"Ariel" doesn't give us much in terms of setting. The poem begins before dawn, in darkness, and ends as the creepy, red sun is rising. We get only little flashes of the scenery around our speaker—the furrowed ground, the dark berries. Still, the poem uses those brief snippets of landscape to underscore the speaker's transformation from frightened rider to unrestrained spirit merging with creation.
You see, at the end of the poem, the speaker is "at one" with the "arrow" that is Ariel (26-27). Along the way, tough, she's also at one with the fields around here: "And now I / Foam to wheat, a glitter of seas" (22-23). Even the sound of the setting is fusing into one with the physical surroundings: "The child's cry / Melts in the wall" (24-25). In the way that the speaker joins the horse in a kind of cosmic fusion, she's anticipates that move by fusing into the setting as well. In a way, the setting is a way for the speaker to deliver the punchline to that old joke: "What did the wise man say to the hotdog vendor? Give up? Make me one with everything."