There's a lot of nature in this poem. Fields, dust, flowers, rivers, suns—it's all over the place. The relationship between the speaker and the natural world is very close, even harmonious. When he dies, he returns to the earth (as dust). Moreover, as a child, he was "washed" and "blest" by the rivers and sun of his homeland (England). When he dies, his heaven will look like the England he knew as a child—including its natural characteristics.
Line 2: The speaker imagines acquiring a "corner of a foreign field" for his home country, England. Nature is endowed with English-ness here, as it will be again soon.
Line 4: The speaker imagines himself as a part of nature, a pile of "dust concealed" in the earth. Dust here is a metaphor for both the speaker's status as a corpse and for his relationship to the natural world.
Line 5: The speaker is a "dust whom England bore, shaped, made aware." England can't really do these things, so this is a case of personification (the attribution of human qualities to non-human things). Dust is a metaphor for the speaker's relationship to nature and for the fact that he may soon be dead.
Line 6: England gave the speaker "flowers to love" and "ways to roam." England can't actually give anything, really (nice try, though), so this is an example of personification, the attribution of human qualities to non-human things.
Line 8: The speaker was "washed" by England's rivers, and "blest" by her suns. Neither the suns nor the rivers can wash or bless, so this is also personification, the attribution of human qualities to non-human things. Both washing and blessing are metaphors for the way England nurtured the speaker.