Scientists are interested in investigating the possibility that plants can be used to remove heavy metals from contaminated soil. They divide a sample of soil with heavy metal contamination into five different pots and plant five different species of grass they believe will take up metals into their roots and leaves. They also plant the same grasses in five pots with uncontaminated soil. After allowing the grasses to grow for a month, they cut the grass and measure the concentration of metals in milligrams per kilogram. Which of the following is the CONTROL group in this experiment?