How does Emerson's quote about good coming from toiling on one's plot of ground relate to Transcendentalist ideals?
To really understand this quote, you need to look at the stuff around it.
The part immediately before and after what you quote says:
There is a time in every man's education when he arrives at the conviction that envy is ignorance; that imitation is suicide; that he must take himself for better for worse as his portion; that though the wide universe is full of good, no kernel of nourishing corn can come to him but through his toil bestowed on the plot of ground which is given him to till. The power which resides in him is new in nature, and none but he knows what that is which he can do, nor does he know until he has tried.
What Emerson is saying is one of the major points of Transcendentalism -- you need to be yourself and not try to be what other people think you should be. That's why he says that you shouldn't envy other people or imitate them. So, he's saying that you need to be yourself and you need to cultivate yourself rather than relying on other people.
I think it's a little bit of a stretch to say that this shows that nature can help us understand life. When you look at the whole quote, it's clear that he's just saying that each person is unique and each person needs to figure out what they are for themselves.
Along with the Transcendental idea of "self-reliance," and individualism, Emerson seems to imply that it is through individual efforts and hard work that one achieves:
...no kernel of nourishing corn can come to him bestowed on that plot of land which is given to him to till.
An individual must achieve goodness through his/her own doing. This concept is the antithesis of the Calvinism of the Puritans who preceded the Transcendentalists, for their ideology held that there are those who are the "elect." That is, they are chosen as those who are good and will go to heaven; they are saved by their faith, not by good works. For the Puritans, good works do not count as penitence for sin, or as anything positive for a person. On the contrary Emerson implies that an individual can achieve goodness only by striving for it. Thus, goodness becomes a goal that lies within any individual's grasp and, as he also has remarked, "The reward of a job well done is the job itself," a statement in accord with the Bible's "As ye sow, so shall ye reap." [Job 4:8]
The implication within the quote is the idea of focusing on the individual's actions. The individual is in the position of power to be able to impact their world with a sense of action and direction. The world's presence of good is only recognizable through the actions of the individual which seek to underscore this reality. The idea of transcendentalist thought sought to establish a more emotional focus to the world and the individual actions within it. I think this is probably where Emerson's quote fits into the idea of the movement. When he suggests that individuals must "till" the land, it indicates that individuals must substantiate the order of the world which is overall good, and they cannot take it for an assumption. In analyzing their own senses of self through an emotional frame of reference, individuals are able to understand that they can be active agents of this world.
Here is the full quote:
That though the wide universe is full of good, no kernel of nourishing corn can come to him but through his toil bestowed on that plot of ground which is given to him to till.
I would say that this is a metaphor. Emerson says that real nourishment/satisfaction in life comes to those who cultivate it themselves. He uses a simple situation - working a plot of land to grow food for yourself and enjoying the pay off when you get to eat what you've grown - to stand in for the bigger picture: anything worth having is worth working for yourself. Emerson's essay places great value on the individual's capacity to be self-sufficient, and he uses an example from nature to expain his view point and to help his readers understand the value in being self-reliant.