A careful analysis of this excellent essay will help you to discover the answers to your question. It is important to realise that he uses imagery in part to convey the force of his message and to also show where his feelings lie. Consider the way in which the "shadow on the wall," for example, is used to cast a negative light on consistency, which it is used to represent:
A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines. With consistency a great soul has simply nothing to do. He may as well concern himself with his shadow on the wall.
Such an image speaks of inane contemplation, boredom and monotony, clearly presenting consistency in a negative light, which is of course Emerson's key belief.
Looking at the use of these phrases in context will therefore help you establish how Emerson uses them symbolically. We can see therefore that planting corn is representative of doing good deeds, the iron string is an image used to suggest the importance of trusting oneself, and the joint-stock company is a powerful image of society and of its various ills.