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Introduction to Soil Descriptions

Part 1 of 3

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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.

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The Goal of Soil Description

1. Determine if the soil can adequately treat the wastewater

2. Determine if the soil can adequately disperse the wastewater

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Accomplishing the Goals

Describe the soilProfile descriptionWetness conditionsRestrictive horizons

Assess suitabilityAerobic conditionsInternal vs. external drainage

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.

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How do you start describing the soil?

What do you see?Can you make measurements?

What is important?Describe/interpret from the top down

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Making Soil Descriptions

Texture

Structure

Consistence

Color

Horizon

Depth

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

Use texture to make inferences into pore sizeFrom pore size begin to estimate water movement and treatmentTexture by itself is not enough information to determine site suitability

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Soil Texture

Mineral material onlyMaterial > 2mm are coarse fragmentsMaterial < 2mm only

Sand - 2.0 - 0.05 mmSilt - 0.05 - 0.002 mmClay - < 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 TextureSand - grittySilt - smooth, velvetyClay - 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)

Sand

Loamy Sand

Sandy Loam

Loam

Silt Loam

Silt

Sandy Clay Loam

Silty Clay Loam

Clay Loam

Sandy Clay

Silty Clay

Clay

In the USDA System there are 12 textural groups.

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Sand + Silt + Clay = 100%

Texture = LOAM

40 % Sand40 % Silt20 % Clay

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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

Group I: Sand, Loamy sand1.2 – 0.8 gpd/ft2

Group II: Sandy loam, Loam0.8 – 0.6 gpd/ft2

Group III: Sandy clay loam, Silt loam, Clay loam, Silty clay loam, Silt0.6 – 0.3 gpd/ft2

Group IV: Sandy clay, Silty clay, Clay0.4 –0.1 gpd/ft2

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Soil StructureRule .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

“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

TypeSize Grade

Soil structure is describing using 3 components: type, size, and grade.

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Natural Soil Structural Units

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Structureless

Single Grain – non-coherentMassive - coherentMassive - 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.

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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.

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SBK to ABK

The type of structure has a profound impact on how water will move through the soil.

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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.