wastewater and soil due today
Soils: Texture and Structure
Soil is a porous natural body of mineral, air, water and organic matter that changes, or has changed, in response to climate, topography, time, and organisms.
Soil and Site. Lindbo et al. DRAFT
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This is the text book definition. Basically, it says that soils are dynamic and reflect the conditions that they formed under. Because they do reflect the environment in which they formed we can use their morphology to understand more about the environment. Thus soils can be like a very smart canary in the coal mine – if we know how to interpret their song.
More discussion of these properties and their interpretation will follow throughout the presentation.
Soil Porosity
Soil Texture
Proportion of particles in sand, silt and clay size ranges
Use texture to make inferences into pore size
From pore size begin to estimate water movement and treatment
Finer texture means slower water movement
Finer texture means greater treatment
Texture by itself is not enough information to determine site suitability
Soil and Site. Lindbo et al. DRAFT
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Texture also impacts chemical adsorption reactions – the finer the texture, the greater the surface area. Unfortunately finer texture also mean slower water movement, thus sizing of OSWW systems using texture has to take both of these competing factors into consideration. For example a clayey soil (fine texture) generally is considered to have a low permeability, but if that soil has a well developed structure its permeability will be greater than just the texture would have predicted.
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
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In the USDA System there are 12 textural groups.
Design Considerations
Soil and Site Evaluation
Soil texture* 7) Landscape position
Soil structure 8) Slope
Soil depth 9) Available space
Soil color 10) Organic soils
Soil mineralogy 11) Restrictive horizons
Soil wetness
*There are 4 soil textural groups, each with a given range of loading rates
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Textural Groups for OSWW
Group I:
Sand, Loamy sand
Group II:
Sandy loam, Loam
Group III:
Sandy clay loam, Silt loam, Clay loam, Silty clay loam, Silt
Group IV:
Sandy clay, Silty clay, Clay
Soil and Site. Lindbo et al. DRAFT
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Long Term Acceptance Rates (LTAR)
Soil Group LTAR (g/day/ft2)
S, LS 1.2 - 0.8
SL, L 0.8 - 0.6
SCL, SiL, CL, 0.6 - 0.3
SiCL, Si
IV. SC, SiC, C 0.4 – 0.1
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Soil Texture
Mineral material only
Material > 2mm are coarse fragments
Material < 2mm only
Sand: 2.0 - 0.05 mm
Silt: 0.05 - 0.002 mm
Clay: < 0.002 mm
Soil and Site. Lindbo et al. DRAFT
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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.
Soil Texture (mineral material only)
Sand - gritty
Silt - smooth, velvety
Clay - slick, sticky
Soil and Site. Lindbo et al. DRAFT
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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.
Particle-Size Distribution
Particle size distribution describes the abundance (by weight) of the various size particles that constitute the mineral portion of soil materials.
Soil and Site. Lindbo et al. DRAFT
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PSD is probably the most important soil property as far as interpreting soils for any land use. Water or effluent movement and treatment are for the most part (soil structure too) a function of the PSD of a soil. Other factors are also important, such as structure and consistence. These factors are often used to modify the LTAR that one first assigns based on the texture.
Determination of Texture
Field procedure (Texture by feel method)
Laboratory procedure
Hydrometer
Pipette
Soil and Site. Lindbo et al. DRAFT
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Texture can be determined in the field by feel or in the laboratory by either the pipette or hydrometer method. Field evaluation requires practice with known samples. Once one has practiced field texturing can be accurate enough to determine the textural class most of the time. The laboratory methods are more accurate and can determine the percentage to 0.1 %. Theses methods are more time consuming. The degree of accuracy of laboratory methods is very high and may at first appear to be unnecessary. However, there are numerous cases where the properties of the minerals in the soil will make the soil appear courser or finer that it truly is. Given these situation errors can be made in the design of the system. Also there may be a dispute between several parties regarding the texture of a given soil. The best way to solve this dispute is with a laboratory test.
Particle Size Distrubution
Stokes’ Law:
V = 2r2g(ps-pl)/(9n)
V = velocity of fall
r = particle radius
g = acceleration due to gravity
ps = particle density
pl = liquid density
n = fluid viscosity
*Larger particles settle faster
than smaller ones
Determining Percent Silt and Clay (Hydrometer)
Based on Stokes Law
Uses a 100 gram sample
Particles dispersed w/sodium solution
Measures concentration of solids in suspension by suspension density
Less time consuming
Less accurate
Plunger for mixing soil in column
Determining Percent Silt and Clay (Pipette)
Based on Stokes Law
Uses a 10 gram sample
Particles dispersed w/sodium solution
Measures concentration of solids in suspension by weight
More time consuming
More accurate
Pipette Method
LOAM (Group II)
LTAR = 0.8 – 0.6 gpd/ft2
40 % Sand 40 % Silt 20 % Clay
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Please explain how to use the textural triangle for determining LTAR range.
25 % Sand 30 % Silt 45 % Clay
Clay (Group IV)
LTAR = 0.4 – 0.1 gpd/ft2
Soil and Site. Lindbo et al. DRAFT
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40 % Sand 30 % Silt 30 % Clay
CLAY LOAM (Group III)
LTAR = 0.6 – 0.3 gpd/ft2
Soil and Site. Lindbo et al. DRAFT
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Soil and Site. Lindbo et al. DRAFT
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Field determination of texture
Soil must be moist, not saturated; moist enough to mold like putty when you try to form a ball in your hand.
