Objective: Students will observe and compare the relative rate and depth of soil infiltration in five soil samples.
Materials: For this activity, you will need
- five large test tubes;
- a 50-ml graduated cylinder;
- sand, gravel, and clay;
- loam or potting soil;
- a sample of schoolyard or neighborhood soil;
- a metric rule;
- graph paper and grease pencils;
- and a test-tube rack.
Procedure: Begin by filling one of the test tubes 3/4 full with sand. Give the test tube a little shake to settle the sand, but don't pack it. With the grease pencil, mark the test tube at the top of the sand and place it in the rack. Prepare the other soil types in the same manner.
Fill the graduated cylinder with 30 ml water. Note the time, and then begin slowly pouring the water into the top of the first test tube. Allow a small amount to soak in at first, then continue pouring until all the water is in the tube. Repeat the process with the remaining test tubes, pouring 30 ml water into each and noting the time you begin pouring.
Observe the test tubes carefully. When it becomes apparent that water in a particular tube is no longer filtering downward, record the time once more.
After infiltration has stopped in all the samples, mark the level to which the water descended. Then, measure the distance between the two markings for each sample. (Compile your data on a chart.)
Time of Infiltration
Depth of Infiltration
| ||(in seconds)||(in centimeters)|
|Sand and Gravel|
The soils with the most visible space between grains should infiltrate the deepest and the fastest. The clay should show the least infiltration.
Ask the children to answer the following questions: Which soil allowed the water to infiltrate the fastest? (The sand and gravel sample should infiltrate most rapidly.) Which sample showed the deepest infiltration? How does your local soil compare to the other types?
The soil found at Hupobi is most similar to the sample of sand and gravel. How did that sample compare to the others? What would this indicate about the crop-growing conditions for the farmers of Hupobi? (During summer downpours at Hupobi, the surface spaces between the sand and gravel grains fill quickly, preventing additional water from sinking below the surface. As a result, most of the rain runs off the surface and never reaches plant roots. In other words, water infiltration did not present difficulties for the ancient farmers of Hupobi; their problems came from the rainfall pattern and the poor water retention properties of the local soil.)
This lesson was developed by Terrill L. Nickerson, the Santa Fe Indian School.
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