Final answer:
The type of soil used in the experiment is the independent variable, which is altered by the scientist to observe its effect on plant growth, while other potential influences should be controlled.
Step-by-step explanation:
In the described lab experiment, the type of soil is the independent variable. This is the factor being deliberately changed by the scientist to observe the effect on the dependent variable, which is plant growth. The independent variable is critical in experimental designs, as it's the cause to which changes in the dependent variable can be attributed. In any proper experiment, other factors such as the amount of water, amount of plant food, amount of sunshine, and individual plant variation, should be controlled to ensure they do not affect the outcome. Additionally, having a control group (plants that do not receive the specific soil in this case) allows the scientist to compare the results and determine the effect of the independent variable more accurately.