Instruction in this lesson should result in students achieving the following objectives:
1
Describe the concept of soil texture and its importance.
2
Identify the classes of soil separates.
3
Use a soil textural triangle to identify fine, medium, and coarse textured soils.
4
Identify the texture of a sample of soil.
5
Explain why it is difficult to change the soil texture.
Anticipated Problem: What is soil texture and why is it important?
I. Soil texture is the comparison of soils due to their separates.
A. Sand is the largest soil particle, as it offers the most drainage.
B. Clay is the smallest soil separate, as it offers the least drainage.
Illinois Physical Science Applications in Agriculture Lesson B1 –5 • Page 2
C. Silt is a medium sized soil particle that drains between fast and slow.
D. Loam 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 material 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.
F. 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?
II. Soil separates are classes of soil material less than 2 millimeters in diameter.
A. Fine texture represents soils that are composed mainly of clay.
B. Medium texture soils have intermediate amounts of sand and clay, with a large representation of silt.
C. 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 letting the particles settle. Once settled the percentages of each can be evaluated using the texture triangle.
B. The triangle will separate the soil into classes as well as coarse, fine and medium textured soils.
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 quick method of determining soil texture. If a long ribbon can be formed the soil is fine in texture. If no ribbon is formed the soil is coarse in texture.
Anticipated Problem: 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 formation. They do not change without major environmental conditions.
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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 texture but takes many years to begin to work.

Part One: Matching
Instructions: Match the word with the correct definition. a. texture c. structure b. bulk density d. consistence
_______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 Two: Fill-in-the-Blank
Instructions: 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 Three: Multiple Choice
Instructions: 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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_______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 area? 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. percolation c. moisture d. color
_______8. A soil that is mostly clay would be considered _______. a. fine-textured b. medium-textured c. moderately coarse-textured d. coarse-textured
Part Four: Short Answer
Instructions: 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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Technical Supplement
1. What are the differences between soil texture, structure, consistence and tilth?
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 meters 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 quite 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 clay 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 sized 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 “natural” 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 describe 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 soil 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 water 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 clay 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 that 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
2
. If that same gram of sand were ground to clay-sized particles the surface area would be approximately
8,000,000 cm
2
. 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 particles 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 24 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 settled 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 colloidal 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?
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 ideal soil will

have approximately 50% of the volume filled with air so the bulk density would be one-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 volume 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 determined. 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 indication 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 scrapers, 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 life. The result is often high plant stress and greatly reduced crop yields or even crop failure.
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References
Prepared by WayneL. Banwart, Professor, Department of Agronomy, University of
Illinois, Urbana, Illinois
Soil Separates
Separate
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 visible
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 the millimeters. The comparison shows the differences by setting a very coarse sand grain equal to three feet in size.
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