The pre-war southern economy was an agricultural economy made possible by slave labor.

From Wikipedia, but any research will back this up:

While about a third of Southern families were slave owners, most were independent yeoman farmers. Nevertheless, the slave system represented the basis of the Southern social and economic system, and thus even non-slaveowners opposed any suggestions for terminating that system, whether through outright abolition or case-by case emancipation.

http://en.wikipedia.org/wiki/History_of_the_Southern_United_...