Rather than noticing the fragrance of the newly cut hay, the "abyss" of odor at his back indicates the wasteland that Frost perceives the hay field to be. He observes that the last evening swallow, although intermittently silenced by Frost's presence and rustle, finds its voice again on its "last sweep." These words do not evoke joy or vibrancy, but instead suggest something worse than discomfort - a numbness of spirit that exists in a wasteland of such gloomy depths that it implies an empty stoniness of the heart.

The poet has brought along to the hay field a book of old treasured songs, not to read and reminisce, but to hold and "freshen in this air of withering sweetness." The songs must hold some former joy for him, but he holds the book only for the memory of the person...

The withering sweetness may imply that, although once happy with this person, her absence has withered the sweetness of their relationship. He has brought the book with him as a bitter reminder, hoping to sweeten it in his sad contemplation.
Frost's wasteland imageries are plentiful in his poetry. His words are beautiful and carefully chosen, but the vacuity of feeling from which they came clearly suggests a person whose battle with his own demons was a lifelong struggle. I believe Frost was a genius at creating a sense of wasteland in his writing because his untreated depression reached a deeper level than simple sadness and despair, and sucked him into an empty space where he was able to write about it more objectively.