Johannes wants to know whether soil, water, or both are needed for plant growth. He chooses to conduct experiments on a willow tree for five years to answer his question. How can his experimental methods be changed to improve his experiment?

Using only one willow tree would make it difficult to know whether the results apply generally or are due to that individual tree. Johannes should use a larger sample size, with multiple plants in each test condition, to make the results more reliable. Asking scientists or testing more environmental factors would not directly improve the comparison of whether soil, water, or both are needed.

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