A biology student is studying the effects of acid rain on tomato plants. He plants four tomato plants in identical pots, using the same type of soil to fill each pot. He places the pots together in the same location. They receive the same amount of sunlight and water each day. The only difference is the pH of the water used to water the plants. The first plant receives water with a neutral pH of 7, which will allow the student to better determine the effects of giving plants water that is more acidic. The second plant receives water with a pH of 5. The third receives water with a pH of 3. The fourth receives water with a pH of 1. Which of the following is a serious flaw in the design of this experiment?