This was a new one for me as well, and thanks for posting it. I'm not sure I'd call it well-known, though.
For what it's worth, there's a
Wikipedia article about it. Stock up on salt before reading the entry...
I read the poem in general as kind of commentary on man's influence on nature.
1) And tall and of a port in air
What does 'of a port in air' mean?
I interpret it as a port (hole - see
second noun definition). It's breaking up (putting a hole in) nature.
It also makes me think of 'port arms', where a soldier holds his weapon sticking up into the air over his left shoulder. This continues the metaphor of the jar being a focal point and thus disrupting the scenery of nature.
2) It did not give of bird or bush
What does 'give of bird and bush' mean?
I agree with Jutfrank's comments above regarding this line - the jar doesn't contribute anything back to nature. In fact, it represents the opposite - taking a natural material (clay), and transforming it (via fire) to a man-made artifact that serves only man, not nature. It's a mark of civilization upon the wilderness.
The entire 2nd stanza (a full third of the poem) talks about how the jar pushes back the wilderness, tames the unruly growth, and tidies up the "slovenly" nature (from the first stanza). Nature now surrounds the hill, instead of covering it.
For what it's worth, Wallace wrote the poem after visiting
Elizabethton, Tennessee which was the site of the first independent American government outside the original 13 colonies. Being in the northeastern corner of Tennessee, it was essentially a gateway to the unexplored wilderness. (You can read about the Cumberland Gap and Daniel Boone's Wilderness Road). That furthers the metaphor of man encroaching upon nature. I can't say for sure that was his inspiration, but it's at least a fitting coincidence.
Despite it being just a jar, it's metaphorically a big ugly flagpole claiming ownership - dominion of man over nature.