Soil Texture

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Soil Texture

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 1


NDWRCDP Disclaimer

This work was supported by the National Decentralized

Water Resources Capacity Development Project

(NDWRCDP) with funding provided by the U.S.

Environmental Protection Agency through a Cooperative

Agreement (EPA No. CR827881-01

01-0) 0) with Washington

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onsite/decentralized wastewater industry. These materials have

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Citation

- Stolt, M. H., Lindbo, D.L., R. Miles, D. L.

Mokma, and S. Greene. 2005. 3. Field

Description of Soils: Texture – Power

Point Presentation. in (D.L. Lindbo and N.

E. Deal eds.) Model Decentralized

Wastewater Practitioner Curriculum.

National Decentralized Water Resources

Capacity Development Project. North

Carolina State University, Raleigh, NC.


Soil Texture

‣ Use texture to make inferences into pore

size

‣ From pore size begin to estimate water

movement and treatment

• Finer texture measn slower water movement

• Finer texture means greater treatment

Texture by itself is not enough information

to determine site suitability


Other factors that combine

with texture

Soil structure

‣ Organic matter and vegetation

Soil mineralogy

‣ Land use

‣ Landscape position

‣ Parent material

Soil wetness


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 Texture (mineral material

only)

‣ Sand - gritty

‣ Silt - smooth, velvety

‣ Clay - slick, sticky


Systems for Classifying Particle

Size Distributions

‣ USDA System (ie.. Sandy Loam)

‣ AASHTO: American Association of State

Highways (ie(

ie. . A-1) A

‣ Unified Engineering (ie.. SM)

‣ Wentworth (phi #)


USDA Particle-Size

Distribution

‣ Fine-earth earth fraction: : Finer than 2 mm:

Used for Soil Textural Class

‣ Rock fragments: 2 mm in diameter or

larger. Particles less than 10” are many

times called “coarse fragments”. Used

to modify textural class.


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


Examples

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 16


40 % Sand

40 % Silt

20 % Clay

Sand + Silt + Clay = 100%

Texture = LOAM


25 % Sand

?? % Silt

45 % Clay


25 % Sand

30 % Silt

45 % Clay


25 % Sand

30 % Silt

45 % Clay

CLAY


65 % Sand

20 % Silt

?? % Clay


65 % Sand

20 % Silt

15 % Clay


65 % Sand

20 % Silt

15 % Clay

SANDY

LOAM


?? % Sand

30 % Silt

30 % Clay


40 % Sand

30 % Silt

30 % Clay


40 % Sand

30 % Silt

30 % Clay

CLAY

LOAM


Particle-Size Distribution

‣ Particle size distribution describes the

abundance (by weight) of the various size

particles that constitute the mineral portion

of soil materials.


Fine-earth earth fraction

‣ Sand - 2.0 - 0.05 mm

‣ Silt – 50 – 2 um

‣ Clay - < 2 um


Sand Fractions

‣ Fraction

Size

‣ ___________________________(mm)

(mm)__

‣ Very coarse sand.......................2.0 to 1.0

‣ Coarse sand..............................1.0 to 0.5

‣ Medium sand..........................0.5 to 0.25

‣ Fine sand..............................0.25 to 0.10

‣ Very fine sand........................0.10 to 0.05


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


Textural Groups for OSWW

‣ Group I:

‣ Group III:

• Sand, Loamy sand

• Sandy clay loam,

• 1.2 – 0.8 gpd/ft 2 Silt loam, Clay

‣ Group II:

loam, Silty clay

loam, Silt

• Sandy loam, Loam

• 0.8 – 0.6 gpd/ft • 0.6 – 0.3 gpd/ft 2

2

‣ Group IV:

• Sandy clay, Silty

clay, Clay

• 0.4 –0.1 gpd/ft2


LOAM

40 % Sand

40 % Silt

20 % Clay

LTAR =

0.8 – 0.6 gpd/ft 2


25 % Sand

30 % Silt

45 % Clay

CLAY

LTAR =

0.4 – 0.1 gpd/ft 2


65 % Sand

20 % Silt

15 % Clay

SANDY LOAM

LTAR =

0.8 – 0.6 gpd/ft 2


40 % Sand

30 % Silt

30 % Clay

CLAY LOAM

LTAR =

0.6 – 0.3 gpd/ft 2


Determining Texture

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 37


Determination of Texture

‣ Field procedure

‣ Laboratory procedure

• Hydrometer

• Pipette


Field Determination of

Texture

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 39


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.

