Key Factor in Fertilizer Experiment

A team of researchers conducted an experiment to study the effect of different fertilizers on the growth of sunflowers. They divided 30 sunflower plants into three groups of 10 plants each. Group A received Fertilizer X, Group B received Fertilizer Y, and Group C received no fertilizer (control group). The researchers measured the height of the sunflowers after four weeks of growth.

What key factor did the researchers control in this experiment to ensure valid results?