To develop an understanding of the relationship between soil water content, texture and particle shape.

Students will measure the amount of water "sticking" to the glass beads of various sizes (representing grains of soil) after the water is emptied from the container. Students will also experiment with the height of water in a column of soil placed in contact with water-saturated soil.

2-3 class periods

All

• porosity
• permeability
• capillary water
• thin film "coat"
• particle size
• sorting

• asking questions
• developing hypothesis
• testing hypothesis
• observing results
• analyzing results
• drawing conclusions
• measuring the weight and volume of objects
• measuring the height of water column
• timing

• large plastic cups
• glass beads or marbles of at least two different sizes
• other small objects (nails, washers)
• sand
• gravel
• large plastic container
• ring stand
• small plastic plate (large enough to cover the plastic cup)
• water
• scale or balance
• ruler
• plastic tube ~3 cm in dia and at least 20 cm long
• 2 Plastic graduated cylinders (at least 100 mL)
• marker

None are required, however Just Passing Through activity is recommended as a starting point

Soils and rocks hold water in the space between grains of the material. That space is called pore space. The more space between grains, the more water it can hold. We call this property porosity. Porosity is calculated as the ratio of the volume of pore space to the volume of sample (soil+air). The porosity of a material depends on many things, such as the shape of particles (round particles have a lot of space between them, whereas flat particles fit together more closely. The porosity also depends on how well the particles are sorted. If the material is well sorted, that is, all particles have the same size, then the pore space between them is the greatest. On the other hand, in poorly sorted materials (for example, a mixture of sand and gravel), small particles fill spaces between larger particles, and the porosity of the material decreases.

Permeability describes the ease at which water can pass through the pore spaces in the material (see Just Passing Through activity). Water passes more quickly through the materials which have large, connected pore spaces such as gravel or coarse sand.

Water fills pore space readily. Most soils experience rapid gravity drainage after a soaking rain but some water passing through the material will "coat" (leave a very thin layer of water) around each particle. Still more water remains behind in tiny spaces between grains. This water is called capillary water. This water can be removed only by evaporation.

Infiltration describes the rate at which water can pass through the soil (see Infiltration protocol)

Part 1. Modeling Porosity

• Prepare plastic cups with materials of various shapes: for example a cup of glass marbles, a cup of small nails, and a cup of metal washers. You will need enough marbles (beads, nails, washers, etc.) to fill the volume of 50 mL.

Part 2. Sticky Water

• Prepare plastic cups with marbles or glass beads of 2 different sizes
• Cut out a large circle out of the plastic plate (large enough to cover the cup)

Part 3. Modeling Capillary Water

• Obtain a plastic tube (for example the kind used to protect the fluorescent lights), cut it into 20 cm sections
• bring samples of sand or soil
• prepare a ring stand with a clamp to hold a plastic tube in place
• fill a plastic container with sand (about half way through), place the plastic tube on the sand surface (do not force it into the sand, but make sure that it touches the surface) and fill it half-way with the sand.
• Prepare a bottle or a beaker of water (you need enough water to fill the container so that the sand at the bottom is completely saturated).

1. Fill a 100 mL graduated cylinder with glass beads (or other material of your choice) to the 50mL mark (if desired, a larger volume can be used, but you must keep track of the value on your Data Sheet. Point out to the students that there are spaces between objects (air/pore spaces)
2. Ask students to predict how much pore space is present in the cylinder and compare that to another cylinder, filled with flat objects, such as buttons of washers. Record students' ideas on the board and/or in the GLOBE notebooks. Note the connection between this three-dimensional prediction and the two-dimensional prediction that is done in the Cloud Cover activity.
3. Carefully pour your glass beads into another graduated cylinder with 50 mL of water and find the volume of the beads by displacement (see Bulk Density protocol).
4. Record the volume of water plus beads on the Data Sheet.
5. Carefully pour water into the cylinder until the glass beads are fully underwater. Gently tap the sides of the cylinder to remove air bubbles. Read out the volume (D) and record it. Calculate the porosity of glass beads using the formula in the table
6. Repeat the measurements for different substances. How is the porosity of the substance related to the shape of grains?
7. To demonstrate the effect of sorting on the porosity, mix glass beads with washers , and repeat the procedure

Prepare samples of sand, gravel, or rocky soil. Ask students to follow the procedure and calculate the porosity of various types of soil. Find the bulk density of these samples (see Soil Characterization).

1. Fill a plastic cup ¾ full with glass beads, then weigh it.
2. Carefully fill the cup with water until the beads are underwater. Tap the sides of the cup to release bubbles, then record its weight. Mark the water line around the outside of the cup with a waterproof marker.
3. Cover the cup with a plastic screen or lid, then pressing on the lid, tilt the cup over a basin or a sink and pour the water out. If students are performing this activity themselves, warn them not to spill out the glass beads.
4. Point out to the students, that the glass beads are still wet, and that not all of the water was removed. Ask them why it is so. Explain that water sticks to grains.
5. Again, weigh the cup and record it. Compare this measurement with the dry weight.
6. Calculate the difference between the two (see table).
7. What would happen if we used smaller glass beads or marbles? Would more or less water "coat" the beads? Older students may be able to compare the surface area of large and small beads.
8. After recording the hypothesis, repeat the procedure with smaller beads and discuss the results.

To help your students make a connection between the beads in the model and particles in the real soil, you can use different size gravel, or a very coarse sand. Water will "coat" the grains of fine sand, and it will be impossible to pour most of it out.

Set up the experiment (as explained above), or have your students do it with your supervision..

1. Pour water into the plastic container, so that the sand is completely saturated. NOTE: start observing immediately after water is poured into the box.
2. Ask students to observe what happens inside the plastic tube. Observe and measure the height of water level in the tube using a ruler.
3. Time the rate of water rise in the tube by marking the known interval (1-3 cm) on the side of the tube, and measuring the time it took for the water to pass the first and the second mark.
4. Ask students what would happen if they have used a different type of soil in the experiment or a different sized tube (like a straw)?
5. Have your students mix up their own soils and see whose column pulls water up fastest and highest.