Understanding this verse requires that we return it to the context of the previous verse. Alma has just said; “for I ought to be content with the things which the Lord hath allotted unto me.” This is the precursor to “I ought not to harrow up….” Alma is continuing the theme of being content with what is, rather than wishing that God had allotted him something else.
The verse is a little difficult to understand both because of the less familiar word “harrow,” and a somewhat convoluted syntax. First, a harrow is a farm implement that turns the soil. Secondly, the phrase “in my desires” might appear to be related to the “harrow up” due to its proximity. It is not, but rather a descriptive insert. The verse might be better understood by relocating the “in my desires” concept. We might “retranslate” this verse as “My desires ought not harrow up the firm decree of a just God.” It is the firm decree that is being harrowed, not the desires. It is rather the desires that would do the harrowing.
Why is it that Alma believes that his desires might even have the possibility of harrowing up the firm decree of God? Apparently Alma has some sense that it is actually possible for him to alter a decree of God. This may be due to a revelation similar to the one to which Joseph Smith alluded when he said: “The Lord once told me that what I asked for I should have. I have been afraid to ask God to kill my enemies, lest some of them should, peradventure, repent.” (Joseph Smith, Teachings of the Prophet Joseph Smith, selected and arranged by Joseph Fielding Smith [Salt Lake City: Deseret Book Co., 1976], 340).
If Alma had the same kind of gift given him, he would be similarly conscious of his responsibility to make righteous choices, and in this case, to school his desires.
The second part of the verse deals with the varying decrees of God to man. Alma proclaims that God’s decrees come to men “according to their wills.” While he doesn’t elaborate it here, this is a foreshadowing of Alma’s doctrine of restoration which we shall see in Alma chapter 41. Most directly, Alma is setting up his argument for the next verse.
Translation: While the instrument known as a harrow would have been unknown to the Nephites, the idea of turning the soil would certainly be part of their agrarian background. Thus the metaphorical use of the term is accurate, even if this cannot have been an absolutely accurate translation of the original term.