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Soil Structure and What it Tells Us

发布时间:2013-11-20 12:01:27  

Tells Us

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.

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.

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.

Virtual Prisms in a Fragipan

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.

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 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

structure classes

Very thin




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.

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