In science class, Ruthie and Heena are conducting a science experiment. They wanted to find out which type of fertilizer would make the plants grow the fastest. The added equal amounts of soil to four identical pots. In three of the pots they added the different types of fertilizer to the soil. They did not add any fertilizer to the fourth pot. Over the next two weeks they measured the height of each of the plants. What is the purpose of the fourth pot that contained no fertilizer?