The imagery continues by contrasting day and night. The farmers are well aware that daytime is when seeds are planted, and during the night “there can be no labor performed.” Of course Amulek has opened this metaphor to life and death as opposed to a literal day and night. Amulek does carry over the imagery by indicating that this life (day) is the time for labor, and the night (death) is not. This would clearly indicate to his audience that they must begin to plant the seed of faith “in the day,” so that they might become the people of the king.
How do we read Amulek’s statement against the knowledge that the gospel is preached in the next world, and that repentance is possible there (see D&C 138:6-37)? All analogies have their limitations, and Amulek’s discourse has a particular end in mind, to urge these people to repentance. The imagery of the night and day are directed to the agricultural mindset, and were not intended to metaphorically cover the entire gospel. We simply must understand that Amulek is teaching a particular lesson, and using this metaphor to emphasize his main purpose. Note that the particular timing implied in the next verse.