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?

In his experiment, first he should conduct an experiment on other seeds instead.
5 years is a long time and plus it's hard to see the difference in a tree. He should use something smaller and more sensitive and obvious such as a green pea seed.
Another way he can improve is to conduct experiments on multiple seeds instead of one.
He should put the pea seeds in different conditions to see whether soil, water, or both is needed.