Squeezing Water from Rocks

By Eve Pranis

"It's so dry in our part of New Mexico that our school gardeners diverted runoff ditches and set up collecting tanks to catch rainwater runoff," reports Santa Fe, NM, parent volunteer Molly Toll. The elementary school gardeners, still challenged to provide enough moisture for their precious plants, decided to set up nine plots to experiment with ways to maintain moisture for plants.

"As a paleoethnobotanist who explores historic people's use of plants, I was able to share with third and fourth grade classes that people 700 years ago had thriving gardens in our cool, dry climate," reports Molly. She also shared photographic evidence that these early people used gravel to mulch their gardens, and told students that researchers were still trying to figure out why. "I invited the students to help figure out this puzzle themselves by thinking and acting like scientists," she adds.

After brainstorming what the Pueblo Indians might have been doing with rocks in their gardens, students went outside to explore conditions under stones outdoors. They recorded their observations: moisture, temperature, bugs, and so on, then discussed how these factors might affect plants. "Students in our area are well aware of the importance of moisture," explains Molly, "so that quickly became an area of interest."

Next, students took a field trip with an archaeologist to see the excavated site of these historic gardens, which prompted even more questions and research possibilities. Before planning experiments in their outdoor plots, students set up some windowsill investigations. First, the classroom teacher put water in two glasses and marked its levels. When students placed a rock in one, they saw the water level appear to rise. Then small groups of students raised corn plants in soil-filled plastic cups, and "mulched" one with rocks. As of this writing, students had found that the corn in the cup with rocks was, in fact, doing better, and they suspected that the rocks increased the available moisture and warmth.

Armed with equipment such as a pH meter, moisture meter, temperature probe, and soil texture kit (purchased through a $200 grant), students designed a gravel mulch garden, then monitored its growing conditions. "Although it's early in the season, the students are beginning to find that the soil is warmer and more moist under the gravel mulch," reports Molly. The student findings are consistent with what the experts assume: that people from long ago used gravel mulches to extend the growing season and conserve water. "The students have loved pursuing these authentic questions, imagining the historical settings, and researching things that most gardeners haven't considered," she adds.

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