Assuming that it is somehow clear what the ancient Greeks saw in their mind's eye when they talked about the elements (substances) Air, Water and Fire, I wonder what they had in mind when they talked about the element Earth. Did they mean soil? And what kind of soil: clayey soil, or humus? Or some unspecified generic kind of soil? Can this be deduced from historical sources? Or must this remain speculative?
See in Wiki : Classical element.
It is clearly an abstract "schema" dating back to the Presocratics but known to us mainly through Plato and Aristotle; Aristotle related the four elements to the four sensible qualities.
It must be read as an explanatory schema devoided of (current) phisycal or chemical interpretation.
In Empedocles we read :
It was Empedocles who established four ultimate elements which make all the structures in the world - fire, air, water, earth. Empedocles called these four elements "roots", which he also identified with the mythical names of Zeus, Hera, Nestis, and Aidoneus (e.g., "Hear first the four roots of all things: bright Zeus, life-giving Hera (air), and Aidoneus (earth), and Nestis who moistens the springs of men with her tears." Empedocles (Arthur Fairbanks, tTranslator), Fragments and Commentary; Scribner, 1898)
Empedocles never used the term "element" (Greek: στοιχεῖον, stoicheion), which seems to have been first used by Plato. According to the different proportions in which these four indestructible and unchangeable elements are combined with each other the difference of the structure is produced. It is in the aggregation and segregation of elements thus arising, that Empedocles, like the atomists, found the real process which corresponds to what is popularly termed growth, increase or decrease. Nothing new comes or can come into being; the only change that can occur is a change in the juxtaposition of element with element. This theory of the four elements became the standard dogma for the next two thousand years.
Today we explain snow and ice as different "transformation" of a basic "element" : water. It is easy to imagine that, for ancient Greek naturalists , also the different "types" of earth can be "transformations" of a basic element common to all.
See also Early Greek Philosophy by John Burnet : From Chapter V, Empedokles of Akragas.
As Mauro pointed out, "earth" is - maybe more than the other elements - an abstract schema or principle. It differs in several respects from the other elements:
Less than the others it doesn't correspond straightforwardly to a (unique) physical substance.
It is the only element which has the name of a godhood: Ge or Gaia. The names of the other elements are not names of godhoods: pyr, hydor, pneuma.
It is the only element with which the soul not has been identified (by some Presocratian philosopher): "[...] each of the four elements except earth has found its supporter.", Arist, de an. I, 2, p. 405 b 8
It's surprising for many modern people that ancient theories were close to today's scientific ones. 4 elements and related theoried were one of these.
Earth (along with other elements, together forming a system of 4), as already suggested, is just an abstract concept, or a scheme, to which other related, concrete things, it was a "pattern". All of the elements have their complementary properties. Earth is of "heavy", "concrete" and other like these.
Therefore Earth is an "essence" of something, to which today we could refer probably as a "solid matter", as opposed to "liquid" water, "thin, scattered" air, or "ever-moving" fire.
Quite a lot on these 4 elements as related to anything in the universe (there's also an example as a human body) can found be e.g. at this site: http://www.greekmedicine.net/b_p/Four_elements.html
BTW it's pretty common to many ancient civilizations philosophy, or cosmology, some basic comparison can be found here https://en.wikipedia.org/wiki/Classical_element
The classical system of the four elements, according to Levi-Straus view, is a "concrete logic", that is a paradigm, which allows to think various things. From 2 binary variables 4 cases are constructed; taking 2 basic substantial priciples "moisture" and "heat" and supposing their presence or absence, the four elements are obtained. Earth is the doubly privative case without moisture and without heat.
It is also a cosmological-spatial schema: a natural vertical axis is defined by gravity with fire-up and earth-down; if the zero point is on the horizon, then water is below and air above.
No need for some concrete instance.(Perhaps only some old fashioned historicising theory would still insist that the cosmogonic schema with 4 element originates from the craft of pottery).