Soil Soil Structure Structure

Presented by:Presented by:Mr. Brian Oram, PG, PASEOMr. Brian Oram, PG, PASEO

Wilkes UniversityWilkes UniversityGeoEnvironmental Sciences and GeoEnvironmental Sciences and

Environmental Engineering Environmental Engineering DepartmentDepartment

Wilkes - Barre, PA 18766Wilkes - Barre, PA 18766570-408-4619570-408-4619

http://www.water-research.nethttp://www.water-research.net

Quick ReviewQuick Review

Sand + Silt + Clay = 100%

Texture = Clay

20 % Sand30 % Silt50 % Clay

Mineral SoilMineral Soil Is never saturated with water for more than Is never saturated with water for more than

a few days and contains less than 20 % (by a few days and contains less than 20 % (by weight) organic carbonweight) organic carbon

If saturated with water for longer periods or If saturated with water for longer periods or artificially drained and excluding live roots, artificially drained and excluding live roots, has an organic-carbon content of:has an organic-carbon content of: Less than 18 % if the mineral fraction contains Less than 18 % if the mineral fraction contains

60% or more clay;60% or more clay; Less than 12 %, if the mineral fraction contains no Less than 12 %, if the mineral fraction contains no

clays; orclays; or Less than 12 + (clay percentage multiplied by 0.1) Less than 12 + (clay percentage multiplied by 0.1)

%, if the mineral fraction contains less than 60%.%, if the mineral fraction contains less than 60%.Soil material containing more than the above

organic carbon is an organic soil.

Why Is Understanding Soil Structure Important ?

Describing Soil MorphologyDescribing Soil Morphologyfor Mineral Soilfor Mineral Soil

Soil Texture and Coarse Soil Texture and Coarse Fragment ContentFragment Content

Soil Structure (Type, Size and Grade)Soil Structure (Type, Size and Grade) Soil ColorSoil Color ConsistencyConsistency Redoximorphic FeaturesRedoximorphic Features BoundariesBoundaries

What is the Soil Structure?

How do we Describe Structure?

What is Going On in this Photo?

Soil StructureSoil Structure ““Structure is the naturally occurring arrangement of soil Structure is the naturally occurring arrangement of soil

particles into aggregates (peds) that result from pedogenic particles into aggregates (peds) that result from pedogenic processes.processes.

The grouping or arrangement of individual soil particles The grouping or arrangement of individual soil particles into a larger grouping. The soil peds are a cluster or into a larger grouping. The soil peds are a cluster or grouping of sand, silt, clay, organic material, biological grouping of sand, silt, clay, organic material, biological slimes, and fungal masses.slimes, and fungal masses.

Three general groupsThree general groups Natural Soil Structural Units (pedogenic structure)Natural Soil Structural Units (pedogenic structure) StructurelessStructureless Artificial Earthy Fragments or ClodsArtificial Earthy Fragments or Clods

DescribingDescribing Soil StructureSoil Structure

Type – shape of the structureType – shape of the structure Size – relative size of the structureSize – relative size of the structure Grade – how well the structure is Grade – how well the structure is

expressed in place.expressed in place.

When described in a soil profile the full description of the structure is describe in the following order – grade, size, shape.

Such as: weak medium subangular blocky (WHAT??)

Soil StructureSoil Structure

TypeTypeSizeSize GradeGrade

Soil Structural TypesSoil Structural Types GranularGranular BlockyBlocky

Angular Blocky Angular Blocky Subangular BlockySubangular Blocky

PlatyPlaty WedgeWedge PrismaticPrismatic ColumnarColumnar MassiveMassive Single GrainSingle Grain

GranularGranular

Small Polyhedrons or SpheroidsBounded by Curved or Irregular SurfacesSymbol for Structure (gr)

Photo – Thank you Dr. Lindo - NCSU

BlockyBlockySubangular or AngularSubangular or Angular

Subangular Blocky – the three dimensions are about the same size, but polyhedrons are subrounded. Symbol (sbk)

Angular Blocky – the three dimensions are about the same size, but edges are shape and faces appear flattened. Symbol (abk)

Platy – Flat and Tabular- Units

Structure has a horizontal dimension that is longer or greater than the verticaldimension. The plates tend to parallel the surface. Typical Symbol (pl or PL)

Photo on the Right – Thank you Dr. Lindo - NCSU

Wedge Structures

Elliptical Interlocking lenses that terminate in acute angles, bounded by slickensides. This structure is Not limited to Vertic martials or Vertisols. Symbol: WEG

Soil: Has a High Clay Content- withExpanding clays.

