Soils TrainingSoil Structure and What it
Grade of Soil Structure
Gradedescribes 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.
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.
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.
Virtual Prisms in a Fragipan
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
always contain some
are physical prisms
that can be extracted
from the soil intact,
and those in some
fragipans that can be seen but not extracted.
The units are blocklike or
polyhedral. They are
bounded by flat or
surfaces that are casts
of the faces of
surrounding peds. The
structure is described
as angular blockyif the
faces intersect at
relatively sharp angles;
as subangular blockyif
the faces are a mixture
of rounded and plane
faces and the corners
are mostly rounded.
Form by wetting and
drying of adjacent aggregates.
Size names and symbols for
Very thick12345Very fineFineMediumCoarseVery coarse
Significance of Soil Structure
1.Water moves rapidly into open macropores
(cracks, channels, and pores). Shrink-swell soils may form slickensides. They swell shut when wetting.
2.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.
Significance of Soil Structure
3.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.
Important properties to observe with a hand lens
1.Connectivity of pores (vesicular or connected)
2.Openness of connected pores (look for clay
3.The number of horizontal planes (macropores)
4.The contrast in particle size between horizontal
planes or soil horizons (clay increase, sand size and uniformity in sandy textures)
Important Properties to Observe
1.Look for vertical ribbons as a sign of restricted water flow through macro and micropores.
2.Look for dense horizons (hard to dig when slightly dry, even though the clay content is not high).
3.Look for pockets or vertical zones of stripped soil (E material) that indicates preferential water movement.
4.Look for horizons that have both stripping and accumulation.
5.Look for a frosting of E material above and beside prismatic structure.