Download Soil and Its Uses

Soil and Its Uses
Chapter 14
1
Geologic Processes
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Crust - Extremely thin, less-dense solid.
Mantle - Makes up majority of earth, and
surrounds a small core of iron.
– Outermost portion is solid.
 Crust and solid outer mantle collectively
known as the lithosphere.
 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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Geologic Processes
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Plate Tectonics - Large plates of crust and
outer mantle slowly moving over surface of
the liquid outer 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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Geological Processes
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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 without
changing the chemical nature of the rock.
 Freezing and Thawing Cycles
 Erosion from Wind and Moving Water
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Physical Fragmentation
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Weathering
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Chemical - Involves the chemical alteration
of rock in such a manner that it is more likely
to fragment or be dissolved.
– 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 that together
support growth of plant life.
– Good Agricultural Soil:
 45% Mineral
 25% Air
 25% Water
 5% Organic Matter
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Soil Formation
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Soil formation begins with fragmentation of
parent material.
– Ancient layers of rock, or more recent
deposits from lava flows or glacial activity.
First organisms to gain a foothold in
weathered parent material also contribute to
soil formation.
– Lichens form pioneer community.
 Decomposition of lichens further alters
underlying rock.
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Soil Formation
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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.
Burrowing animals bring nutrients up from
deeper soil layers, improving soil fertility.
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Other Factors Influencing Soil Formation:
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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.
 Gravel
>2.0
mm
 Sand
0.05 - 2.0
mm
 Silt
0.002-0.05
mm
 Clay
< 0.002
mm
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Soil Texture
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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 intra-soil spaces
contain air after 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 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.
– Each recognizable layer of the profile is
known as a horizon.
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Soil Horizons
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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
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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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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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Major Soil Types
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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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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.
 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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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
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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Soil Conservation Practices
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Agricultural Potential
– Worldwide:
 11% land surface is suitable for crops.
 24% 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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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.
– Useful on gentle slopes.
Strip Farming - Alternating strips of closely
sown crops to slow water flow, and increase
water absorption.
Terracing - Level areas constructed at right
angles to the slope to retain water.
– Good for very steep land.
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Soil Conservation Practices
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Waterways - Depressions in sloping land
where water collects and flows off the land.
– Channels movement of water.
Windbreaks - Planting of trees or other plants
that protect bare soil from full force of the
wind.
– Reduces wind velocity decreasing the
amount of soil that can be carried.
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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 Tillage - Tilling narrow strips.
– Ridge Tillage - Leave ridge.
– No Till - Place seeds in slits.
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Positive Effects 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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Drawbacks of Conservation 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 Non-Farm 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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Fig. 14.1
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Fig. 14.2
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Fig. 14.4
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Fig. 14.5
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Fig. 14.7
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Fig. 14.8
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Fig. 14.10
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Fig. 14.11
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Fig. 14.12
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Fig. 14.15
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Fig. 14.16
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Fig. 14.17
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Fig. 14.18
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Fig. 14.19
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Fig. 14.20
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Fig. 14.21
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Fig. 14.22
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Fig. 14.23
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Fig. p.324
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Fig. p.326
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Table 14.2
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