Soil Structure and What it Tells Us

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Soil Structure and What it Tells Us

• Soils Training

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Grade of Soil Structure

• Grade describes the distinctness of units.
• Structureless (0) Structural units are absent.
  Structureless soil material may be either single
  grain or massive.
• Single grain (sg) limited to sands and loamy
  sand with no adhesion Massive (m) particles
  adhere together
• Weak (1) Structural units are barely observable
  in place. When gently rubbed, the soil material
  parts into a mixture of whole and broken units.
  Most material exhibits no structural surfaces.
• Moderate (2) Structural units are well formed
  and evident in undisturbed soil. When rubbed, the
  soil material parts into a mixture of many whole
  units, some broken units, and material that is
  not in units. Aggregate surfaces have properties
  distinct from those of fractured surfaces.
• Strong (3) Structural units are distinct in
  undisturbed soil. They separate cleanly when the
  soil is rubbed. When removed, the soil material
  separates mainly into whole units. Aggregate
  surfaces have properties distinct from those of
  fractured surfaces.

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Granular Shape

• The units are approximately spherical or
  polyhedral and are bounded by curved or very
  irregular faces that are not casts of adjoining
  peds. Found only in A horizons. Formed by organic
  carbon, some clay, and organisms.

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Platy Shape

• The units are flat and platelike. They are
  oriented horizontally. Found mainly in E and B
  horizons, unless the surface is compacted. Formed
  by freeze expansion or compaction (vertical
  pressure), or remain from bedding planes.

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Virtual Prisms in a Fragipan
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Prismatic Shape

• The individual units are bounded by flat to
  rounded vertical faces. Units are distinctly
  longer vertically, and the faces are casts or
  molds of adjoining units. Form by wetting and
  drying of adjacent aggregates, and always contain
  some substructure. There are physical prisms that
  can be extracted from the soil intact, and those
  in some fragipans that can be seen but not
  extracted.

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Blocky Shape

• The units are blocklike or polyhedral. They are
  bounded by flat or slightly rounded surfaces that
  are casts of the faces of surrounding peds. The
  structure is described as angular blocky if the
  faces intersect at relatively sharp angles as
  subangular blocky if the faces are a mixture of
  rounded and plane faces and the corners are
  mostly rounded. Form by wetting and drying of
  adjacent aggregates.

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Slickensides
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Size names and symbols for structure classes

• Platy structure symbol Other shapes
• Very thin 1 Very fine
• Thin 2 Fine
• Medium 3 Medium
• Thick 4 Coarse
• Very thick 5 Very coarse

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Significance of Soil Structure

• Water moves rapidly into open macropores (cracks,
  channels, and pores). Shrink-swell soils may form
  slickensides. They swell shut when wetting.
• Water moves more slowly into micropores (pores
  between soil particles). As clay increases, so
  does the total porosity, but also the adhesion
  because pore size decreases. Soils that have a
  mixture of particle sizes and little porosity
  have restricted water movement.

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Significance of Soil Structure

• Strong, very fine or fine angular blocky
  structure with a non-expansive clay type promotes
  water movement because of the very high amount of
  vertical macropores. Thus as nonexpansive clay
  content in a soil increases, the smaller the
  aggregates, the better for water movement.
• Nonexpansive clays include Kaolinite and oxides
  of Al and Fe.

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Important properties to observe with a hand lens

• Connectivity of pores (vesicular or connected)
• Openness of connected pores (look for clay
  plugging)
• The number of horizontal planes (macropores)
• The contrast in particle size between horizontal
  planes or soil horizons (clay increase, sand size
  and uniformity in sandy textures)

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Important Properties to Observe

• Look for vertical ribbons as a sign of restricted
  water flow through macro and micropores.
• Look for dense horizons (hard to dig when
  slightly dry, even though the clay content is not
  high).
• Look for pockets or vertical zones of stripped
  soil (E material) that indicates preferential
  water movement.
• Look for horizons that have both stripping and
  accumulation.
• Look for a frosting of E material above and
  beside prismatic structure.

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