To what does Emerson compare the ordinary event of planting corn?
I assume that you are talking about the passage in which Emerson says that each man must come to understand that
... 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...
In this passage, Emerson is comparing the process of planting, growing and eating corn to the process of working hard to understand who you are as a person -- what it is that essentially defines you.
Emerson is saying that it is important for people to know who they are and to act accordingly. Each person is unique and must act in the ways that they are uniquely meant to act. But (and this is the point of the metaphor of the corn) no one can understand who they are without working at it. It is not easy to gain understanding of yourself, just as it is not easy to grow corn -- both take hard work.