Earth was an element according to the Greek philosopher Aristotle, along with Air, Fire, and Water. (PROSE: The Burning)
According to one account, Earth, Fire, Water, Wood, and Metal were five elemental forces. The Tenth Doctor and Gabby Gonzalez encountered an Earth monster as one of the five elemental beings guarding the Red Jade General's walled village in ancient China. (COMIC: Sharper Than a Serpent's Tooth)