Pentacles are typically associated with Earth in occult and tarot usage. This applies to the Tarot of the Old Path, which is a Pagan-flavoured deck. Just about any contemporary book on Wicca or the elements will explain the association as will many tarot books by writers such as Greer and Pollack. You could also review Golden Dawn materials.
In "Magick, Liber ABA, Book 4", Crowley says:
There is, therefore, nothing movable or immovable under the whole firmament of heaven which is not included in this pantacle, though it be but 'eight inches in diameter, and in thickness half an inch.' Fire is not matter at all; water is a combination of elements; air almost entirely a mixture of elements; earth contains all both in admixture and in combination. So must it be with this Pantacle, the symbol of earth.
I hope that helps.