U.S. DEPARTMENT OF THE INTERIORBUREAU OF LAND MANAGEMENT
 
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WATER RETENTION

Objective: Students will demonstrate gravel mulching and build a model of a waffle garden to observe ways in which altering the soil can affect the amount of moisture the soil retains.

Materials: For this activity, you will need

  • three plastic laboratory trays (45 cm x 60 cm, 5 cm deep) or soda-pop flats lined in plastic;
  • soil, clay, and gravel;
  • construction paper;
  • a 500-ml beaker;
  • a plastic bucket;
  • water;
  • and scissors.

Procedure: Fill the three trays with soil to within 2.5 cm of the top. Pack the soil a bit, but don't overdo it.

Leave one tray as it is, containing just the soil.

For the second tray, evenly spread a thin layer of gravel over the soil.

The remaining tray will represent a waffle plot. Make it by filling the bucket halfway with soil and adding enough water to moisten it - until the soil is tacky, but not runny. With a handful of the moist soil, form a wall on the surface of the soil in the tray. The wall should be about 5-8 cm tall, 10 cm long, and 1 cm thick. Continue making mud walls to form a waffle pattern of interlocking squares, about 8 cm per side, that covers the soil surface. Let the mud walls dry cimpletely before continuing the experiment. Cut the construction paper into 1 x 7-cm strips. Then, in the first tray, make a hole in the soil that extends to the bottom of the tray. Continue making holes, spacing them about 10 cm apart, until the entire surface has been punctured. Insert the construction-paper strips, one per hole, securely packing the soil around them. Repeat the process for the other two trays, making one hole per square in the waffle-garden model.

Next, fill the beaker with water and gently pour it over the surface of the first tray. Cover the entire surface evenly, and try not to pour water directly onto any of the construction-paper strips. Repeat with the other two trays.

Every hour, pull one of the paper strips from each of the trays, taking care not to tear it. Measure the part of the strip that is still moist. Continue to pull strips and measure them until they are gone or the soil has dried completely. Record the time that each strip is pulled, which tray it comes from, and how much of it is moist.

The data will show that the waffle-garden model retains moisture the longest, followed by the gravel mulch-covered soil. The uncovered surface is the least efficient at water retention.

Discuss the following questions: How do the three models differ? Why does the waffle garden work so well? How could the waffle garden be improved? (A gravel mulch would further increase the efficiency of the waffle garden.) What specific materials would the inhabitants of Hupobi need in order to make waffle gardens? (They needed clay and water to build the low walls surrounding the plots.)

You could try conducting this experiment on a realistic scale, if you have enough space on the school grounds. Or you can try the exercise with live plants over a long period of time, testing for productivity as well as water retention.

This lesson was developed by Terrill L. Nickerson, the Santa Fe Indian School.

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Last updated: 11-13-2009