Depending on the time period, here are three major crops:
1) Tobacco: Back when tobacco business was booming, the South grew large amounts of Tobacco, as the need for it was growing. However, Tobacco prices started to drop, and Tobacco played out the land to quickly, so farmers switched to the next one.
2) Cotton: One of the cash crops in the South, cotton grew well in hot, drier climates, and combined with the then-industrial revolution, the South economy was able to boom because of the great demands. In fact, the South was called "Cotton-King" because of the amount of cotton they created.
3) Rice: A stable food, Rice was eating by many varieties of people. It became extremely popular after the Asians moved to the US (mostly to the western south coast).
hope this helps
