The sense of solitude, in the first place, is evoked by the title itself - The Solitary Reaper. As the poem starts, the poet urges us to look at the maiden reaper, who is “single in the field.” The expressions “Yon solitary Highland Lass” and “singing by herself,” leave no doubt among the readers that the girl is all alone. She is lost in her thoughts and busy humming a song.
Besides, it is the girl’s melodious song that is the only sound audible in the fields. We notice the poet is extremely cautious so as not to make any unnecessary noise that might distract the girl. He is heard saying to the passersby either to stop or pass gently. He is afraid that their movement might make her conscious about their presence and thus cause her to stop singing.
The poet’s fear that even the slightest noise might distract the girl implies that except for the mellifluous voice of the maiden the fields are completely silent and still. This further emphasizes the sense of solitude.
The expression “Vale profound / Is overflowing with the sound” underscores the point that the girl’s is the most dominant sound that flows unobstructed throughout the deep valley.
The line, “Breaking the silence of the seas / Among the farthest Hebrides,” further elicits the sense of solitude and stillness. The poet suggests that up to the vast expanse of land reaching as far as the Hebrides islands the wistful song of the highland girl could be heard. Rather it's her song that resonates far up to the islands "breaking the silence of the seas."
Although the poet is unable to make out what the girl sings, he is sure that the song is about some sad topic. It might be possible that she must be musing about some misery of her life and hums a sad note. This hints at the sense of isolation and deprivation she might be experiencing in her life.
In the final stanza the poet says the voice of the girl fades away only when he has mounted up the hill. It is distance only that causes it to fade away. Thus there is no human activity around or anything else to interfere with the profound sense of solitude in the fields.
Thus it can be said that the sense of solitude permeates the poem and makes the maiden’s song all the more poignant and resonant.
The speaker happens upon a "single" and "solitary" reaper in a field. She is alone, unaware that the speaker is watching her. So, she believes and behaves as if she is totally alone, and the speaker meditates upon her solitary behavior. Perhaps if she knew she was being watched, she would not sing so freely and/or behave as if no one were watching. The sound of her song fills the "Vale" (valley); the valley is "overflowing with the sound." This could mean she is singing loudly and/or the sound fills the valley because she is alone and the only living being making any sound.
Her song is so full and singular that the speaker begins to imagine that her song has a universality to it. It's as if her song is one that all songs aspire to. The speaker also intuits a melancholy to her song. This might also have to do with the loneliness of the singular singer. At the end of the poem, the speaker continues to hear the song "Long after it was heard no more." He, now in solitude himself, is overcome with the song.