In the case of watering and fertilizing, too much water causes anoxia, which damages roots and can ultimately kill the plant. Similarly, too much fertilizer can become toxic, damaging or killing the plant by causing osmotic problems or through other mechanisms.
Work Step by Step
Anything in an excess becomes a problem— your marble collection, if it grows to 1,000,000 marbles in your 500 square foot home becomes a huge problem. Scan Concept 37.1 for mentions of the problems caused by excess water and/or fertilizer.