In the opening line of the poem, Keats uses metonymy when he writes that "The Poetry of the earth is never dead." Metonymy is a poetic device whereby the name of something is replaced by a different word that represents that same something in a symbolic way. In the example in the opening line of this poem, the word poetry is used as a replacement for the word life. The meaning of the line is that the life of the earth never dies, but by using the word poetry instead of life, Keats suggests that that life is beautiful, musical, intricate, and creative.
Throughout the poem, Keats uses juxtaposition to emphasize the point that the life or "poetry" of the earth continues at all times, through all seasons and climates. For example, in the second and third lines of the poem, Keats says that this life endures during "the hot sun" as well as in the "cooling trees." Later in the poem, he writes that the life of the earth endures "in summer luxury" and also in "lone winter evening[s]." The juxtaposition here emphasizes how robust and indefatigable is the life that pulses through the earth.
To further emphasize the same point, Keats also uses repetition. For example, he says that the pulse of life will "run/ From hedge to hedge," and he also uses the word never three times in the poem. In the first line, he writes that the life of the earth is "never dead"; in the sixth line, he writes that this life is "never done"; and in the ninth line, he says that it is "ceasing never."