many people fertilizer to their house and garden plants. make a hypothesis about whether you think fertilizers really help plants grow. next design an experiment to test your hypothesis.
H0: Fertilizers do not increase plant growth. H1: Fertilizer do increase plant growth. Methods: Buy 10 (or more, ideally) plants of the same species, all about the same size. Randomly assign half to the Control group, and the other half to the Fertilizer group. Mark (with small tape, or the like) a couple or so leaves on each plant, and measure their length and/or width. Record the data (be sure to label the pots). Fertilize the appropriate plants on a regular basis, following the instructions on the label -- in all other regards, treat all plants equally. Place them on a large shelf near a window, and occasionally change their positions along the shelf. Every so often, remeasure the same leaves on each plant. Calculate mean values for each plant (the individual leaves on each plant are not independent observations). Compare gain in leaf area or length between the two groups with a one-tailed t-test or one-way ANOVA, because you expect a priori that the Fertilizer group will have bigger/longer leaves than the Control group.