Feb 22, 2014 (7 years and 4 months ago)




Student Learning Objectives.
Instruction in this lesson should result in

achieving the following objectives:

Describe the concept of soil texture and its importance.

Identify the classes of soil separates.


Use a soil textural triangle to identify fine, medium, and coarse textured soils.

Identify the texture of a sample of soil.

Explain why it is difficult to change the soil texture.

Anticipated Problem:
What is soil texture and why is it important?


Soil texture
is the comparison of soils due to their separates.

is the largest soil particle, as it offers the most drainage.

is the smallest soil separate, as it offers the least drainage.

Illinois Physical Science Applications in Agric
ulture Lesson B1

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is a medium sized soil particle that drains between fast and slow.

is a mixture of relatively equal amounts of sand, silt or clay.

E. The soil texture also affects
pore space
, a portion of the soil not occupied

by solid

but filled with air or water. The larger the soil particle the larger the pore spaces.

Small particles have more spaces but they are smaller in size.

Particle density
is the mass per unit volume of the soil and is determined largely
by the

texture. The
Bulk density
of the soil is the particle density after the water has been dried.

The density is a major factor in dealing with root movement and stability of the soil.

Anticipated Problem:
What are the classes of soil separates?

oil separates
are classes of soil material less than 2 millimeters in diameter.

Fine texture
represents soils that are composed mainly of clay.

Medium texture
soils have intermediate amounts of sand and clay, with a large

of silt.

Coarse texture
represents those soils that are largely composed of sand.

Anticipated Problem:
How can soil textural triangle determine fine, medium and coarse

textured soils?

III. A soil texture triangle is a tool that is used to determine soil texture.

A. The soils can be separated into their separates by mixing the soil with water and

the particles settle. Once settled the percentages of each can be evaluated using the


B. The triangle will separate the soil into classes as w
ell as coarse, fine and medium


Anticipated Problem:
How can you identify the texture of a soil sample?

IV. Soil texture can be determined in many ways.

A. The sedimentation method using the texture triangle is the most detailed method.

B. The ribbon method where soil is pressed between the thumb and index finger is a

method of determining soil texture. If a long ribbon can be formed the soil is fine in

If no ribbon is formed the soil is coarse in texture.

Anticipated Prob
Why is it difficult to change soil texture?

V. Soil texture is a natural characteristic of soil.

A. The relative components of the soil separates are established by the soil early in

They do not change without major environmental conditio

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B. Agricultural land is very large and an attempt to mix soil separates manually would be

impossible. Texture cannot be mixed without nature. Organic matter can help change

ure but takes many years to begin to work.


Part One: Matching

Match the word with the correct definition.

a. texture

c. structure

b. bulk density

d. consistenc

_______1. A soil’s response or resistance to pressure.

_______2. Mass of oven dry soil in relation to volume.

_______3. The arrangement of soil particles into aggregates or peds.

_______4. The relative proportion of soil separates in a soil.

Part Tw
o: Fill

Complete the following statements.

1. The smallest of the soil separates is _________.

2. A relatively even mixture of sand, silt and clay is _________.

3. The largest of the soil separates is ______________.

Part Th
ree: Multiple Choice

Write the letter of the correct answer.

_______1. The size of soil particles affects _______.

a. color

b. water
holding capacity

c. mineral rocks

d. phosphorus

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n B1

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_______2. Which of the following is the smallest soil particle?

a. sand

b. clay

c. silt

d. quartz

_______3. The soil texture class where sand, silt, and clay contribute almost equally to soil
properties is called


a. onieda

b. tama

c. muscatine

d. loam

_______4. The size and number of pore spaces in a soil is dependant on _______.

a. humidity

b. erodability

c. particle size

d. soil separates

_______5. Soils comprised of which particle size would have the greatest material surface


a. clay

b. sand

c. silt

d. loam

_______6. Common types of soil structure are prismatic, platy and _______.

a. sand grains

b. colloids

c. granular

d. clay

_______7. Texture, structure and _______ are major soil characteristics.

a. infiltration

b. p

c. moisture

d. color

_______8. A soil that is mostly clay would be considered _______.

a. fine

b. medium

c. moderately coarse

d. coarse

Part Four: Short Answer

Answer the following questions.

1. How does soil texture affect water holding capacity?

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2. Describe the ribbon method of finding soil texture.

3. Provide two characteristics of a loam soil.

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cal Science Applications in Agriculture Lesson B1

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Technical Supplement



1. What are the differences between soil texture, structure, consistence and


Soil texture refers to the size of
the individual particles that make up a soil. By definition

it is the relative amount of sand, silt, and clay in a soil. Clay particles are

extremely small particles, less than .000002 meters in size. Silt particles range from

.000002 meters to .00005 mete
rs in diameter and sand sized particles range from

.00005 meters to .002 meters (.05 to 2 mm). If the soil is in a very moist condition

these particle sizes feel differently when rubbed between ones fingers. Sand feels

“gritty,” silt sized particles feel q
uite smooth or a “flour like” and clay
sized particles

are quite sticky when wet. Particles larger than .002 meters (2 mm) such as gravel or

rocks are not taken into consideration when soil texture is assigned. Based on the

percentage of sand, silt, and cl
ay in the sample, one of twelve textural classes is

assigned. Soil texture measurements are concerned only with particle size and not

with chemical composition. Chunks of a pure metal, like iron, could be crushed and

ground into sand, silt, and/or clay siz
ed particles. Equal percentages of sand, silt and

clay sized iron would make a clay loam “soil.”