Soil Texture Class Key*

‣ Does soil form a ball or cast?

‣ No - the texture is SAND


Soil does not form a cast:

Textural class is SAND


Field determination of texture

‣ Can the ball be handled

‣ No - the texture is LOAMY SAND.

‣ OR

‣ 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.


Forms a cast of moist soil material.

Textural class is LOAMY SAND


The length of the ribbon will depend on

mineralogy as well as clay content

Making a ribbon


Field determination of texture

‣ If the soil forms a ribbon that that extends

past the forefinger, note the length of the

ribbon.

‣ Next excessively wet a small sample in the

palm and rub with the forefinger.


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.


Laboratory

Determination of

Texture

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 50


Laboratory Determination of

Particle-Size Distribution

‣ Percent sand and rock fragments are

determined by a weight percent remaining

in a sieve.

‣ Silt and clay fractions are determined

based on how fast a particle of a given

size falls in a sedimentation column.


Determining Percent Silt and

Clay (Hydrometer)

‣ Based on Stokes Law

‣ Uses a 100 gram sample

‣ Particles dispersed w/ calgon

‣ Measures concentration of solids in

suspension by suspension density

‣ Less time consuming

‣ Less accurate


Determining Percent Silt and

Clay (Pipette)

‣ Based on Stokes Law

‣ Uses a 10 gram sample

‣ Particles dispersed w/ calgon

‣ Measures concentration of solids in

suspension by weight

‣ More time consuming

‣ More accurate


Stoke’s Law

‣ The settling time of a particle of a given

density falling in a liquid of a given

viscosity is proportional to the square of

the particles radius.

‣ What does this mean? – Bigger particles

fall faster!!!

and

‣ If we know the viscosity of the liquid and

the density of the particles we can figure

out their size.


Stokes’ Law:

V = 2r 2 g(p s -p l )/(9n)

V = velocity of fall

r = particle radius

g = acceleration due to gravity

p s = particle density

p l = liquid density

n = fluid viscosity


At 20 o C how long does it take a 2um particle

to fall 1 cm in a 0.5 g/l solution?


At 20 o C how long does it take a 2um particle

to fall 1 cm in a 0.5 g/l solution?

8.41h/10h = .841h or 50.5 min.


1000 ml

Sedimentation

Column


Plunger for

mixing soil in

column


Pipette Setup:

Lowy Pipette

Pump

Stand

Bell Jar

Sieve

Funnel


25 ml Lowy

Pipette for silt

and clay

determination


Is there texture beyond

just Sand, Silt and

Clay?

And so what if there is.

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 72


Textures Classes with Sand-Size

Size

Modifiers

‣ Coarse sand (CoS(

CoS): : >25% very coarse and

coarse sand

‣ Fine sand (FS): : >50% fine sand

‣ Very Fine sand (VFS): : >50% very fine sand

‣ Loamy coarse sand (LCoS(

LCoS): : >25% very coarse

and coarse sand

‣ Loamy fine sand (LFS): : >50% fine sand

‣ Loamy very fine sand (LVFS): : >50% very fine

sand


Textures Classes with Sand-Size

Size

Modifiers

‣ Coarse sandy loam (CoSL(

CoSL): : >25%

very coarse and coarse sand

‣ Fine sandy loam (FSL): : >30% fine

sand

‣ Very Fine sandy loam (VFSL): : >30%

very fine sand


Terms for describing rock fragments.

