wastewater and soil due today

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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

2/11/2016

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

2/11/2016

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

Soil and Site. Lindbo et al. DRAFT

2/11/2016

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

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

2/11/2016

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

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

2/11/2016

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

2/11/2016

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

2/11/2016

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

2/11/2016

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

Soil and Site. Lindbo et al. DRAFT

2/11/2016

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

2/11/2016

40 % Sand 30 % Silt 30 % Clay

CLAY LOAM (Group III)

LTAR = 0.6 – 0.3 gpd/ft2

Soil and Site. Lindbo et al. DRAFT

2/11/2016

Soil and Site. Lindbo et al. DRAFT

2/11/2016

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

2/11/2016

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

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.