Does soil form a ball or cast?
No - the texture is SAND
Soil and Site. Lindbo et al. DRAFT
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The soil texture key should be tested for samples within ones own region. Soil materials behave in different ways depending upon the shape and size of the sand fraction, and the mineralogy of the clay fraction. Some sand fractions are flat in shape and will feel smooth (mica particles for example). Fine and very fine sand will behave in a manner similar to silt-sized particles. Clay fractions with appreciable smectite minerals will form long ribbons even with less than 40% clay.
Some people listen for fine and very fine sand when rubbing the sample. Some form a puddle in the palm of their hand and successively wash away the silt and clay until nothing but sand is left. Some folks look for a glistening or waxy look to the sample when forming a ribbon to distinguish loams from clay loams and silt loams from silty clay loams.
Field determination of texture
Can the ball be handled
No - the texture is LOAMY SAND.
When pressing the soil between thumb and forefinger does the soil form a ribbon that extends beyond your forefinger?
No - the texture is LOAMY SAND.
Making a ribbon
The length of the ribbon will depend on mineralogy as well as clay content
Field determination of texture
If the ribbon was < 1 inch long when it broke and the excessively wet sample feels:
gritty, the texture is SANDY LOAM;
smooth, the texture is SILT LOAM;
neither gritty nor smooth, the texture is LOAM.
Field determination of texture
If the ribbon was between 1 and 2 inches long when it broke and the excessively wet sample feels:
gritty, the texture is SANDY CLAY LOAM; smooth,
the texture is SILTY CLAY LOAM;
neither gritty nor smooth, the texture is CLAY LOAM.
Field determination of texture
If the ribbon > 2 inches long when it broke and the excessively wet sample feels:
gritty, the texture is SANDY CLAY;
smooth, the texture is SILTY CLAY;
neither gritty nor smooth, the texture is CLAY.
Clay %
Sand %
Particle Size and Reactive Surface Area
Soil Structure
Soil structure is the arrangement of the primary soil particles (sand, silt, and clay) and other soil materials into discrete aggregates.
Structural units are called peds, and have distinct boundaries and well-defined planes of weakness between the aggregates.
Peds consist of primary particles bound together by cementing agents like organic matter, clay, and hydrous oxides of iron and aluminum.
Peds can take several shapes.
Granular Structure
Resembles cookie crumbs and is usually less than 0.5 cm in diameter.
Commonly found in surface horizons where roots have been growing.
http://soil.gsfc.nasa.gov/pvg/granular.gif
http://soils.usda.gov/technical/manual/images/fig3-30_large.jpg
Blocky Structure
Irregular blocks that are usually 1.5 - 5.0 cm in diameter.
Can be subangular or angular blocky.
http://soil.gsfc.nasa.gov/pvg/blocky.gif
http://www.dpi.vic.gov.au/dpi/vro/glenimages.nsf/Images/gl167_profile/$File/gl167_profile.jpg
Prismatic Structure
Vertical columns of soil that might be a number of cm long.
Usually found in lower horizons.
http://soil.gsfc.nasa.gov/pvg/prismatic.gif
http://soils.usda.gov/technical/manual/images/fig3-27_large.jpg
Columnar Structure
Vertical columns of soil that have a salt "cap" at the top.
Found in soils of arid climates.
http://soil.gsfc.nasa.gov/pvg/columnar.gif
http://soils.usda.gov/technical/manual/images/fig3-28_large.jpg
Platy Structure
Thin, flat plates of soil that lie horizontally.
Usually found in compacted soil.
http://soil.gsfc.nasa.gov/pvg/platy.gif
http://soils.ag.uidaho.edu/soilorders/i/Arid_03.jpg
Single-grained Structure
Soil is broken into individual particles that do not stick together.
Always accompanies a loose consistence.
Commonly found in sandy soils.
http://soil.gsfc.nasa.gov/pvg/singlegrained.gif
Massive Structure
Soil has no visible structure, is hard to break apart and appears in very large clods.
http://soil.gsfc.nasa.gov/pvg/massive.gif
http://soils.usda.gov/technical/manual/images/fig3-31_large.jpg
How does structure affect water movement in soils?
In soils with good structure, the pore space that occurs between peds is relatively large and facilitates water and air movement.
Well-developed structure is very important in clayey soils.
Clayey soils with poor structure restrict water and air movement.
Soil Structure and Water Movement
Unsuitable soil structures for onsite wastewater treatment systems: platy, columnar, pristmatic, massive, angular blocky
Bulk Density vs. Particle Density
http://soil.gsfc.nasa.gov/pvg/pd3.htm
(Hodges et al., 2000)
Organic Soils
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A-Horizon- surface layer, typically
darkened with organic matter from roots,
litter layer, etc.
E-Horizon- zone of eluviation, where
iron, aluminum, organic matter, and/or
clay was translocated to a deeper layer
B-Horizon- layer of accumulation of clay,
Iron, aluminum, organic matter
Soil Series- classification system for
soils. Describe characteristics of soils
including texture, structure, depth, color,
drainage characteristics, etc.,
Soil Horizons- layers of soil with distinct characteristics
Published Soil Survey
Tract 13, Field 2433
Soil surveys include aerial photographs with overlays of the different
soil series in the area. Soil surveys provide important information about
soil properties such as depth to water table, permeability, and texture.
Soil surveys are great resources for many different applications concerning
land use.