How does the poet describe nature in "The Solitary Reaper?"

The poet describes nature here in a characteristically Romantic way. The vale in which the reaper sings and works is "profound" as it overflows with the melancholy sound of her voice. He describes the welcome felt by travelers when they hear the nightingale and know, then, that it is almost time to rest, as well as the thrill of hearing the cuckoo sing when one is at sea and its song lets one know that land is nearby. The poet compares the voice of the maiden to these birds' voices to show how much he is affected by her voice, likening her, then, to these natural singers. Nature, then, has the ability to affect us powerfully and profoundly, and the speaker shows how much he is affected by the song of the solitary reaper by suggesting that her voice is as, if not more, welcoming and thrilling than the birds' voices. The voice of the young woman and this natural setting combine to create an even greater impression of beauty on the poet than either would alone.

Unusually for Wordsworth, nature takes a back seat in "The Solitary Reaper." His main focus is on the Scottish lassie as she sings her beautiful song rather than the natural landscape in which she lives and works. It's notable in this regard that the speaker doesn't tell us much about the vale, except that it is "profound"—i.e., deep—and that its depths are overflowing with the sound of the reaper's song.
Later on, the speaker will compare the reaper's sweet, melodious song with certain features of the natural world, much to the latter's detriment. We are assured, for instance, that "A voice so thrilling ne'er was heard / In spring-time from the Cuckoo-bird". And the song of the nightingale, that bird so beloved by successive generations of poets, never gave

More welcome notes to weary bands
Of travellers in some shady haunt,
Among Arabian sands.

Nature may be a source of incredible beauty, but it cannot compare with the beguiling song of the solitary reaper.
https://www.poetryfoundation.org/poems/45554/the-solitary-reaper

In "The Solitary Reaper," Wordsworth describes nature in terms that are meant to trigger imagination and wonderment.
Wordsworth finds a vast amount of mystery in the natural world as the speaker in "The Solitary Reaper."  One example of this would be in the girl's song. Wordsworth considers her to be a part of the natural setting. The song of the "Highland Lass" has captured his imagination. Wordsworth is not clear as to what she is singing:  "Will no one tell me what she sings?"  In asking the meaning of her song and reflecting on what it might represent, Wordsworth expands his imagination to embrace what might be as opposed to what is.  In this way, the natural setting that includes the solitary reaper's song initiates wonderment.  Her song is a part of the beautiful mystery that is the natural world.
Once the speaker of the poem hears the song, his imagination begins to take over his sensibilities.  It does not matter that he lacks a clear understanding of the song's meaning.  The song inspires his mind to drift to alluring settings such as "Arabian sands," "the silence of the seas," and "the farthest Hebrides."  He does not think of the dreariness of the urban landscape or the blight of a slum.  The song moves Wordsworth's imagination to consider places in nature far removed from daily life. These natural settings initiate wide open thought.  When standing on the "Arabian sands" or in the midst of "the silence of the seas," one is able to engage in expansive thought.  This pondering might very well include the world and a person's place in it.  Wordsworth believes that broad level of thought is only possible when standing in the midst of nature.   In this way, the natural world is linked with wonderment and awe.  
Wordsworth believes that nature holds the key to unlocking our moral imagination.  Simply interacting with it in a meaningful way, as he does in "The Solitary Reaper," can unlock doors of thought and perception which embrace transformative possibilities.