The Middle Colonies were composed of what is today the states of New York, Pennsylvania, New Jersey and Delaware.
Because the soil was rich in the Middle colonies, farmers were able to grow more grain than they could use. They began selling grain to other colonies and to England, shipping out of the big ports in Philadelphia and New York. Because of this, the middle colonies became known as the Breadbasket colonies.
http://www.slideshare.net/ajroets/61-the-middle-colonies-55805719
Source: http://13colonies.mrdonn.org/middlecolonies.html
