What was the country’s staple in the south before the great migration?

Cotton--the southern farming staple was cotton grown on large farms or plantations. 

Cotton requires large amounts of labor and most of the farm labor from the Civil War until World War I was former black slaves. The Great Migration began with the beginning of World War I as demand increased for labor in factories. Blacks left the South for better opportunities and more social freedom. This migration of labor forced the South to change their farming staple.