This passage has to be understood in the context of the descriptions of Anne’s state of mind earlier in the scene. Recall that Anne is in love with Captain Wentworth, but she believes that he is in love with Louisa Musgrove. Circumstances have resulted in Anne being on a walk in the country with her sister Mary, the Musgroves, and Wentworth. This is an uncomfortable situation as she has to suffer the sight and sound of Louisa flirting with Wentworth. Accordingly:
Her [Anne’s] pleasure in the walk must arise from the exercise and the day, from the view of the last smiles of the year upon the tawny leaves, and withered hedges, and from repeating to herself some few of the thousand poetical descriptions extant of autumn, that season of peculiar and inexhaustible influence on the mind of taste and tenderness, that season which had drawn from every poet, worthy of being read, some attempt at description, or some lines of feeling. She occupied her mind as much as possible in such like musings and quotations; but it was not possible, that when within reach of Captain Wentworth’s conversation with either of the Miss Musgroves, she should not try to hear it; yet she caught little very remarkable.
Jane Austen (1817). Persuasion, chapter 10. Project Gutenberg.
In other words, Anne has been trying to distract herself by silently reciting poetry, but the works she has selected are of a melancholy character. Note that this is a double displacement of description: the poets that Anne has been quoting used autumn as a metaphor for sadness, age, and death; and the narrator of Persuasion does not tell us directly how Anne is feeling about the situation, but lets the reader deduce it from the description of Anne’s selection of verses. The effect of this displacement is to place Anne’s emotions at a double remove, allowing the reader to consider them more objectively.
A couple of paragraphs later, Anne hears Wentworth saying to Louisa, “I honour you”, and Austen writes:
Anne could not immediately fall into a quotation again. The sweet scenes of autumn were for a while put by, unless some tender sonnet, fraught with the apt analogy of the declining year, with declining happiness, and the images of youth and hope, and spring, all gone together, blessed her memory.
The use of “blessed” here is ironic: clearly Anne is actually miserable with jealousy and despondent with unrequited love. At this point we get the paragraph quoted in the question. In “the ploughs at work, and the fresh made path spoke the farmer”, the verb “speak” is used in the sense “indicate, denote, or betoken; to reveal, make known” (OED): that is, the ploughing of the fields and the maintenance of the path indicate the intentions of the farmer.
The “sweets of poetical despondence” are the emotions that Anne has been indulging in. The narrator’s use of “sweets” (joys, pleasures) is ironic in the same way as “blessed” was above. The use of irony indicates the position that Austen wants us to take: namely, that Anne’s despondency has an element of self-indulgence and is not wholly admirable, and that the attitude of the farmer, of getting on with the work of ploughing in preparation for spring, is the better one.
