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Soils
What is soil?
• Thin layer on the earth’s surface that is
made by the interaction of five factors:
rocks, sunlight, water, air, and living
organisms
Soil Composition
The average soil is mainly composed of
mineral particles that come from rocks.
Below is the composition of an average
soil.
Taken from http://www.physicalgeography.net/fundamentals/10t.html
Soil Formation/Composition
• The formation of 1 meter of soil can take
from 100 – 100,000 years to form
(depending on the conditions present)
• Climate affects rate
• Higher temp. and more rain = faster soil
formation
• Rain provides water for chemical reactions
to occur and warmer temp. increases
speed of reactions
Soil formation/Composition
• Rocks are the “parents” of the soil
• Parent material affects the kind of soil that
is formed
• Different rocks contain different minerals
• Color of soils is partially dependent on the
minerals the soils contain
Soil formation/Composition
• Weathering – the process of breaking
rocks apart or removing minerals from
them.
• Weathering agents can be physical and
chemical
• Weathering agents include water, ice,
wind, temperature changes, sand,
glaciers, and plant roots
Soil formation/Composition
Weathering
• Temp. changes cause rocks to expand and contract
cracking rocks and releasing minerals
• Water in cracks can turn to ice further splitting rocks
• Roots can act as physical wedges. They can also
produce chemicals that dissolve minerals from the rocks
• Sand and rocks carried by moving water scour the soil
and rocks beneath
• Glaciers that carry much bigger rocks can file and scrape
rocks below
• Wind-blown sand acts like a sandblaster on the surfaces
of rocks
Soil formation/Composition
Chemical Changes in Rocks
• Equation for ferric oxide reaction (rust)
Significance of reaction:
Soil formation/Composition
Chemical Changes in Rocks
• Equations for Carbonic acid formation and
calcium and bicarbonate ion formation:
• Significance of reactions:
Soil Composition
• The average soil consists of 25% air and 25%
water.
• Water and air are extremely important
-for plant roots
-for many of the bacteria, protists, fungi,
and animals that live in the soil
-All of the organisms that live in the soil interact
together to form the soil ecosystem
Soil Composition
Organic Matter
• An average soil contains 5% of organic
matter
• Although organic matter is only a small
percentage of the overall soil it is
extremely important
• In an average soil, 80% of the organic
matter is humus, 10 % is roots, and 10% is
organisms
Humus
• Humus-partly decomposed organic matter
that was once living or was produced by a
living thing.
• Provides nutrients to many organisms
• Increases water-holding capacity of soil
• Provides nutrients to plants
Image taken fromhttp://urbanext.illinois.edu/soil/SoilBiology/soil_food_web.htm
Taken from http://permaculturenews.org
Soil texture
• Mineral particles in soil are classified by
size
• Size of particles determines soil texture
• See soil texture chart on page 213
Soil Texture
Soil Triangle
Spaces in soil
• Normally 40-60% of volume of soil is pore
space
• Important for water and air to travel
through soil
• Determine infiltration and percolation rates
• Size of spaces is dependent on texture
• Smaller particles=smaller spaces
• Larger particles=larger spaces
Key terms
• Soil texture – size of mineral particles
• Infiltration – the ability of water to soak into
the soil
• Water holding capacity – the ability of soil
to store water
• Aeration – the ability of air to move
through the soil
Soil Comparison
Soil Texture
Infiltration
Water-Holding Capacity
Aeration
SAND
Good
Poor
Good
SILT
Medium
Medium
Medium
CLAY
Poor
Good
Poor
LOAM
Medium
Medium
Medium
*Loam is considered an ideal soil for many crops due to its mix of different
soil particles (sand, silt, and clay)
Soil Structure
• Structure-when individual particles are
“glued” together to form larger pieces
• The “glue” is produced by organisms in the
soil
• Granular structure-rounded clumps with a
diameter of less than 1.5 cm
• Crumb structure-irregular shaped clumps
• Platy structure-soil particles glued together
into thin horizontal plates
Soil Profile (in descending order)
• Topsoil-top layer of humus rich soil
• Subsoil-layer of soil beneath the topsoil
(does not contain humus)
• Parent material –the pieces of rock that lie
on top of solid rock
• Bedrock-solid rock
Soil Conservation
• Erosion-the process in which wind or
water move soil to new locations
• Erosion occurs naturally but can also be
caused by human activities
• To much erosion is bad for farming
• Displaced soil can also be a major
pollutant of aquatic ecosystems
Some causes of erosion
•
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•
•
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Overgrazing by livestock
Repeated years of monocultures
Plowing land
Construction
Surface mining
logging
Preventing erosion
• Contour planting-planting across a slope
(rather than up and down it)
• Strip cropping-strips of close growing
plants are planted next to crops that are
planted in rows
• Diversion terraces-ridges of soil that are
constructed along the contours
• Waterways-wide ditches that are planted
with a permanent grass cover
Contour planting
http://www.extension.iastate.edu/agdm/crops/html/images/a141/image5.jpeg
Strip Cropping
http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=10888010
71&topicorder=12&maxto=16
Diversion Terraces
http://www.wi.nrcs.usda.gov/programs/solutions/diversion.html
Waterways
http://milford.nserl.purdue.edu/weppdocs/overview/practices.html
Preventing erosion
• Crop rotation
• Windbreaks
• Conservation tillage-methods to reducing
the amount of tilling and avoid the use of
the moldboard plow
• Cover crops-grasses or legumes planted
to hold soil in place
Crop Rotation
http://www.marionswcd.net/crop-rotation/
Windbreaks
http://www.mda.state.mn.us/protecting/conservation/practices/fieldw
indbreak.aspx
Conservation Tillage
http://nfrec.ifas.ufl.edu/programs/impacts_conservation_tillage.shtml
Cover Crops
http://biocontrol.ucr.edu/irvin/research/wsare.html
The dust bowl
• Dust bowl- Area in the southern part of the
great plains which experienced massive
amounts of soil erosion during the 1930s
• Caused by poor farming practices
combined with drought and winds
• As a response the federal government
created several agencies and policies to
prevent soil erosion