Sarah is testing different types of planting soil to determine which type is most effective in growing blueberry bushes. She purchases two different brands of planting soil from a local store. Sarah applies Brand A to 1 area of the yard and Brand B to another area of the yard, both of which receive full sunlight. She waters the area with Brand B daily; she waters the area with Brand A every other day. At the end of the study, Sarah concludes Brand B is more effective in growing blueberry bushes. Why is her conclusion not valid?