The Soil/Water Equation

By Eve Pranis, June 23, 2008

Soil and water have a dynamic and important relationship. Different-sized soil particles create air spaces that can hold air or water. When rain falls, water and dissolved nutrients fill the soil spaces and become available to plant roots.

Water tends to drain quickly through sandy soils (made up of large particles), and slowly through clay (composed of small particles) and compacted soils. If the water drains too quickly from the soil, plants will wilt, yet if drainage is poor, the soil spaces will have limited oxygen that plants and soil organisms need to survive. If soil is compacted, excess water can collect on top, then run off and take precious soil with it. (Plant roots help protect the soil from being washed or blown away.) Organic matter improves how both clay and sandy soils absorb and hold water.

Invite your students to use soil samples indoors, outdoor gardens, or schoolyard sites to examine the soil/water relationship.

Indoors: Dirty Drains

Invite students to examine how quickly water moves or percolates through different types of soils. Bring in or have students collect soil from different settings, then leave the samples at room temperature until each feels dry. Ask students to examine each sample, then predict and explain which they think will drain water most quickly and defend their predictions.

Next, have small groups set up one plastic soda bottle or clear funnel and filter for each soil sample, and place an equal amount of soil in each. Have someone from each group pour an equal amount of water through each sample while another person tracks how long it takes for the first drop of water to emerge from each soil into the container. By comparing the water volume before being poured with the volume that drips out, students can also compare how much each soil absorbed. Ask: Were there any surprises? How did results relate to your predictions? Which soil would you choose for growing plants and why? Consider challenging your students to use marbles and sand as props to figure out, and then explain why bigger soil particles drain water faster.

Outdoors: Perc Test

If you want to compare how different types of soils drain outdoors, remove the bottoms from several tin cans, then insert each one about an inch into dry soil in different locations. (Try to choose at least one area that is compacted.) Have students predict which they expect to drain water most quickly and why, then pour water in until each can is half full. Time how long it takes the water in each can to completely percolate.

Discuss what factors students think accounted for the differences. Examine how soil compaction affects plant/root growth and life in the soil by choosing a high- and low-impact area near your school, then comparing drainage, evidence of soil life, and so on.

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