The path to a stronger faith depends upon the person, upon the farmer in this analogy. If the farmer does not care for the plant, it will never grow to the point where it bears fruit. The farmer’s reason for planting will be thwarted, not because of the seed, but because of the farmer himself.
Notice that Alma blends one agricultural analogy with another with which the people must have been familiar. All of a sudden this is no simple fruit tree, but the ultimate fruit tree; the Tree of Life. Alma wants to make sure that they understand that this is an analogy, not a lesson in farming. This is a way to grow faith until it becomes to them a tree of life. The gospel is the promise of this eternal life, not just in enduring through time, but in the quality of the life throughout time and eternity. That is the tree they are attempting to grow, and the one that they would be discarding should they decline to care for it properly. Alma is warning them against destroying the most valuable of end results by inattention to the beginnings.