A research summary reported a pattern, but two variables changed together. Bean plants near a simulated ash fall grew more slowly than uncovered plants. The ash coating shaded the leaves and also released small amounts of soluble aluminum into the soil. Explanation 1 says ash reduced growth by blocking light from leaf surfaces. Explanation 2 says aluminum released from ash reduced root function and slowed growth. Which follow-up design would most directly distinguish the explanations while controlling the major confounding factor?