Soil structure refers to how these sand, silt, and clay sized soil particles are arranged

into stable structural units called aggregates or peds. These are “na
tural” peds that

develop as soils form over long time periods. As the soil breaks apart structural units

will be formed that take on different shapes, described with names such as platy (flat

like), blocky, granular, and prismatic (prism like shapes). Red
surfaces form “natural

cleavage planes.” Plant roots and water can move more readily and air exchange to

plant roots is enhanced. Ground up iron would form no natural peds and its structure

would be termed massive.

Consistence is a soil term used to descri
be how easily a soil deforms under pressure.

The official terms used depend on how moist the soil is when this attribute is tested

but would include terms like loose, friable, firm or extremely firm if the soil was

moist. In a very wet soil the terms used
to describe soil consistence would be terms

like nonsticky, sticky, plastic, and very plastic. The consistence of a soil is determined

by squeezing the soil, while structure is determined by carefully breaking a

soil apart to expose the shape of natural so
il peds.

Soil tilth is a very qualitative term. Good tilth implies a soil that is easy to till, forms

an excellent seedbed for planting seeds, and does not impede seedling emergence or

root penetration. A soil with ideal texture, structure, and consistence

will have

excellent tilth.

2. Why is soil texture important?

Many important chemical and physical properties of soils are a function of the particle

size of soils. The amount of sand, silt, and clay in a soil will determine such properties

as total wa
ter holding capacity, plant available water holding capacity, cationexchange

capacities, soil surface area, the amount of total pore space and relative

proportion of large (macro) and small (micro) pores. In any given climatic zone the

relative amount of c
lay in a soil will greatly influence the organic matter content of

that soil. Soil texture is also important in determining soil management practices

such as proper herbicide and fertilizer rates. These differences in properties are

related to the fact tha
t smaller particle sizes result in much larger surface areas. For

example, one gram of very course sand will have a surface area of 11 cm
. If that same

gram of sand were ground to clay
sized particles the surface area would be approximately

8,000,000 cm

The tremendous influence of particle size on surface area and

soil reaction makes texture a very significant soil property.

3. What is the principle involved in measuring soil texture by sedimentation?

The principle is simply that different sized particl
es settle in water at different rates.

Stokes Law of Physics says particles will fall at a velocity which is proportional to the

square of the radius of the particles, the force of gravity, the density of the particles

and water, and the viscosity of water
. Since the rate of settling is proportional to the

square of the radius of the particles, large particles will fall much faster than small

particles when suspended in water. Using the procedure outlined in PSAA 124, a

depth of material is measured after 2
4 hours settling. That reading represents sand

+ silt 4

clay. Another reading is taken exactly 40 seconds after mixing. At this time

the laws of physics (Stokes Law) can be used to mathematically determine that all

the sand sized soil particles have settle
d to the bottom of the jar. The depth of material

in the bottom of the jar is proportional to the percentage of sand in the sample

being tested. A reading is also taken after 30 minutes. Again the theory of physics

determines that after 30 minutes all the
sand and all the silt will have settled out. The

depth of soil after 30 minutes represents the amount of sand and silt in the sample.

By difference the clay sized particles settle out between 30 minutes and 24 hours.

Extremely fine clay particles called co
lloidal clay may still be suspended even after 24

hours. Stokes Law, upon which this theory is based, assumes that all soil particles are

of equal density and spherical. Since soil particles are mainly quartz based, their density

is quite uniform. However,

not many soil particles are perfect spheres and clay

particles tend to be plate like in shape. In spite of this limitation, texture by sedimentation

provides very good results.

4. Why is soil density important and how does one physically measure soil


Density of any object is the weight of the object (mass) divided by the volume. We

can determine particle density and bulk density of soils. Particle density is the density

of the soil solids or soil minerals only. Particle density is approximately 2
.65 g/cc

and is quite uniform for most soils. Bulk density, on the other hand, measures the

density of soil as it exists in the field and includes pore space or air volume that is

very important for air exchange to roots and for storing soil water. An idea
l soil will

have approximately 50% of the volume filled with air so the bulk density would be

half of the particle density or about 1.3 g/cc. We are most interested in the bulk

density of soils, which is the weight of the soil divided by the total volu
me of the soil.

One way to measure bulk density would be to drive a coring ring into the soil. The

ring is removed and placed in an oven to dry. The volume of the coring ring can be

calculated and the weight of the soil that occupied that ring is determine
d. Simple

division gives us bulk density (g/cc). Other methods can be used to determine bulk

density in soils but all must measure the oven
dry weight of the soil and the volume

the soil occupies in its “natural state.” Bulk density is important as an indi
cation of

air and water movement and especially root penetration.

Bulk density is the main problem in attempting to reclaim prime farmland that has

been strip mined for coal. That process often involves replacing topsoil after mining

with rubber tired scra
pers, which severely compact the soil resulting in high bulk

densities (often greater than 1.6

1.8 grams/cubic centimeter). High water cannot

penetrate bulk densities and plant roots cannot penetrate the dense soil to get water

and nutrients to sustain lif
e. The result is often high plant stress and greatly reduced

crop yields or even crop failure.

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Prepared by WayneL. Banwart, Professor, Department of Agronomy, University


Illinois, Urbana, Illinois

Soil Separates


Diameter (mm)

Comparison Feel

Very coarse sand 2.00

1.00 36" Grains easily seen, sharp, gritty

Coarse sand 1.00

0.50 18"

Medium sand 0.50

0.25 9"

Fine sand 0.25

0.10 4½" Gritty, each grain barely visib

Very fine sand 0.10

0.05 1¾"

Silt 0.05

0.002 7/16" Grains invisible to eye, silky to touch

Clay < 0.002 1/32" Sticky when wet, dry pellets hard, harsh

The United States Department of Agriculture System of Soil Separates. The diameter of particles is in

millimeters. The comparison shows the differences by setting a very coarse sand grain equal to three feet in size.

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