Final answer:
The student's experiment lacks a control group and introduces an additional variable, which may affect plant growth.
Step-by-step explanation:
The student is conducting an experiment to determine the relationship between the growth rate of plants and the presence of detergent in the soil. However, there are a few flaws in the experiment. Firstly, the student should have included a control group with no detergent in the soil to compare with the pots that contain detergent. Secondly, placing the pots with detergent in the sun and the pots without detergent in the shade introduces another variable which may affect plant growth. To ensure accurate results, all conditions other than the presence of detergent should be kept the same for both groups of plants.