Photo – Thank you Dr. Lindo - NCSU

Prismatic StructurePrismatic Structure Symbol pr or PR Symbol pr or PR

Typically associated with fragipan and other similar horizons. These are prismlikeStructural units where the length and width is significantlyLess than the depth. (Vertical Structures)

These structures have flat tops or caps. Water movement tends to be associated with movement along the vertical contact between units.

A soil with a prismatic structure may also have secondary structural units – Photo 2 has a platy secondary structure.

Photo 2Photo 1

Photo on the Left – Thank you Dr. Lindo - NCSU

Z

Y

X

ColumnarColumnarSymbol cpr or COLSymbol cpr or COL

Prism-like structural units where the length of the unit in the x and y direction are much less than the z direction.

The units tend to have rounded tops.XY

Photo on the Right – Thank you Dr. Lindo - NCSU

Z

Structureless (0)Structureless (0)

Single Grain (non-coherent)Single Grain (non-coherent) Massive (coherent mass)Massive (coherent mass) Massive - Rock Controlled Fabric- Massive - Rock Controlled Fabric-

May be a saprolite.May be a saprolite.

Single GrainSingle GrainSymbol sg or SGRSymbol sg or SGR

Lacks Structure- LooseOn rupture- more than 50 % of the mass is single Grain – example beach sand.

Material is NON-Coherant.

Massive Massive Symbol m or MASymbol m or MA

Structureless material that is coherent,but is not cemented.

Massive Massive Rock Controlled Rock Controlled

FabricFabricStructureless material that is coherent,but is not cemented or weakly cemented

In this case the material contains the original apperance of the bedrock material, but lacks the strength.

Possible – Cr Horizon – Saprolite

Photo on the Right – Thank you Dr. Lindo - NCSU

Bx horizons

E HorizonsCompacted

Horizonsand Bx

Horizons

Soil StructureSoil Structure

TypeTypeSize Size GradeGrade

Soil Structure SizeSoil Structure Size Very Fine (Very Thin)Very Fine (Very Thin) Fine (Thin)Fine (Thin) MediumMedium Coarse (Thick)Coarse (Thick) Very Coarse (Very Thick)Very Coarse (Very Thick)

Section of Size Criteria Depends on Section of Size Criteria Depends on the Structural Units.the Structural Units.

Soil Structure Size Soil Structure Size (Granular and Blocky)(Granular and Blocky)

Image Source:NRCS, Ver. 2.0, 2002

Soil Structure Size Soil Structure Size (Platy and Prismatic)(Platy and Prismatic)

Image Source:NRCS, Ver. 2.0, 2002

Soil Structure GradeSoil Structure Grade Structureless = 0Structureless = 0

Single Grain (non-coherent)Single Grain (non-coherent) Massive (coherent)Massive (coherent) Massive - Rock Controlled Fabric

Weak = 1Weak = 1 Moderate = 2Moderate = 2 Strong = 3Strong = 3

Description of StructureDescription of Structure Three class of grade (Excluding Structureless (0) Three class of grade (Excluding Structureless (0)

):):1)1) Weak (1)– the units are barely observable in Weak (1)– the units are barely observable in

place and when gently disturbed, the soil parts place and when gently disturbed, the soil parts into a mixture of whole and broken units.into a mixture of whole and broken units.

2)2) Moderate (2) – The units are well formed and Moderate (2) – The units are well formed and evident in undisturbed soil. When disturbed, the evident in undisturbed soil. When disturbed, the soil material parts into a mixture of whole with soil material parts into a mixture of whole with some broken units.some broken units.

3)3) Strong (3) – The units are distinct in undisturbed Strong (3) – The units are distinct in undisturbed soil. They separate cleanly when the soil is soil. They separate cleanly when the soil is disturbed and when removed the soil separates disturbed and when removed the soil separates into whole units.into whole units.

DescribingDescribing Soil StructureSoil Structure

Grade – how well the structure is Grade – how well the structure is expressed in place.expressed in place.

Size – relative size of the structure.Size – relative size of the structure. Type – shape of the structure.Type – shape of the structure.

Soil Structure Soil Structure Presented by:Presented by:

Mr. Brian Oram, PG, PASEOMr. Brian Oram, PG, PASEOWilkes UniversityWilkes University

GeoEnvironmental Sciences and Environmental GeoEnvironmental Sciences and Environmental Engineering DepartmentEngineering DepartmentWilkes - Barre, PA 18766Wilkes - Barre, PA 18766

570-408-4619570-408-4619

http://www.water-research.nethttp://www.water-research.net

Soils and Hydrogeological Work Jack Frost