Activity:Make and test hypotheses about the effect of nutrient levels on plant growth
Level: Grade 4-6 (easily adapted to older students)
Time: 1-2 hours for introduction and setup; 10 minutes a day, 3-5 days a week, for 2 weeks for data collection; 1-2 hours for graphing data and discussing results
Objectives: After successfully completing this activity, the student will be able to:
Author: Rebecca Burton, feel free to email questions and comments from you and your students. Creation and maintenance of this web site was made possible by NSF-ILI Grant DUE 9750658.
This activity allows students to explore the relationship between nutrient levels and plant growth. The actual data collection is quite straightforward. Mature plants reproduce by budding new "leaves" (not true leaves, but that's a subtle distinction). Growth can be quantified simply by counting the leaves. Depending on light, water, temperature, and nutrient conditions, the number of leaves may double in less than three days. If your growth rates are slower, you may wish to run the experiment longer. Duckweed is a source of food for waterfowl and fish. It is also used to remove nutrients from water at some sewage treatment plants. For more information on duckweed, see: The charms of duckweed, The Lemnaceae, and Lemna minor, and Lemna minor.
The following outline is provided as a source of ideas. It is not intended as a script.
All living things are made of similar "building blocks" called nutrients.
Where do we get them?
Where do plants get them?
Where do wastes (dead leaves, animal droppings, etc.) go?
Where do nutrients go when things die?
How do activities like fertilizing lawns, keeping animals, and putting food down the drain affect the nutrient levels in water?
[Draw simple nutrient cycle: consumption, fertilizer, sewage, decomposition, erosion, removal, harvest]
How much is enough?
What happens if plants have too few or too many nutrients? [excess can be toxic, low levels can lead to disease, slow growth, death]
How could we test the effect of nutrient levels on duckweed?
Make hypotheses about how much fertilizer your plant needs. Is any needed? Is more better? Is too much bad?
How would we test these hypotheses?
[Might discuss independent and dependent variables.]
What things other than nutrients might affect plant growth? [light, temperature, amount of water, etc.]
How can we make sure any differences are due to fertilizer only?
Why do we need more than one cup of each type?
Do all cups need to start with the same number of leaves? Why?
Lab Safety - Keep stuff away from eyes and mouth, use things only as directed, ask if you're unsure.
What is a concentration?
what does "percent" mean?
How much plant food would we have to mix with 1 liter of water to get a 1% solution?
Mix the four solutions.
Label a clear plastic cup with group name and type of water (0, 0.5%, 1%, 2% fertilizer).
Add duckweed until you have 10 leaves (not 10 plants).
Set cups in appropriate light.
Observe cups daily and record changes by counting leaves and noting other things.
Take a look at the cups of other groups.
Working with Data
Did all the groups find the same thing?
Why might there be differences?
Why did we say it was important to have more than one cup of each type?
How can we combine the results of different groups? [If students are new to dealing with data, you might just look at the last day for all of the groups.]
What is the "story" our data tell?
How can we make a graph to tell that story?
Each group can take the data of all of the groups to find the average leaves for a given solution on a given day. For students who are new to graphing, you may wish to graph the final day only. More advanced students can produce a graph with all of the data (Fig. 1).
Figure 1. Graph of hypothetical data. Students with little graphing experience might start with a simpler graph, such as a bar graph comparing the four treatments on the last day.
What did we learn?
If we were to repeat this experiment, what should we do differently?
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Last update: 5/20/02 by Rebecca Burton, Dept. of Biology, Alverno College