Which would be needed to compare the effect of two different fertilizers on plant growth?
a cardboard box, a way to measure plant growth, a selection of soil types or a variety of plant species

A way to measure plant growth. The issue with an experiment like this, is there is no way of telling if the plant would have grown differently if you had not added the extra fertilizers. To properly test this you must have "control" plants that would grow in the same conditions just without any fertilizer. You have to have multiple plants as subjects to get a good amount of data and then take the averages of each fertilizer and compare them that way. (Just as a headsup if you are going to do this as a science fair project, don't use raw egg or unprocessed chicken poop. The egg turns the soil into rock and the chicken feces smells horrible and starts to mold. >< )