Activity: There is always space -
Soil Pores and Particle Size

Purpose

The purpose of this activity is for students to understand that soil has different size particles which can be packed together in many ways. How they are packed will determine how much air or water can fit between the pieces.

Overview:

Students will use large gravel and pebbles to represent soil particles. They will measure the space the 'soil particles' use and the space left over for air or water.

Time Needed: 1 class period

Level:

K-3, intermediate (advanced investigation)

 

Key Concepts:

  1. There is space between soil particles
  2. The amount of space and soil bulk density changes with:
  • soil packing (compaction)
  • particle size

Key Skills:

  1. Subtraction/addition
  2. Measuring with a ruler
  3. comparing amounts
  4. (Advanced Investigation) Calculating volume and percent

Materials:

  • Synthetic Soil
    • 2 kg, gravel (2-3 cm pieces, oblong sizes work best)
    • 2 kg, pebbles (5-10 mm)
  • Clear plastic cups (250 ml (10 oz)), (2-4 for each group of students)
  • Rulers
  • Measuring beakers or cups
  • Tap Water, 4 L

Key Words

  • GLOBE 3; Soil Moisture; Soil; Bulk Density; Physical Model;

Background:

packing Two terms that you will learn when talking about soil moisture are POROSITY and PERMEABILITY. Since soil is made of tiny pieces of broken rock and organic matter, there is always space between the pieces. The amount of space depends on the shape and size of the particles and how tightly they are packed (see Figure to right) Porosity is the term used for the amount of space between soil particles. Permeability is the ease with which water can flow through these spaces. If the particles are lined up like dominoes so that straight channels are made, the water can flow through quickly. If the water has to wind around, it flows more slowly. Permeability is determined by the properties of the soil and how it clumps together (aggregates).

The soil's porosity and permeability can be changed by adding things to the soil like mulch (dead plants which are ground up). It can also be changed by plowing or tilling, which 'fluffs up' the soil.

Soils are different around the world. Farmers in some places need to dry out their soil, while others need to add more water. Each needs to use different techniques to provide the proper soil moisture for the plants that grow there. Can you think of things to add to the soil to change the way it holds moisture?

Preparation:

Lay out workspaces for teams of 2-3 students with a container, synthetic soils, water and measuring instruments

Demonstration:

  1. Hold up a new box of cereal (ex: cornflakes).
  2. Ask students to guess how much of the box will be filled with cereal when you open it. Mark their predictions with a marker on the outside of the box.
  3. Open the box. Mark the level of cereal with a different color marker.
  1. Discuss with students why the box is not full. was the company just trying to fool you? was it more full before?
  2. Ask students if they think you could make the box more full without adding any more cereal. How?
  3. Close the top of the box and shake it for about 10 seconds. Ask students how much cereal will be in the box now.
  4. Open the box and mark the level of cereal.
  5. Ask students to explain why there is more cereal now: is there really more cereal? how could we test to see if there is really more now than before? why does the box seem fuller?
  6. If possible weigh the amount of cereal in the box to show that the same amount is there.
  7. Ask students what is taking up the space between the flakes. Air? Could that space be filled with milk or water?
  8. Hold up a clear plastic cup filled soil. Ask students if there could be space between the soil particles.

What to Do and How to Do It

  1. Have your students imagine that they have been able to greatly magnify common sand and silt grains to the sizes before them. Ask them how they could fill their container with 'soil particles' so that there would be the most room for air or water: what size particles do they want to use? why? how will they put them in?
  2. Have students fill their cups with gravel to leave the most room for water.
  3. Give each student a cup filled with water that is the same size as their soil cup.
  4. Ask students to guess how much of the water they think they can pour into the soil cup: all of it? half of the cup? less than half? Have them mark the water cup with a marker to show how much water they think will be left.
  5. Pour the water into the gravel cup until it reaches the top.
  6. How much of the water is left in the water cup? Use a different color to mark the level of the water left in the cup. Discuss the results: Is it close to what they guessed? What was the variation between groups of students? Why were some students able to use more water than others?
  7. Remove the gravel from the cup, leaving the water. Mark the level of the water on the soil cup.
  8. Ask students to repeat the experiment by planning how to pack the gravel so as to have the least amount of room for water in their 'soil column'.
  9. Pack the gravel in the empty soil cups to give the least amount of space.
  10. Have students mark the filled water cup to show how much water they think will be left.
  11. Pour the water into the gravel until it reaches the top.
  12. Mark the water cup to show the water left. Compare to the previous marks.How close was their prediction? Was it closer than the time before?
  13. Remove the gravel and mark the top of the remaining water. Did the student use more or less water than the time before? Why was there less room (or more room) for water?
  14. Have each group report their method and show their results. What methods resulted in leaving the least space for water? What was the smallest amount of water that was used? How could you make the amount of water held in the 'soil' even smaller?

    THINK BOX:
    How do you compact soil? How do you fluff it up? Why would you want to fluff it up?

    Would you expect the soil to be the same all the way down? Where would it be compacted most? What happens to worms as the soil is compacted?

    Intermediate and Advanced Activities:

    Figure the volume of your container.

    • Volume = pi x (radius of can)squared x (length of can)

    Figure the percentage of the volume which is taken up by gravel and water. How does this compare with the percent of water that is found in your soil moisture samples?

    Estimate the bulk density of the different soil columns. Measure the weight of the dry material (bulk density) and compare it with the volume of water for each soil column. How is bulk density related to water content?

    Find examples of sites with high amounts of soil water content from the GLOBE server. Look at soil data from these sites to see if the soil characteristics are consistent with what students associated with high soil water content. Use GLOBEmail to contact other schools and ask questions about sites where soil moisture is different than what you expect.

Further Investigations

  1. Use mixtures of gravel, pebbles, and soil? Can you predict how much water is required to fill your container?
  2. Devise experiments to test erosion on soil that has been compacted or tilled.
  3. Devise experiments to test how much water a soil will hold if compacted or tilled.
  4. Research how and why soils are compacted for construction projects.

Evaluation:

  1. Which soils would hold more water, those in a plowed field or on a road?
  2. Why do farmers plow fields or homeowners aerate (poke holes in) their lawns?
  3. Why don't plants grow on old dirt roads, even years after no one drives on them?

Last updated: 5/3/97 Comments? globe@hwr.arizona.edu

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