Download chapt13_lecture-Fall-2011

Environmental Science
A Study of Interrelationships
Twelfth Edition
Enger & Smith
Chapter 13
Soil and Its Uses
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Soil and Its Uses
Outline









Geologic Processes
Soil and Land
Soil Formation
Soil Properties
Soil Profile
Soil Erosion
Soil Conservation Practices
Conventional Versus Conservation Tillage
Protecting Soil on Nonfarm Land
Geologic Processes
Structure of the Earth
Layered
Earth
Geologic Processes
 The crust of the Earth is an extremely thin, lessdense 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.
Plate Tectonics
Fig. 2-32
Geologic Processes
Tectonic plates
Geologic Processes
Tectonic plates
Plate tectonics
Fig. 2.5
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.
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
Geologic Processes
Physical fragmentation by freezing and thawing
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.
Geologic Processes
An eroded landscape
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
Soil and Land
The components of soil
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.
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.
Soil Formation
 Other factors influencing soil formation include:
•
•
•
•
•
•
•
Plant roots
Bacteria and fungi (decomposers)
Position on slope
Climate
Time
Rainfall
Soil pH
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.
Soil Properties
Soil texture
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 waterholding 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.
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.
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.
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.
Soil Profile
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.
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.
Soil Profile
Major soil types
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.
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.
Soil Erosion
Worldwide soil erosion
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.
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.
Soil Conservation Practices
 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.
Soil Conservation Practices
 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.
 Strip farming is the practice of alternating strips
of closely sown crops to slow water flow, and
increase water absorption.
Soil Conservation Practices
Contour farming
Soil Conservation Practices
Strip farming
Soil Conservation Practices
 Terracing is
the practice of
constructing
level areas at
right angles to
the slope to
retain water.
• Good for very
steep land.
Terraces
Soil Conservation Practices
 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.
Soil Conservation Practices
A well-maintained, uncultivated waterway
Soil Conservation Practices
Windbreaks
Conventional vs. 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.
Conventional vs. 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 15-30% of soil surface covered
with crop residue after planting.
Conventional vs. 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.
Conventional vs. 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.
Conventional vs. 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.
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.
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.
Summary
 The ability of soil to grow crops is determined by
the inorganic matter, organic matter, water, and
air spaces in the soil.
 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.
Summary
 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.