Make and Take Soil Experiment for the Classroom

Objective:

To determine the amount of sand, silt, and clay particles in a given soil sample.

Introduction:

In science, matter is referred to as either organic or inorganic. Organic matter is of plant or animal origin. Examples of organic matter would be compost, leaves, and wood chips. Inorganic matter is of mineral origin. Iron, salts, and sand are examples of inorganic matter. Soil is composed mostly of inorganic matter. This inorganic matter is classified by particle size, as can be seen in the chart below:

Particle Size (mm) / Less than 0.002 / 0.002 to 0.06 / 0.06 to 2.0 / Greater than 2.0
Particle Name / Clay / Silt / Sand / Gravel

When suspended in water, the particles will settle according to their size. The larger particles will settle first, and the smallest particles will settle last. In this way, the different soil components of a given sample can be separated out and measured.

Soils needed:

A sample of soil that contains:

Sand

Silt

Clay

Other equipment:

Clear jars with lids

Water

Felt tip marker

Ruler

Procedure:

1.  Fill a jar half full with soil.

2.  Add water to the jar until it is full.

3.  Tightly close the jar with the lid.

4.  Shake the jar until the soil and water are mixed well.

5.  Allow the jar to sit undisturbed for 24 hours.

6.  After the 24 hours have passed, you should see layers. Use a felt-tipped marker to mark the layers on the jar.

7.  Identify each layer as sand, silt, or clay and write it in the chart.

8.  Using a ruler, take measurements to determine the composition of the soil in percents. (See sample calculation below the chart on next page.)

Soil Data

Layer / Particle Type / Height of Layer / Total Height / Height of Layer ÷ Total Height x 100 = %
High
Mid
Low

To figure percent use this example:

The height of the lowest layer is 7 inches.

The height of the entire soil sample is 10 inches.

7 inches of the 10 are composed of the lowest layer.

Divide 7 by 10

7 ÷ 10 = .7 .7 X 100% = 70% of the soil is composed of the lowest layer.

Questions:

  1. List the following soil components in order from smallest to largest: silt, clay, and sand.
  1. Which particle type was in the lowest layer in your jar? Which one was in the highest layer?
  1. Which particle type made up the highest percentage of your soil sample?

The Soil Triangle

Introduction:

Using the percentages you found and the soil triangle below, you can determine what kind of soil you have. Just find the spot in the triangle where two of your percentages line up. For example, if you found that the soil contained 40% sand, 40% silt, and 20% clay:

  1. Start along the bottom of the triangle and find the line marked “40.”
  2. Follow that line up and to the left until it crosses the horizontal line marked “20.”
  3. Read the name of the soil type. In this case, the soil is “loam.”
  4. Note that the point at which the line for 40% sand and 20% clay cross is also the point where the line for 40% silt crosses. (That line goes up and to the right.) You can find your soil type by looking at just two of your percentages. The third one will line up with the other two every time.

Questions:

  1. What type of soil do you have?

Soil for Plants

Soil type is important for the success of plants because it determines the amount of water the soil can hold, the amount of oxygen that will be available to plant roots, the relative temperature of the soil, the fertility of the soil, and how likely the soil is to become compacted.

Soil with a lot of sand dries out quickly, but allows ample oxygen to the roots. Because sandy soil does not hold much water, the temperature changes quickly. Sandy soil is relatively infertile, but is not likely to become compacted.

Soil with a lot of clay can hold water for a long time, but does not allow much oxygen to reach the roots. The temperature of clay soils does not change very quickly because the water in the soil has a high specific heat. Clay soils have a greater surface area per volume and so are able to hold more ionized minerals and nutrients. Because clay soils have the finest particles, they are more likely to become compacted.

Since silt particles are smaller than sand and larger than clay, soils high in silt will have qualities in between those described for sandy soil and clay soil.

Plants need access to loose, fertile soil that can hold plenty of water while allowing the roots access to oxygen. The best soil for a garden is loam, which has about 40% sand, 40% silt, and 20% clay. After that are silt loam, sandy loam, sandy clay loam, silty clay loam, and clay loam.

Questions:

  1. How well do you think plants would grow in your soil sample?
  1. What about your soil is good for plant growth?
  1. What about your soil might be bad for plant growth?