Rounded, subrounded, and irregular:

_____________________________________

Size (mm) Type Adjective

2 to 76...........Gravels.......gr.............gravelly

2 to 5.............Gravels......grf.......fine gravelly

5 to 20...........Gravels.....grm....med. gravelly

20 to 76.........Gravels......grc...coarse gravelly

76 to 250........Cobbles......cb...............cobbly

250 to 600......Stones.........st................stony

>600..............Boulders.....by............bouldery


Terms for describing flat rock fragments.

_____________________________________

Size (mm) Type Adjective

2 to 150.........Channers....cn............channery

150 to 380......Flagstones...fl.................flaggy

380 to 600......Stones.........st................stony

>600..............Boulders.....by............bouldery


Using Rock Fragment Modifiers

‣ Less than 15 percent: : No adjective or modifier

terms are used.

‣ 15 to 35 percent: : The dominant kind of rock

fragment is used as an adjective ie. . "gravelly

loam”.

‣ 35 to 60 percent: : An adjective term with the

word "very" is used ie. . "very gravelly loam“.

‣ More than 60 percent: : An adjective term with

the word “extremely" is used ie. . "extremely

gravelly loam."


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


Relating Texture

to Hydraulics

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 79


Pore

Space


Median percolation rates for 220 subsoil horizons grouped by textural class.

Textural Class

cm/hr

in/hr

Number of

Observations

Silty Clay Loam

1.3

0.5a*

22

Silt Loam

2.3

0.9a

33

Clay Loam

2.5

1.0a

18

Silty Clay

3.3

1.3ab

9

Loam

4.1

1.6ab

71

Clay

4.3

1.7ab

16

Fine Sandy Loam

12.2

4.8b

19

Sandy Loam

14.7

5.8b

17

Loamy Coarse Sand

27.7

10.9b

5

Loamy Sand

47.2

18.6b

4

Sandy Clay Loam

48.8

19.2b

6

Coarse Sand

83.8

33.0b

2

Data taken from Matelski 1975.

*Medians with different letters are significantly different at the 0.05 level.


Interpreting Textural

Modifiers

Rules of thumb:

‣ As coarse fragments (CF) increase

loading rates decrease

‣ As CF becomes platy loading rates

decrease

‣ As sands become finer loading rates

decrease


Organic Matter

3/19/2005

Soil and Site. Lindbo et al.

DRAFT 89


Organic Matter in Soils

‣ Coloring agent

‣ Water holding capacity

‣ Fertility

‣ Cementing agent for aggregation

‣ Organic matter may feel smooth (like silt)

and sticky (like clay) and therefore

interfere with your texture by feel


Organic Matter Subdivisions

‣ Organic Soil Materials

‣ Mineral Organics

‣ Mineral


Organic Matter Subdivisions

‣ Muck

‣ Mucky Mineral

‣ Mineral


Organic Carbon Content

% Organic Carbon

21

18

15

12

9

6

3

0

Muck

Mucky Mineral

Mineral

0 30 60

% Clay


Identifying Organic Material

by “Feel”

‣ Difficult to do without practice

‣ Need to practice on samples with known

carbon contents


Types of Soil Organic

Materials

‣ Sapric (Oa) – Very decomposed, 40%

rubbed fibers


Types of Soil Organic

Materials

‣ Sapric (Oa) – Muck

‣ Hemic (Oe) – Mucky Peat

‣ Fibric (Oi) – Peat

‣ Mineral Organic – Mucky Sandy Loam


Type of Organic Soil

‣ Muck – Highly decomposed. < 1/6 fibers

remaining after rubbing. Sapric material

‣ Mucky Peat – Moderately decomposed.

Between 1/6 and ¾ fibers remaining after

rubbing. Hemic material

‣ Peat – Slightly decomposed. > ¾ fibers

remaining after rubbing. Fibric material.


Identifying Organic Soil Type

‣ Rub moist sample between fingers 10

times

‣ Examine material with hand lens

‣ Look for fibers…not live roots... and

estimate percent

‣ Fibers are smaller than 2 cm (approx. 1”)

and show cellular structure


Knowing about texture can

keep you from getting stuck

if nothing else

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