And for all this, nature is never spent. (9)
Spent is an interesting word choice. It makes us think of "trade" and money. The line means nature can’t run out, but also perhaps that it can’t be bought – no amount of money can completely destroy it. It is this line that gives us a glimmer of hope about the state of the natural world.
[…]dearest freshness deep down things […] (10)
Yep, as we said, the damage is only on the surface of things, underneath all is fresh and sweet, but buried and hidden away.
[…] last lights off the black West went (11)
This is a fancy way of saying that the sun set in the west, and then the sky become dark. To the speaker of this poem, this natural phenomenon is no small miracle. The "morning" that follows it is an even bigger miracle. In spite of all the doom and gloom of life, the light just keeps on following the darkness. This could serve as a metaphor for human treatment of the earth. After a period of darkness and damage, there can still be lightness and healing.