Introduction to Soil
Descriptions
Part 1 of 3
1
Acknowledgement
This work was supported [in part] by the National Decentralized
Water Resources Capacity Development Project with funding
provided by the U.S. Environmental Protection Agency through a
Cooperative Agreement (EPA No. CR827881-01-0) with
Washington University in St. Louis. The results have not been
reviewed by EPA or Washington University in St. Louis. The
views expressed in this presentation are solely those of NCSU, and
University of Arkansas and EPA and Washington University in St.
Louis do not endorse any products or commercial services
mentioned in the presentation.
2
The Goal of Soil Description
Determine if the soil can adequately treat
the wastewater
2. Determine if the soil can adequately
disperse the wastewater
1.
3
Accomplishing the Goals
ƒ Describe the soil
ƒ Profile description
ƒ Wetness conditions
ƒ Restrictive horizons
ƒ Assess suitability
ƒ Aerobic conditions
ƒ Internal vs. external drainage
ƒ Use to assist in evaluating the site
Understanding and interpreting soils is a iterative process that begins with a
soil description and leads towards an assessment of the soils suitability to
carry out its proposed land use. Evaluation of the soil is just one component
of fully assessing a site. There are additional site factors and characteristics
that must also be evaluated. These are discussed in Chapter 4.
4
How do you start describing
the soil?
What do you see?
Can you make measurements?
What is important?
Describe/interpret from the top down
5
Making Soil Descriptions
¾Texture
¾Color
¾ Structure
¾Horizon
¾Consistence
¾Depth
A soil description should include all of the following. Each component of the
description will aid in the overall interpretation, however, depending on
landuse some will be more important than others.
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Soil Texture
Rule .1941 (a1)
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Soil Texture
¾ Use
texture to make inferences into pore
size
¾ From pore size begin to estimate water
movement and treatment
¾ Texture by itself is not enough information
to determine site suitability
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Soil Texture
¾ Mineral
material only
¾ Material > 2mm are coarse fragments
¾ Material < 2mm only
z
z
z
Sand - 2.0 - 0.05 mm
Silt - 0.05 - 0.002 mm
Clay - < 0.002 mm
When considering texture it is important to remember that soil texture refers
only to the mineral component of less the 2 mm. Roots etc are ignored as
are particles above 2 mm. These are added to the descriptive textural term
as modifiers. The size ranges above illustrate that a clay particle is 3 orders
of magnitude smaller than the coarsest sand grain.
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Soil Texture
¾Sand
- gritty
¾Silt - smooth, velvety
¾Clay - slick, sticky
Trying to remember the size ranges by their particle diameter range is not
really helpful in the field. One of the best ways to determine texture is by
recall how each of the separates feels. Sand – gritty; Silt - smooth, velvety
like baking flour; Clay - slick, sticky.
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USDA Textural Classes (12)
¾ Sand
¾ Sandy
Clay Loam
¾ Loamy
Sand
¾ Silty
Clay Loam
¾ Sandy
Loam
¾ Clay
Loam
¾ Loam
¾ Sandy
¾ Silt
¾ Silty
¾ Silt
Loam
Clay
Clay
¾ Clay
In the USDA System there are 12 textural groups.
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Sand + Silt + Clay = 100%
40 % Sand
40 % Silt
20 % Clay
Texture = LOAM
12
This is a good starting point, but you will need to develop your own feel based on
known samples and your region.
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Textural Groups for OSWW
¾ Group
I:
z Sand, Loamy sand
2
z 1.2 – 0.8 gpd/ft
¾ Group II:
z Sandy loam, Loam
2
z 0.8 – 0.6 gpd/ft
¾ Group
III:
z Sandy clay loam,
Silt loam, Clay
loam, Silty clay
loam, Silt
2
z 0.6 – 0.3 gpd/ft
¾ Group IV:
z Sandy clay, Silty
clay, Clay
2
z 0.4 –0.1 gpd/ft
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Soil Structure
Rule .1941 (a2)
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.
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Soil Structure
ƒ
“Structure is the naturally occurring
arrangement of soil particles into
aggregates (peds) that result from
pedogenic processes.
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.
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Soil Structure
ƒ
Type
Size
ƒ Grade
ƒ
Soil structure is describing using 3 components: type, size, and grade.
17
Natural Soil Structural Units
18
Structureless
¾ Single
Grain – non-coherent
¾ Massive - coherent
¾ Massive - Rock Controlled Fabric
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.
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Single Grain – No structural units;
entirely noncoherent; e.g. loose sand
Single Grained
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Massive – No structural units;
material is a coherent mass (not
necessarily cemented)
Massive – dark gray- glacial till.
21
Massive – Rock Controlled Fabric –
No structural units; material is a
coherent mass with the original rock
fabric is still identifiable
Rock controlled fabric. Unlike massive a preferred orientation of the
minerals can be seen. The material may easily break into the individual
mineral grains.
22
SBK to ABK
The type of structure has a profound impact on how water will move through
the soil.
23
GRANULAR
SUB-ANGULAR BLOCKY
PRISMATIC and ANGULAR BLOCKY
ANGULAR BLOCKY
SUB-ANGULAR BLOCKY
MASSIVE
Looking at this profile some horizon will be suitable for structure while others will
not. When your system is designed the whole profile needs to be considered.
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Texture Plays a Major Role with
Micropores.
Structure Plays a Major Role with
Macropores.
Another fact to keep in mind is that texture relates to the microporosity
whereas structure relates to macroporosity.
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