This Is Control Calling

Purpose: To determine why a control is necessary for experiments

Background

A variable is a changeable factor in an experiment and when manipulating variables during an experiment, a control is necessary. A control is a standard used for comparison. A control is used to rule out other factors that might be influencing the outcome of the experiment.

Materials

• 3 cups
• potting soil
• spoon
• bean seeds
• marker
• measuring cup
• metric ruler
• science journal
• 2 different plant fertilizers

Procedure

1. Discuss the basic needs of plants (food, air, water, and light)
2. Define fertilizer and brainstorm ideas on how it benefits plants. Record ideas in your science journal.
3. Use a marker to label each cup “A,” “B,” and “C.”
4. Use a measuring cup to measure the same amount of potting soil for each cup. This amount will depend on the size of your cup.
5. In each cup, place 4 bean seeds in separate positions about 1 cm under the soil.
6. Measure 50 ml of water and add the number of drops of fertilizer “A” recommended by the directions on the fertilizer package.
7. Pour this solution on top of the soil in cup “A.”
8. Record in your science journal the date, time, and name of fertilizer for cup “A.”
9. Repeat steps 6-8 for fertilizer “B” and cup “B.”
10. For cup “C,” measure 50 ml of water and pour on top of the soil and record.
11. Place the cups in a sunny location.
12. At approximately the same time each day observe the cups for any growth and record your observations.
13. If the soil becomes dry and needs additional water, be sure that you repeat the above steps exactly as you did the first time.
14. Once the plants begin to sprout, use a metric ruler to measure the height of each plant and record in your science journal.
15. At the end of 3 weeks or time determine by your teacher, compare each plant and its growth.
16. Determine which plant grew the tallest, had the most number of leaves, and/or had the greenest color.
17. Discuss why.

 

Conclusion

1. Which plant grew the “best?” Why?
2. Did the plants that were given fertilizer grow better than the plant that did not receive fertilizer? Why or why not?
3. The plant that did not receive fertilizer is called a “control.” Why is it necessary to have a control when conducting a science experiment?
4. Why was it necessary to keep all the variables the same for each cup? Where there any factors that you could not control?
   

Extension

Conduct this experiment making your manipulated variable different types of soil instead of fertilizer.

Answer Key

1. Answers will vary.
2. Answers will vary, but it is to be expected that if conditions were right, then the plants that received fertilizer grew better than the plant that did not. Fertilizer is a plant food and when added to the soil, it gives additional nutrients that help to enhance the growth of the plant.
3. It is necessary to have a control to have it for comparison. If you did not have a control plant, then you would not know how the plant would have grown normally without fertilizer, therefore, you would not have had anything to compare it to.
   

It was important to keep all the variables the same in each cup so that you know that the fertilizer is what made the plants grow differently. Answers will vary.