Verse 52, “The Spotted Hawk Swoops By,” completes Whitman’s “Song of Myself.” What do lines 7-16 suggest about the theme of Whitman’s...

Much has been said about the end of "Song of Myself" . It is true that Whitman is asserting his essential unity with everything, not just nature. From the beginning of the poem he asserts his union with his reader: 

I Celebrate myself, and sing myself,
And what I assume you shall assume,
For every atom belonging to me as good belongs to you.

So, at the end of the poem, when he says that 

I too am not a bit tamed, I too am untranslatable,
I sound my barbaric yawp over the roofs of the world.

He means that like the "spotted hawk" he also is of the wild, his poem an "untranslatable" cry. But the lines you mention give a slightly different twist on the ending of the poem, and perhaps Whitman's purpose in writing it. Lines 7-8:

I effuse my flesh in eddies, and drift it in lacy jags.
I bequeath myself to the dirt to grow from the grass I love,

Here Whitman describes the dissolution of his body into the air and dirt. His body permeates everything. Then,

If you want me again look for me under your boot-soles.
You will hardly know who I am or what I mean,
But I shall be good health to you nevertheless,
And filter and fibre your blood.

Because of his dissolution into everything, his call to the reader to look for him "under his boot soles" is as good a place to seek the poet as anywhere. If we examine the dirt for him, probably we will "hardly know" who he is, or even if we have found him; nevertheless the good will of the poet will "filter and fibre" our blood. In the next lines:

Failing to fetch me at first keep encouraged,
Missing me one place search another,

the poet encourages us to keep looking for him, even if we cannot at first find a trace; for

I stop somewhere waiting for you. 

Whitman's words point to something different than simply uniting with nature; first, this is a physical unity, not just a spiritual or metaphorical one. The essence of the poet, his "barbaric yawp," not only can be found in the dust beneath our feet or the clouds overhead, but somehow offers us protection. Whitman's title, "Song of Myself," is a bit of a joke because his point is that it is the song of everyone. Whitman's poem is an attempt to articulate the deep essential union of all things: man, nature, machinery, art, sex, work. These things form the essential and inescapable, and ultimately "untranslatable," fabric of existence. Whitman's final line, in which he is waiting somewhere for us, is a perfect end: it evokes the image of the poet, a person just ahead, waiting, but also of course works in the symbolic sense that, since poet and reader are the same spirit, it is really ourselves that is waiting for us, and the search we make for the poet really is best done if we look inside ourselves.