Soil Structure, Density, and Porosity

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Soil Structure, Density, and Porosity

• Laboratory 4

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Objectives

• 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.

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Definition

• Soil structure is the arrangement of the primary
  soil particles (sand, silt, and clay) and other
  soil materials into discrete aggregates.

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Peds

• 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.

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Ped Shapes

• Granular
• Blocky
• Prismatic
• Columnar
• Platy
• Single-grained
• Massive

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Granular Structure

• 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
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Blocky Structure

• 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
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Prismatic Structure

• 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/manual/images/fig3
-27_large.jpg
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Columnar Structure

• 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
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Platy Structure

• 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.jp
g
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Single-grained Structure

• 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
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Massive Structure

• 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
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Grade of Soil Structure

• 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
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Class of Soil Structure

• The size or class of the peds is described as
  fine, medium, or coarse.

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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.

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Degree of Water Movement
Structure
Water Movement
http//ohioline.osu.edu/b905/images/006.jpg
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Altering Soil Structure

• 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/programs/images/tracto
r-tillin.jpg
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Bulk Density

• Soil bulk density is the mass per unit bulk
  volume of soil that has been dried to a constant
  weight at 105 C.

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Bulk Density Example

• 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?
• It would be 50 g/30 cm3 or 1.67 g/ cm3.

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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

• 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.

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Bulk Density vs. Particle Density
http//soil.gsfc.nasa.gov/pvg/pd3.htm
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Porosity

• 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.

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Soil Permeability

• 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.

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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

• Which soil would have the greater soil strength?

http//www.wy.blm.gov/botany/pics/cob-duckswamp-50
0wfo.jpg
http//www.research.noaa.gov/spotlite/archive/imag
es/drysoil.jpg
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Soil Texture Soil Strength

• 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.

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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

• 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 (gt1.7
  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.

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Penetrometers

• 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/phot
os/May0505a-lr.jpg
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Questions?