What sort of environment (hypotonic, hypertonic, isotonic) did the extra fertilizer create around the roots of the corn?

Extra fertilizer would cause a hypertonic environment to your corn plant. Hypertonic means the membrane that has the most solute. Note that water always favors the hypertonic environment. Meaning to say water will be drawn by the extra fertilizer from the roots of the corn and leaving the corn plant dehydrated.