In this class, you will occasionally be asked to create a hypotheses, to interpret data, and to design experiments with proper controls.
From what I understood, in the experiment outlined above, soil composition is assumed to be a control variable. A few weeks ago, I remember reading an article about how light exposure can change soil composition (pH, microorganism diversity, some nutrient levels, etc). So, investigating this aspect may also lead to interesting insights. But again, sometimes we need to make assumptions to make our lives easier!