Presence

A random Google search for Sotah turned up an article entiteld "The Meaning of the Trial of the Sotah" by Tilia Klebenov. The article offers a good summary of the source text, as well as deeper exploration into the meaning and symbolism of the Sotah process. In particular, the article draws a connection between the Adamah (Earth) used to dirty the bitter waters and the Adamah used to create Man:

In addition to the ink, it contains holy water, meim kiddoshim, mixed with some of the earth that is on the floor of the Tabernacle.(Num 5:17) This act contains multiple layers of meaning...[The Tabernacle] was the place where heaven and earth were linked by the just and powerful presence of the Divine. Because of this, the woman's drinking the earth and water is nothing less than her completing a circuit. She is becoming physically and visibly connected to the earth--it is now in her body--the same earth which God enters at this same location.

This is especially powerful when we realize that this is a culture which believed that mankind sprang from the soil. Adam, of course, is fashioned from the earth; and the Hebrew word for soil is adamah. The two words have the same root. In this sense, then, the woman is doing nothing less than imbibing her own essential nature, she is earth, and she drinks the earth. In so doing, she is placing herself utterly in God's presence, in the hands of the one who fashioned the first humans from that same soil.

I think this may help in understanding the focus on Eglah Arufah towards the end of the tractate.