In a field I am the absence Of field (1-3)
True to poetic tradition, this poem opens up in nature. We're "[i]n a field" but beyond that, there's no description. This could be to reinforce the speaker's isolation from the natural environment, as if to say, "just the facts, ma'am, just the facts!" But what's different is the speaker's relationship to that field. In lines 2 and 3 the speaker says he is, "the absence / of field," which creates a sense of isolation from his environment. He's both there and not there. The short lines reinforce this sense of fragmentation. Instead of a place where the speaker goes to clear his thoughts or feel one with with nature, the landscape becomes a backdrop for his feelings of being separate from the world.
Wherever I am I am what is missing. (6-7)
Okay, no natural imagery here, but we do get the all-encompassing, vague, "[w]herever." Just like the speaker is caught up in the duality of being present and absent, "wherever" can be read as an unknown place—like saying, "I don't know where I am"—and as a word that means everywhere. No matter where the speaker goes, he's separated from his location, and that means there's no escape from his feeling left out. Boo hoo.
When I walk I part the air And always The air moves in (8-11)
Back to nature, back to the basics. Our speaker really likes to keep it simple. As for descriptions of nature, we get "field" and "air." Can't be more general than that. What seems important here is that "air" parts for him. Think of someone walking through a crowd and the whole crowd splits out of the way while that person passes, and then the crowd reforms. It's sort of like that. The speaker feel so isolated from the world that he can't even be a part of the air—or the party. It's like his body is some sort of protective shell that keeps him separate from his environment. Makes you wonder how he can even breathe, right?