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:

Johannes is conducting an experiment to determine the factors (soil, water, or both) needed for plant growth. In experimental design, a larger sample size reduces the impact of individual variability and random errors. Asking scientists' opinions is not part of the experimental method itself. Conducting the experiment for a longer period (five years is already long) may not address the core issue of sample - size - related reliability. Testing more environmental factors is not relevant to answering the specific question about soil and water.

Answer:

He needs to use a larger sample size.