movement.
 Plates are pulling apart in some areas,
and colliding in others.
Building processes counteracted by
processes tending to tear down land.
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Tectonic Plates
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Weathering
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Weathering—Factors bringing about
fragmentation or chemical change of parent
material.
– Mechanical —Results from physical forces
reducing size of rock particles.
 Temperature changes and abrasions are
primary agents.
– Chemical—Rock fragments exposed to
atmosphere may oxidize, or otherwise
chemically change.
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Soil and Land
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Land—Portion of world not covered by water.
Soil—Mixture of minerals, organic material,
living organisms, air, and water.
 Role of organisms in soil development is
very important.
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Soil Formation
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Parent material—Ancient layers of rock, or more
recent deposits from lava flows or glacial activity.
Humus—Decaying organic material.
– Mixes with top layers of rock particles, and
supplies needed nutrients to plants.
– Creates crumbly soil which allows adequate
water absorption and drainage.
Soils can take 100’s to 1000’s of years to fully
develop. “non-renewable” resource?
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Soil Components
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Soil Components
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Other Factors Influencing Soil Formation:
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Earthworms
Burrowing animals
Plant roots
Bacteria and fungi (decomposers)
Position on slope
Climate
Time
Rainfall
Soil pH
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Soil Properties
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Texture—Determined by the size of mineral
particles within the soil.
– Too many large particles leads to extreme
leaching.
– Too many small particles leads to poor
drainage.
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Pore Spaces and Particle Size
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Soil Properties
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Structure—Refers to the way various soil
particles clump together.
– In good soils 2/3 of the intra-soil spaces
contain air after the excess water has
drained.
 Friable—Crumbles easily.
– Protozoa, nematodes, earthworms,
insects, algae, bacteria, and fungi are
typical inhabitants of soil.
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Soil Properties
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Soil Profile
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Soil Profile—A series of horizontal layers of
different chemical composition, physical
properties, particle size, and amount of
organic matter.
Horizon—Each recognizable layer of the
profile.
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Soil Horizons - “Topsoil”
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O Horizon— (Litter)—Un-decomposed or
partially decomposed organic material.
A Horizon—(Topsoil) Uppermost layer—
contains most of the soil nutrients and
organic matter.
E Horizon—Formed from leaching darker
materials.
– Not formed in all soils.
– Usually very nutrient poor.
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Soil Horizons - “Subsoil”
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B Horizon—(Subsoil) Underneath topsoil.
Contains less organic matter and fewer
organisms, but accumulates nutrients
leached from topsoil.—Poorly developed in
dry areas.
C Horizon—Weathered parent material, very
little organic material.
R Horizon—Bedrock.
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Soil Profiles
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Mixtures
– Over 15,000 separate soil types have been
classified in North America. However,
most cultivated land can be classified as
either grassland or forest soil.
Grassland Soils—Usually have a deep A
Horizon—Low rainfall limits topsoil leaching.
A Horizon supports most root growth.
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Soil Profiles
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Forest Soils—Topsoil layer is relatively thin,
but topsoil leachate forms a subsoil that
supports substantial root growth. (High
rainfall areas)
– Tropical Rainforests
 Two features of great influence:
 High temperatures
 Rapid decomposition, little litter.
 High rainfall
 Excessive leaching of nutrients.
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Soil Erosion
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Erosion—Wearing away and transportation
of soil by wind, water, or ice.
– Worldwide removes 25.4 billion tons/yr.
– 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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Worldwide Soil Erosion
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Soil Erosion
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Most current agricultural areas lose topsoil
faster than it can be replenished.
Wind erosion may not be as evident as water
erosion, but is still
common.
QuickTime™
and a
TIFF (Uncompressed) decompressor
are needed
see thistreeless
picture.
– Most common
intodry,
areas.
– Great Plains have had four serious bouts
of wind erosion since late 1800s.
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Soil Degradation
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Soil Conservation Practices
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When topsoil is lost, fertility is reduced or
destroyed, thus fertilizers must be used to
restore fertility.
– Raises food costs and increases sediment
loads in waterways.
– Over 20% of U.S. land is suitable for
agriculture, but virtually all of it requires
some form of soil conservation practice.
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Soil Conservation Practices
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Contour Farming—Tilling at right angles to
the slope of the land. Each ridge acts as a
small dam.
Strip Farming—Alternating strips of closely
sown crops to slow water flow.
Terracing—Level areas constructed at right
angles to the slope to retain water.
– Good for very steep land.
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Terraces
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Soil Conservation Practices
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Waterways—Depressions in sloping land
where water collects and flows off the land.
Windbreaks—Planting of trees or other
plants that protect bare soil from full force of
the wind.
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Conventional vs. Conservation Tillage
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Plowing has multiple desirable effects:
– Weeds and weed seeds are buried or
destroyed.
– Crop residue is turned under.
 Decays faster and builds soil structure.
– Leached nutrients brought to surface.
– Cooler, darker soil brought to top and
warmed.
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Problem:
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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— Uses less cultivation to
control weeds and prepare soil, but
generally leaves 15-30% of soil surface
covered with crop residue after planting.
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Problem:
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Conservation Tillage—Further reduce
amount of disturbance and leaves 30% or
more of soil surface covered with crop
residue.
 Mulch Tillage—Tilling entire surface just
prior to planting.
 Strip & Ridge Tillage—Tilling narrow
strips.
 No Till—Place seeds in slits.
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Pros of Reduced Tillage
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Wildlife gain food and cover.
Less runoff—reduced siltation of waterways.
Row-crops can be planted in sloped areas.
Fewer trips means lower fuel consumption.
Double-cropping
Fewer trips means less soil compaction.
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Cons of Reduced Tillage
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Plant residue may delay soil warming.
Crop residue reduces evaporation and
upward movement of water through the soil.
Accumulation of plant residue can harbor
plant pests and diseases requiring more
insecticides and fungicides.
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Protecting Soil on Nonfarm Land
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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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Chapter Summary
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Geologic Processes
– Weathering
Soil Formation
Soil Properties
– Profile
– Horizons
Soil Erosion
Soil Conservation
– Tillage Practices
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