An agricultural scientist investigated the rate of decomposition of two types of biodegradable seedling pots: Pot X (made from compressed starch) and Pot Y (made from recycled paper pulp).
The scientist placed 30 g of shredded Pot X into a soil-filled container. She placed 30 g of shredded Pot Y into an identical container with the same mass and type of soil. Both containers were kept in a laboratory incubator for 60 days, and the remaining mass of the pots was measured at regular intervals.
State two variables, not already specified in the method, that would need to be controlled in this investigation to ensure a valid comparison.
