Answer:
Both groups should have been tested in the same soil type at each humidity level.
Step-by-step explanation:
From the experimental process described above, the student failed to control for bias which could creep in by not controlling for the soil type used, controlling for possible error as a result of the soil type requires that each group is tested using the same soil type, this way, the student can be confident that variation in the result is not due to the non-uniformity in soil type the plants were subjected to. At each humidity level, the groups should be tested using the same soil type. This way, observed result would have been largely due to different humidity levels.