Soil Structure,
Density, and Porosity
Laboratory #4
Objectives
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Understand the concept of soil structure and
how it differs from soil texture.
Understand how soil structure influences other
soil properties.
Learn how soil structure affects other soil
properties and why it is important to maintain soil
structure.
Learn how land use and management affects
soil structure.
Definition
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Soil structure is the arrangement of the
primary soil particles (sand, silt, and clay)
and other soil materials into discrete
aggregates.
Peds
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Structural units are called peds, and have
distinct boundaries and well-defined planes of
weakness between the aggregates.
Peds consist of primary particles bound together
by cementing agents like organic matter, clay,
and hydrous oxides of iron and aluminum.
Peds can take several shapes.
Ped Shapes
Granular
 Blocky
 Prismatic
 Columnar
 Platy
 Single-grained
 Massive
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Granular Structure
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Resembles cookie
crumbs and is usually
less than 0.5 cm in
diameter.
Commonly found in
surface horizons
where roots have
been growing.
http://soil.gsfc.nasa.gov/pvg/granular.gif
http://soils.usda.gov/technical/manual/images/fig3-30_large.jpg
Blocky Structure
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Irregular blocks that
are usually 1.5 - 5.0
cm in diameter.
Can be subangular or
angular blocky.
http://soil.gsfc.nasa.gov/
pvg/blocky.gif
http://www.dpi.vic.gov.au/dpi/vro/glenimages.nsf/I
mages/gl167_profile/$File/gl167_profile.jpg
Prismatic Structure
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Vertical columns of
soil that might be a
number of cm long.
Usually found in lower
horizons.
http://soil.gsfc.nasa.gov/
pvg/prismatic.gif
http://soils.usda.gov/technical/man
ual/images/fig3-27_large.jpg
Columnar Structure
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Vertical columns of soil that have a salt "cap" at
the top.
Found in soils of arid climates.
http://soil.gsfc.nasa.gov
/pvg/columnar.gif
http://soils.usda.gov/technical/manual
/images/fig3-28_large.jpg
Platy Structure
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Thin, flat plates of soil
that lie horizontally.
Usually found in
compacted soil.
http://soil.gsfc.nasa.gov/pvg/platy.gif
http://soils.ag.uidaho.edu/soilorders/i/Arid_03.jpg
Single-grained Structure
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Soil is broken into individual particles that do
not stick together.
Always accompanies a loose consistence.
Commonly found in sandy soils.
http://soil.gsfc.nasa.gov/pvg/singlegrained.gif
Massive Structure
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Soil has no visible structure, is hard to break
apart and appears in very large clods.
http://soil.gsfc.nasa.gov/pvg/massive.gif
http://soils.usda.gov/technical/manual/images/fig3-31_large.jpg
Grade of Soil Structure
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The terms weak, moderate, or strong are used
to describe the grade or how stable the peds are
and how hard they are to break apart.
What do you think
the grade would be
for this picture?
http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg
Class of Soil Structure
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The size or class of the peds is described
as fine, medium, or coarse.
How does structure affect water
movement in soils?
In soils with good structure, the pore
space that occurs between peds is
relatively large and facilitates water and air
movement.
 Well-developed structure is very important
in clayey soils.
 Clayey soils with poor structure restrict
water and air movement.

Degree of Water Movement
Structure
Water
Movement
http://ohioline.osu.edu/b905/images/006.jpg
Altering Soil Structure
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Unlike texture, structure
can be altered by tillage
or traffic.
Tilling soils that are too
wet, or compacting soils
with heavy equipment
can break down the
natural structural units.
http://www.ny.nrcs.usda.gov/progra
ms/images/tractor-tillin.jpg
Bulk Density
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Soil bulk density is the mass per unit bulk
volume of soil that has been dried to a constant
weight at 105 °C.
Bulk Density Example
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If we have a soil that weighs 50 grams after
being oven-dried and has a volume of 30
cm3, what will be the bulk density?
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It would be 50 g/30 cm3 or 1.67 g/ cm3.
Bulk Density in the Field
Bulk density of organic soils can be very
low, such as 0.5 g/cm3.
 Clayey soils have higher bulk densities.
 Compacted clay soils have very high bulk
densities, between 1.6 and 1.8 g/cm3.
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Particle Density and Porosity
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Particle density is the mass per unit volume of
soil particles.
Particle density is a relatively constant
parameter and is sometimes assumed to be
2.65 g/cm3.
Bulk Density vs. Particle Density
http://soil.gsfc.nasa.gov/pvg/pd3.htm
Porosity
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Soil porosity is the percentage of a soil that is pore
space or voids.
The average soil has a porosity of about 50%, and the
pores are filled with air or water depending on the
moisture content.
Sands have larger pores, but less total pore space
than clays.
If both bulk density and particle density are known, the
total porosity can be calculated using these values.
Soil Permeability
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Soil permeability is the ease with which air,
water, or plant roots penetrate or pass through
soil.
Soils with large pores that are connected are
more permeable.
Rainwater soaks in readily and moves down
through the soil profile.
Clayey soils can have greater total porosity than
sand and still be less permeable than sand since
the pores are small.
Soil Strength
Soil strength is defined as the amount of
force required to move or rearrange soil
particles.
 Soil strength is an important physical
property affecting plant growth and
seedling emergence.
 It is affected by three main factors;
moisture content, soil texture, and bulk
density.
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Moisture Content & Soil Strength
Water content is the most important factor
determining soil strength.
 The lower the soil water content, or the
drier the soil, the greater the soil strength.
 Soils that are saturated, or waterlogged
have weak soil strengths.
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Moisture Content
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Which soil would have the greater soil
strength?
http://www.wy.blm.gov/botany/pic
s/cob-duckswamp-500wfo.jpg
http://www.research.noaa.gov/spot
lite/archive/images/drysoil.jpg
Soil Texture & Soil Strength
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Soil texture also affects soil strength.
Soil strength of aggregated soils increases as clay
content increases.
Poorly aggregated or single grain soils (sands, loamy
sands, sandy loams) usually have the weakest soil
strengths unless they are cemented or compacted.
Individual particles of single grain (sandy) soils are easy
to rearrange, but these soils are susceptible to
compaction, sometimes resulting in the formation of hard
pans.
Bulk Density & Soil Strength
Bulk density is the third factor affecting soil
strength.
 As bulk density of a given soil increases
soil strength also increases.
 Remember that soil is composed of solids
and pores, and the greater the bulk
density the greater the amount of solids,
and the smaller the amount of pore space.
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Soil Management Problems
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Two examples of management problems caused
by increasing soil strength are soil crusts and
tillage pans.
A soil crust is a thin soil layer that forms at the
soil surface following heavy rains.
Tillage or hard pans are high bulk density (>1.7
g/cm3) layers that occur within the Ap and E
horizons.
Compaction caused by traffic of equipment,
vehicles, or even foot traffic often increases soil
strength to levels that restrict root penetration
and plant growth.
Penetrometers
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The force required to push
a rod into the soil is a
measure of soil strength.
Penetrometers are devices
used to measure the
resistance of a soil to
penetration to estimate the
effect of compaction on
growth, and to detect layers
of different soil strength.
http://agnews.tamu.edu/dailynews/stories
/SOIL/photos/May0505a-lr.jpg
Questions?