Another custom little seen anymore outside of the most traditional circles is that of having a cup or bowl of dirt or sand at a Brit Milah (circumcision) ceremony. The practice was that immediately after the Milah is performed, the foreskin is placed in the earth and covered over. This now-neglected custom is yet another example of the aborginal dimension of Judaism that (pardon the pun) lies just below the surface.
In fact, the earliest forms of this ritual disposal of the foreskin varied from community to community. Apparently in the early middle ages, Jews in Babylon performed the circ over a bowl of water, sometimes mingled with myrtle and spices (Otzer ha-Geonim, vol 2), then the foreskin was deposited there, while the Jews in the Land of Israel placed it in earth.
Why water? The symbolism, even the source of the custom, is not so clear to me. Perhaps it refers to the water used to ritually bathe the child prior to the circumcision, as required by some authorities. The child is circumcised "over" this cleansing water, after which the removed tissue is placed in the pan. If any readers have insight into this from sources in the Mesorah, I would love to hear it.
The rationale for the dirt is esoteric, yet easier to account for. It assumes that the foreskin signifies the future virility of the male child. By placing it in dirt, the community symbolically reminds God of the promise He made regarding the fertility of the Jewish people, Your descendants will be as the dust of the earth (Gen. 28:14; as explained in PdRE 29).
Many discussions of the practice refer to using sand, rather than earth. This not only comports well with another text, Gen. 32:13, Your descendants shall be as the sand of the sea, but the sand also represented a compromise between the water custom and the earth custom, as sand is the liminal material between the two realms.