The Biodegradability of an Allelopathic Chemical in Soil

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Updated on May 17, 2013

Grade Level: 6th to 10th; Type: Life Science: Botany


Determine if the allelopathic chemical produced by the Black Walnut tree biodegrades immediately or can persist in the soil over several days.

Research Questions:

  • What is Allelopathy and Juglone?
  • What is the meaning of the term biodegradable?
  • Does the allelopathic chemical produced by the Black Walnut tree biodegrade immediately once it enters the soil?
  • Does the allelopathic chemical produced by the Black Walnut tree tend to persist in the soil over an extended period of time?
  • How did the allelopathic chemical juglone react with the tomato plants?


  • Tomato plant seeds
  • Small flower pots (or plastic cups)
  • Distilled water
  • Black walnut (leaves, bark, or roots), which can be obtained at
  • Metric measuring cup
  • Plastic tablespoon
  • Potting soil

Experimental Procedure

  1. Setup 10 small flower pots (or cups with holes made in the bottom for drainage), and fill with soil.
  2. Using a marking pen label each pot as follows:
    • Soil + Control day 1 and Soil + juglone day 1
    • Soil + Control day 2 and Soil + juglone day 2
    • Soil + Control day 3 and Soil + juglone day 3
    • Soil + Control day 4 and Soil + juglone day 4
    • Soil + Control day 5 and Soil + juglone day 5
  3. Place 20 black walnut leaves, bark (or roots) in a large pot and fill the pot half full with distilled water.
  4. Bring the leaf-water mixture to a boil. Boil the leaves and water for about 10 minutes. After 10 minutes, use a large spoon to remove the leaves from the water.
  5. Continue to boil the now extracted juglone solution for 20 minutes.
  6. After 20 minutes, allow the liquid to cool completely.
  7. Once the liquid has cooled, pour equal amounts of the solution into the five juglone labeled flower pots.
  8. Pour the same amount of distilled water into each control soil pot.
  9. Do not add excessive amounts of juglone to the point where the soil becomes muddy!
  10. Using a plastic spoon stir the mixture.
  11. Add one or two tomato seeds to both the control and the juglone soil pots labeled “day one” place them in a sunny location.
  12. The next day place a tomato seed in the soil labeled juglone “day two” and its corresponding control.
  13. Repeat the same procedure with the remaining soil pots over the next three days.
  14. If the soils appear to be drying out moisturize with a small amount of distilled water.
  15. Record in which soil pot the tomato seed began to germinate if any, and compare it to the growth seen in the control.
  16. Enter the results of the observation in a data table.
  17. As an extension project on day six begin measuring the heights of the tomato plants that have begun to grow, if the juglone did not prevent their germination.
  18. The data in the table can be visually displayed by plotting a bar graph of allelopathic chemical soil biodegradable vs. time on graph paper or a computer equipped with Excel, by listing the height along the vertical axis verses the length of time the juglone was in the soil along the horizontal axis.

Terms/Concepts: biodegradable, allelopathy, juglone, allelopathic effect, walnut toxicity, Black Walnut Tree, Do allelopathic substances tend to biodegrade in soil over time?


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