Multitype changes an Arceus holding this item to its Ground-type form. From Generation V onward, item-manipulating effects (such as Trick and Fling) cannot give Arceus a Plate or remove a Plate from Arceus. (In Generation IV, item-manipulating effects cannot affect Pokémon with Multitype.)
The Earth Plate, alongside the other 15 Plates at the time, appeared in Arceus and the Jewel of Life, where Arceus used them to change its type, and almost died when it lost all of the Plates after destroying a meteor. Damos found one of these lost Plates and returned it to Arceus, giving it the strength to recall the other 15 Plates. This act earned its trust and prompted it to detach the Earth Plate, Splash Plate, Meadow Plate, Zap Plate, and Draco Plate in the form of the Jewel of Life so Damos could make the land that would become Michina Town flourish. The Earth Plate was included into the Jewel of Life to represent the soil of the land.