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.
Petrel was collecting the Plates for Team Rocket's plan. After Silver stole the few Plates he had managed to find, Petrel decided to let him gather all 16 Plates, just to get them back at once later. With his Weavile leaving messages for Sneasel and Weavile across Johto, Silver soon indeed gathered all 16 Plates for himself. At the Sinjoh Ruins, Gold surrendered the Plates to Archer, who used them to seemingly take control of Arceus, ordering it to recreate Dialga, Palkia, and Giratina. When the Pokédex holders used their Pokémon's ultimate attacks to stop the creation, all the Plates were absorbed by Arceus.
The Earth Plate was one of the two Plates that Petrel had found and were then stolen by Silver. After figuring out that the Plates stood for Pokémon types, Silver used the Earth Plate in Raising the Stakes with Rhyperior in order to power up his Rhyperior's Earth Power.