Soil Sizes - Some Surprises!

Purpose

To understand how larger diameter particles like sand settle more quickly than silt and clay, and to understand what Particle Size Distribution means. To understand how the distribution of different sized particles in the natural environment affect how the landscape appears and how soil can and cannot be used. To learn about soil texture.

Overview

The size of particles is a very important characteristic of soil. Smaller particles, like clay, will not let water pass through as easily as larger particles, like sand. Soil that allows water to drain, and not pond, is best for growing gardens, building buildings, and many more uses. However, soils that retain water create good habitats for fish and waterfowl and are good for recreational uses, but not for roads and buildings.

Time

One class period, and allowing filled bottles to stand overnight once completing all steps.

Level

Beginner to Intermediate

*(Intermediate to Advanced students can learn more in depth information by performing the ÒMaking Sense of the Particle Size Distribution MeasurementsÓ activity.)

Key Concepts

• How larger diameter particles settle more quickly than smaller diameter particles
• How different sized particles are distributed to create specific soil textures
• How soil particle size distribution affects the movement of water, heat, and nutrients in soil.
• How soil texture impacts the use of soils

Skills

• Identifying variations of particle sizes Acknowledging the behavior of soil particles in water
• Analyzing the rate of settling of soil particles
• *Understanding that gravity, velocity, particle size, temperature, and liquid are factors that influence settling of soil particles.

Materials and Tools

• Potting Soil - 1 1/2 cups
• Sand - 1/2 cup
• 2 juice bottles with wide mouths and lids
• Paper or plastic cups
• Masking tape
• Pen
• Metal teaspoon
• Water

Preparation

Conduct a discussion about sand, silt, and clay, where they are found in the natural environment.

Discuss that knowing how much sand, silt and clay are in a soil help determine how much water and heat will be held in soil and move through soil, and also how well nutrients will be held in soil for plant use.

Background

Existing in the natural environment are mineral materials that make up soil. There are three particle sizes of mineral material known as sand, silt and clay.

The United States Department of Agriculture (USDA) defines particle sizes as:

sand 2.0 mm - 0.05 mm in diameter silt 0.05 mm - 0.002 mm in diameter clay less than 0.002 mm in diameter

The International Soil Science Society (ISSS) defines particle sizes as:

sand 2.0 mm - 0.02 mm silt 0.02 mm - 0.002 mm clay less than 0.002 mm

The amount of each size particle in soil is called particle size distribution. Since it is very hard to determine the approximate amount of each particle size in soil, certain tests are done to determine their amounts. Here is a simplified test that gives students the basic ideas pertaining to the soil particle size distribution test soil scientists use.

Preparation

Use your masking tape and pen to label one bottle ÒsoilÓ and the other bottle Òsoil and sandÓ

Put about one cup of soil in a cup

What to Do and How to Do It

1. Fill the soil bottle about 3/4 full of water. Very slowly pour the soil into the water.

What do you notice about the way the soil particles sink in the water? Do the different sizes seem to sink at different speeds?

2. After all the soil is in the water, fill the rest of the bottle with water if the water is not to the top already. Put the lid tightly on the bottle. Shake the bottle back and forth several times. Place the bottle on a flat surface where it will not be disturbed.

3. Take your remaining 1/2 cup of soil and add 1/2 cup of sand to the soil. Mix thoroughly with your spoon.

4. Repeat steps 1 and 2 with the soil and sand bottle and the soil and sand mixture. Wash hands after handling soil. Allow both bottles to stand overnight. Observe the bottles the next day. Look at the bottles from the side.

class investigation- Do you notice any layers in the bottles? How would you describe the particles that make up these layers? Are some layers thicker than others? Do you think this is a good way of finding out the amount of different size particles in soil? Why or why not?

Further Investigations

1. Using the Textural Triangles 1 and 2 in the Soil Characterization Field Protocol, determine the texture of soil sample(s) by feeling the moistened soil sample(s).
2. Students can practice determining the percent sand, silt, and clay in samples using the hand ÒtexturingÓ method along with extural Triangle 3. They can accurately verify their estimations by performing the Particle Size Distribution Protocol, which is more quantitative.
3. Develop a set of 12 standard soil texture samples that students can use to practice determining soil texture. List the percent sand, silt, and clay (from actual values determined using each standard soil sample in the settling method) next to each appropriate standard.
4. Once the students have had enough practice referring to the list, have them determine the soil texture of each standard without referring to the list.
5. Challenge students to think of and write about the different uses of soils, based upon soil particle size distribution characteristics.

More Advanced

Advanced students can use the StokeÕs Law procedure to calculate the velocity and settling time of different sized particles (in cm).

where:

V= settling velocity (in cm/second)

d= particle diameter in cm (such as 0.2 cm - 0.005 cm for sand, 0.005 cm - 0.002 cm for silt, and <0.002 cm for clay)

k= a constant which depends on the liquid in which the particle is settling, particle density, the force of gravity, and the temperature (8.9 x 10³ cm ^-1 sec^-1 for soil in water at 20 degrees C).

Student Assessment

Students should be able to acknowledge that sand, silt, and clay are found in soil and that because of their difference in composition they are distributed differently not only in the bottles, but in the natural environment.

Advanced students should be able to understand that these characteristic differences impact land use and determine why vegetation, roads, buildings, etc., are located where they are. Advanced students should be able to understand and/or discuss what happens when buildings and roads, for instance, are built on soil that is not good for these purposes. They should also be able to understand and/or discuss why lakes and wetlands are located where they are.

Acknowledgments:

Adapted from Soil Sizes-Some Surprises!, WonderScience magazine on soil science. Also adapted from L.J. Johnson. 1979. Introductory Soil Science: A Study Guide and Laboratory Manual., MacMillan Publishing Co., Inc., N.Y.