In this moment, the speaker is trying to get a grip on Ariel. She's trying, desperately, to rein in nature. It's an epic fail, though.
Berries cast dark Hooks—
Black sweet blood mouthfuls, (11-13)
It's not just Ariel who represents the wilds of the natural world in this poem. Even the berries are threatening. We're gonna think twice next time we reach for those blackberries in the supermarket aisle.
I unpeel— Dead hands, dead stringencies.
And now I Foam to wheat, a glitter of seas. The child's cry
Melts in the wall. (20-25)
In this moment, the speaker begins to transform. She throws off her human cares—those "dead stringencies" and "the child's cry"—and imagines herself as "a glitter of seas." She's finding peace with this wild ride by seeing herself as a part of nature, not as something opposed to it. This is some radical acceptance of the world that we're witnessing.
And I Am the arrow,
The dew that flies Suicidal, at one with the drive Into the red
Eye, the cauldron of morning. (26-31)
In this final, crazy moment of the poem, the speaker imagines that she and Ariel have become one. Her horse's drive is her drive, and they ride off as one into the sunset, sharp as an arrow. The "suicide" is the death of her former, human-bound self. The speaker is now a horse-woman. Awesome sauce.