A student is performing an experiment to see if fertilizer will help a plant grow. He/She sets up 2 containers to grow the plants in. In both containers the student puts the same type and the same amount of soil, the same type of plant seed, and then gives each container the same amount of water and sunlight each day. The only difference between the 2 containers is in container 1 the student gives the plant the fertilizer. During the next 60 days the student measures the growth (height) of the plant and records the data. What is the experimental variable is this experiment?