Question

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?
he needs to ask scientists what they think.
he needs to conduct the experiment for a longer period.
he needs to use a larger sample size.
he needs to test more factors in the environment.

Explanation:

A single willow - tree as a sample is too small. A larger sample size reduces the impact of individual - plant variability and increases the reliability and generalizability of the experiment results.

Answer:

He needs to use a larger sample size.