In an experiment, Joey and Elena are trying to determine whether Super Grow fertilizer works better than Mo' Grow fertilizer on pepper plants. They buy 10 pepper plants that are the same size and water them the same amount three times per week. They use Super Grow on half of the plants and Mo'Grow on the other half. All of the plants receive the same amount of sunlight. Joey and Elena measure the height of the plants at the beginning of the experiment and every 6 days for one month (30 days).
 Which of the factors in the experiment are controlled variables?