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TITLE 5 SOIL
EVALUATOR
CERTIFICATION
TRAINING
SOIL STRUCTURE
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Prepared for: Commonwealth of Massachusetts
Department of Environmental Protection
By L.A. Spokas, PhD, Stockbridge School of Agriculture
University of Massachusetts Amherst
Presented by:
New England Interstate Water Pollution Control Commission
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STRUCTURE OF MINERAL SOILS
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“Soil structure refers to the aggregation of
individual soil particles into larger units with planes
of weakness between them”1
Particles are held together (by organic matter, iron,
and clay) to form distinct units called peds. Peds
are individual, natural soil aggregates, which have
units of mm to m in size. An example of a ped is a
granule, a prism, or a block.
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Peds are NOT clods.
1. Buol, Southard, Graham, and McDaniel,
Soil Genesis and Classification, 5th Ed. 2003
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http://homeguides.sfgate.com/break-up-plowed-soil
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Clods are chunks of soil formed artificially, often
compressed due to plowing or other disturbance, or
excavation.
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STRUCTURE RANGES FROM “STRUCTURE
LESS” TO HIGHLY DEVELOPED.
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Structureless Soils
Soil Structures
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Single Grain
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Granular
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Massive
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Blocky
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Prismatic
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Angular
Subangular
Columnar
Platy
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Soil structure forms by wetting and drying cycles.
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During periods of desiccation soil compression occurs
due to capillary tension, causing cracks to form.
 When this soil becomes wet again, it swells.
 The protruding points on adjacent ped faces touch,
causing pressure or shearing forces at these points.
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The shear forces cause coarse particles (sand) to move
away from the pressure, leaving the platy clay particles
behind.
The clay particles are forced into parallel orientation
along the ped face by the pressure of swelling.
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Clay Skins (clay
coatings along a
natural plane of
weakness)
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This clay skin on the ped face causes the soil to “crack”
in the same place when it next dries.
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 The
formation of structure requires
TIME and the presence of shrinking
and swelling clays
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 New
England Soils are very young,
and
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 There
is very little clay in our soils, and
no shrink swell clays
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GRANULAR STRUCTURE
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2 cm
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Granular Structure
talk.newagtalk.com
Notice the
small peds,
held together
by organic
matter.
Granular
structure is
most
prevalent in
O, A and Ap
horizons
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a granule
BLOCKY STRUCTURE
a block
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Horizontal and Vertical extents of structure are equal.
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An example of good blocky structure
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academic.emporia.edu
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Prismatic Structure
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Vertical extent of structure is greater
than the horizontal extent.
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a prism
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Prismatic
Structure
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colbydigssoil.com
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Columnar Structure
www.geog.ucsb.edu
passel.unl.edu
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Structure should be easily
visible on the pit wall.
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PLATY STRUCTURE
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Horizontal extent of the
structure is greater than
the vertical extent.
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a plate
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2 cm
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PLATY STRUCTURE
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www.omafra.gov.on.ca
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Typical platy structure on a
dense till C horizon
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STRUCTURELESS SOILS
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Single Grain
Massive
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STRUCTURE – SIZE
Size†
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Shape of Structure
Prismatic/
Platy‡
Columnar
Blocky Granular
------------------ size (mm) --------------------
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Very fine
<1
<10
<5
<1
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Fine
Medium
1-2
2-5
5-10
10-20
20-50
50-100
5-10
10-20
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20-50
1-2
2-5
5-10
>10
>100
>50
>10
Coarse
Very Coarse
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† size refers to the smallest dimension of plates, prisms, and columns.
‡ Substitute thin for fine and thick for coarse for platy sizes.
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STRUCTURE – GRADE
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Grade refers to the strength of firmness of the peds.
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Weak – Peds are observable in place, but cannot be
removed.
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Moderate – Peds can be removed from the profile for
hand examination.
Strong – Removed peds are rigid and durable in
hand.
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Most structure in MA Soils is weak!
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STRUCTURE DESIGNATION – GRADE, SIZE, SHAPE
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moderate, thick, platy structure
strong, medium, prismatic structure
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CONSISTENCE – A MEASURE OF THE SOIL
MATERIAL’S RESPONSE TO PRESSURE
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Class
Test
Loose
Clod cannot be obtained (e.g. sands)
Very friable
Clod fails under very slight pressure between thumb and
forefinger
Friable
Clod fails under slight pressure between thumb and forefinger
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Firm
Clod fails under moderate pressure between thumb and
forefinger
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Very firm
Clod fails under strong pressure between thumb and
forefinger
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Extremely firm Two hands are required to rupture clod
Slightly rigid
Ruptures when placed underfoot with full body weight
Rigid
Ruptures when struck with 1 kg hammer
Very rigid
Cannot be ruptured with hammer
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Single Grain, Loose
(Structure, consistence)
STRUCTURE &
CONSISTENCE
Weak fine granular, friable
(grade, size, shape;
consistence)
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Massive, friable
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Note: Structureless soils
(single grain and massive)
have no size or grade.