More slave labor is needed to harvest more cotton.
Although Eli Whitney's invention of the cotton gin in 1793 doubled the production of cotton in the 1800's, since such machine reduced the labor of removing seeds, it also had an impact on the growth of slavery, since there was no machine to pick the cotton: a slave had to do it.
As the cotton production grew faster and faster, it also became more profitable, so the planters demanded more of both land and slaves. So much so, that by 1860, one in three people in the Southern States was a slave.
