1. Soil Structure Presented by:
Mr. Brian Oram, PG, PASEO Wilkes University GeoEnvironmental Sciences and Environmental Engineering Department Wilkes - Barre, PA

2. Quick Review

3. Sand + Silt + Clay = 100%
20 % Sand
30 % Silt
50 % Clay
Texture = Clay

4. Mineral Soil
Is never saturated with water for more than a few days and contains less than 20 % (by weight) organic carbon
If saturated with water for longer periods or artificially drained and excluding live roots, has an organic-carbon content of:
Less than 18 % if the mineral fraction contains 60% or more clay;
Less than 12 %, if the mineral fraction contains no clays; or
Less than 12 + (clay percentage multiplied by 0.1) %, if the mineral fraction contains less than 60%.
Soil material containing more than the above organic carbon is an organic soil.

5. Why Is Understanding Soil Structure Important ?

6. Describing Soil Morphology for Mineral Soil
Soil Texture and Coarse Fragment Content
Soil Structure (Type, Size and Grade)
Soil Color
Consistency
Redoximorphic Features
Boundaries

7. What is Going On in this Photo?
What is the Soil Structure?
How do we Describe Structure?
Soil structure is another aspect of soil description that must be considered in order to evaluated a soil for its ability to treat and dispose of wastewater.
What is Going On in this Photo?

8. Soil Structure
“Structure is the naturally occurring arrangement of soil particles into aggregates (peds) that result from pedogenic processes.
The grouping or arrangement of individual soil particles into a larger grouping. The soil peds are a cluster or grouping of sand, silt, clay, organic material, biological slimes, and fungal masses.
Three general groups
Natural Soil Structural Units (pedogenic structure)
Structureless
Artificial Earthy Fragments or Clods
Description of structure (as with texture) follows the NRCS description on categories. “Structure is the naturally occurring arrangement of soil particles into aggregates (peds) that result from pedogenic processes. Three general groups: Natural Soil Structural Units (pedogenic structure); Structureless; Artificial Earthy Fragments or Clods. It is the first two that we will spend the vast amount of time discussing.

9. Describing Soil Structure
Type – shape of the structure
Size – relative size of the structure
Grade – how well the structure is expressed in place.
Soil structure is describing using 3 components: type, size, and grade.
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??)

10. Soil Structure Type Size Grade
Grade refers to how well expressed or how stable the structure is. There are 4 groups of structure grade.

11. Soil Structural Types Granular Blocky Platy Wedge Prismatic Columnar
Angular Blocky
Subangular Blocky
Platy
Wedge
Prismatic
Columnar
Massive
Single Grain
Verbal description of types.

12. Photo – Thank you Dr. Lindo - NCSU
Granular
Small Polyhedrons or Spheroids Bounded by Curved or Irregular Surfaces
Symbol for Structure (gr)
Typical granular structure. Most often found in the surface layers.
Photo – Thank you Dr. Lindo - NCSU

13. Blocky Subangular 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)
Typical granular structure. Most often found in the surface layers.

14. Platy – Flat and Tabular- Units
Platy structure is wide than deep.
Structure has a horizontal dimension that is longer or greater than the vertical
dimension. The plates tend to parallel the surface. Typical Symbol (pl or PL)
Photo on the Right – Thank you Dr. Lindo - NCSU

15. Photo – 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- with
Expanding clays.
Wedge structure has eliptical interlooking peds the often show slickensides.
Photo – Thank you Dr. Lindo - NCSU

16. Prismatic Structure Symbol pr or PR
Photo 1
Photo 2
Typically associated with fragipan and other similar horizons. These are prismlike
Structural units where the length and width is significantly
Less 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. X Y Z
Prismatic structure is vertically elongated.
Photo on the Left – Thank you Dr. Lindo - NCSU
A soil with a prismatic structure may also have secondary
structural units – Photo 2 has a platy secondary structure.

17. Columnar Symbol 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. Y X
Columnar structure is similar to prismatic but the unit tops are frequently rounded and bleached due to the high salt content. Z
Photo on the Right – Thank you Dr. Lindo - NCSU

18. Structureless (0) Single Grain (non-coherent) Massive (coherent mass)
Massive - Rock Controlled Fabric- May be a saprolite.
Structureless soils are broken into 2 official groups; Single Grain, Massive and one suggested (by the author) group Massive - Rock Controlled Fabric. Single grain refers to sands whereas massive refers to any soil that does not break apart into any predictable and repeatable type or shape. Massive rock controlled structure is used for soil developed from saprolite.

19. Single Grain Symbol sg or SGR
Lacks Structure- Loose On rupture- more than
50 % of the mass is single
Grain – example beach sand.
Material is NON-Coherant.
Single Grained

20. Massive Symbol m or MA Structureless material that is coherent,
but is not cemented.
Massive – dark gray- glacial till.

21. Massive Rock Controlled Fabric
Structureless 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
Rock controlled fabric. Unlike massive a preferred orientation of the minerals can be seen. The material may easily break into the individual mineral grains.
Photo on the Right – Thank you Dr. Lindo - NCSU

22. E Horizons Compacted Horizons
and Bx Horizons
Bx horizons
Angular blocky structure. The faces are sharp and they fit together well.

23. Soil Structure Type Size Grade
Grade refers to how well expressed or how stable the structure is. There are 4 groups of structure grade.

24. Section of Size Criteria Depends on
Soil Structure Size
Very Fine (Very Thin)
Fine (Thin)
Medium
Coarse (Thick)
Very Coarse (Very Thick)
Size is broken into 5 groups.
Section of Size Criteria Depends on
the Structural Units.

25. Soil Structure Size (Granular and Blocky)
Image Source: NRCS, Ver. 2.0, 2002
Size is broken into 5 groups.

26. Soil Structure Size (Platy and Prismatic)
Image Source: NRCS, Ver. 2.0, 2002
Size is broken into 5 groups.

27. Soil Structure Grade Structureless = 0 Single Grain (non-coherent)
Massive (coherent)
Massive - Rock Controlled Fabric
Weak = 1
Moderate = 2
Strong = 3
All structureless soil have a grade of 0. The others range from 1 to 3.

28. Description of Structure
Three class of grade (Excluding Structureless (0) ):
Weak (1)– the units are barely observable in place and when gently disturbed, the soil parts into a mixture of whole and broken units.
Moderate (2) – The units are well formed and evident in undisturbed soil. When disturbed, the soil material parts into a mixture of whole with some broken units.
Strong (3) – The units are distinct in undisturbed soil. They separate cleanly when the soil is disturbed and when removed the soil separates into whole units.

29. Describing Soil Structure
Grade – how well the structure is expressed in place.
Size – relative size of the structure.
Type – shape of the structure.
Soil structure is describing using 3 components: type, size, and grade.

30. Soil Structure Presented by:
Mr. Brian Oram, PG, PASEO Wilkes University GeoEnvironmental Sciences and Environmental Engineering Department Wilkes - Barre, PA
Soils and Hydrogeological Work Jack Frost