Washing Detergent & Hydrophobic Soil

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Updated on Mar 05, 2014

Grade Level: 6th -8th; Type: Physical Science


This project evaluates the ability of a biodegradable dish detergent to increase the penetration of water in hydrophobic soils.

The goal is to have the student formulate and test a hypothesis about the feasibility of using a washing detergent as a wetting agent for soils that resist penetration by water.

Research Questions:

  • How do biodegradable dish detergents behave on wax surfaces?
  • Do dish detergents improve the penetrability of water in hydrophobic soils?

When plants decompose in the ground, they may produce a waxy coating on soil particles. This waxy material makes the soil water repellent. Soils that contain large amounts of sand or that contain large amounts of organic matter tend to be hydrophobic. When water is applied to these soils, it runs off without penetrating it. The passage of water into these soils can be improved by using a wetting agent that reduces the surface tension of the water.

Poor water penetration is also a problem in compacted soils and in soils containing large amounts of clay. Wetting agents do not improve water penetration in these kinds of soils.

Wetting agents reduce the surface tension of water, allowing it to spread on the waxy surface coating soil particles and into the pores in the soil. Wetting agents consist of molecules that are attracted to water on one side and to waxes on the other.

Wetting agents and washing detergents work the same way. The addition of ordinary washing detergent can improve the penetration of water in soil, but the effect does not last very long. Also many of these detergents contain compounds that are harmful to growing plants. They may also interfere with the life-cycles of some aquatic organisms. In high concentrations they may be poisonous.


  • Biodegradable dish detergent
  • Wax

Materials can be found at a Wal-Mart type store.

Experimental Procedure

  1. Read about the surface properties of hydrophobic soils, and the role of wax in making them impermeable to water.
  2. Place a drop of water on a dry wax surface. Observe whether the water drop spreads on the surface or “beads” up.
  3. Place a drop of biodegradable (environmentally friendly) dish detergent on a second (identical) wax surface. Observe whether the detergent spreads or beads up on the surface.
  4. Repeat this test for two more biodegradable (environmentally friendly) detergent formulations.
  5. Rank the abilities of water and the detergents to spread on the wax surfaces.
  6. Use your observations to formulate a hypothesis about the possibility of using a detergent as a wetting agent to improve the penetration of water into hydrophobic soils.
  7. Identify a source of hydrophobic soil. These soils can be identified by the tendency of water to bead up on them.

When attempting to locate the soil source, you may have to dig down one or more inches below the surface. The upper few inches of soil are often hydrophilic.

  1. Mix up water-detergent solutions for each of the three detergent formulations. Use the same amount of water and detergent in each solution.
  2. Place a drop of the first solution on the soil. Observe whether the water beads up or spreads out on the surface.
  3. Repeat this test for the two other detergent solutions.
  4. Evaluate your hypothesis in light of your observations. If necessary, revise it and conduct additional tests. Finally make recommendations about using the detergents as wetting agents in hydrophobic soils.

Beading behavior on wax surface


Detergent 1

Detergent 2

Detergent 3

Beading behavior on soil surface


Water/Detergent 1

Water/Detergent 2

Water/Detergent 3

Terms/Concepts: Hydrophobic; Hydrophilic; Detergent; Wetting agent; Soil


Dr. Frost has been preparing curriculum materials for middle and high school students since 1995. After completing graduate work in materials science at the University of Virginia, he held a postdoctoral fellowship in chemistry at Stanford. He is the author of The Globalization of Trade, an introduction to the economics of globalization for young readers.

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