A biology class is conducting an experiment to determine how soil pH may affect the germination of pine seedlings. Three pine seeds are placed in a pot of soil with a pH of 6.5, and three pine seeds are placed in a pot of soil with a pH of 4.0. After one week, the students count the number of seeds which have germinated and determine the mass of each pine sprout. Which of these is the BEST way to improve the accuracy and validity of the data in this experiment?