Fire is usually a symbol of transformation. Think "trial by fire." One goes through difficulty to become something new. Much of art is driven by an artist’s desire to grow, though transformation doesn’t always mean growth, as we see in the poem.
[…] shares man's smell […] (7)
We cringe a tiny bit here. We aren’t stinky, but we get the point. The purest perfume is bad if too much used. The earth has been transformed. People too. The speaker sounds downright cranky about it here, but transforms to a lighter mood, thick with the possibility of rejuvenation and renovation.
[…]the soil […] (7)
This poem shows two sides of the soil, and by extension two sides of nature. The soil on the surface of the earth is lonely and sad, with no plants growing in it. It’s been transformed by what people have done with it. But, the soil under the earth is just as fertile as ever, ready to counteract this transformation by urging another.
And though the last lights off the black West went Oh, morning, at the brown brink eastward, springs—(11-12)
These lines describe night transforming into day. They might also be a metaphor for the human heart, or the speaker’s heart, moving from the confusion of darkness, to the light of clarity.