The previous verse’s mention of being “content with the things which the Lord hath allotted unto me” explains why he says, “I ought not to harrow up.… ” Alma is continuing to stress the need to be contented with what is, rather than wishing that God had allotted him something else. A harrow is a farm implement that, when dragged over rough soil, breaks up clods and smoothes it out (see commentary accompanying Alma 14:6).
The verse may be difficult to understand because of its convoluted syntax. “In my desires” might seem to refer to the “harrow up” due to its proximity. It is best read as a descriptive insertion: “My desires ought not to harrow up the firm decree of a just God.” It is the firm decree that is being harrowed, and the desires that do the harrowing. Alma is suggesting that there are times when he might desire that one of God’s decrees might be altered.
The second part of the verse deals with Yahweh’s decrees, which, Alma proclaims, come to men “according to their wills.” While he doesn’t elaborate it here, this statement foreshadows Alma’s doctrine of restoration (Alma 41). Most directly, Alma is setting up his argument for the next verse.
Translation: Nephites would not have had a harrow. It is Joseph’s translation of some similar metaphor from the plates. (See commentary accompanying Alma 14:6.)