Final answer:
The independent variable in the plant growth experiment with different soils is the type of soil. Control of other variables is essential to ensure valid results, and examples of uncontrolled factors could include individual plant variation and soil temperature differences.
Step-by-step explanation:
The independent variable in the plant growth experiment is the type of soil. This is the factor that the student is intentionally changing to observe its effect on plant growth, which is the dependent variable. In experiments, it's crucial to change only one independent variable at a time while keeping other potential variables constant to ensure that any differences in the outcome can be attributed to the change in the independent variable.
Problems may arise in a setup if there are factors that were not controlled effectively, which could influence results. Possible uncontrolled factors include individual plant variation, changes in soil temperature due to different colors of containers, water loss due to evaporation from the soil, or the effect of insects which may attack one set of plants but not another.
When setting up an experiment, factors such as the size of the containers and the amount of water given to the plants must be the same for all, as Gary has done, to ensure these do not introduce additional variables into the experiment.