Neruda sure loved him some nature. In the first two lines of "Love Sonnet 17," the speaker contrasts the love he has for his paramour with the kind of love one has for flowers and precious gems. In the second stanza of the poem, he elaborates on the flower theme, suggesting that truly powerful love is not based on external beauty. The speaker is torn. He says his love is not like one’s love for natural objects, and yet he compares his feelings to an "aroma" that "rose from the soil." He relies on nature to explain love, but at the same time he realizes it doesn’t really work as an explanation. Confounding man and nature is a common practice in literature, and Neruda uses this technique wisely.
Love is like an "aroma that rose from the soil." It is like a plant, and must be nurtured, cared for, and cultivated.
Neruda is confused: in the first stanza the speaker says that he can’t describe his love in natural terms, but in the second stanza he compares it to an "aroma" that resembles a flower.