The key cash crops grown by American slave labor before the 19th century were tobacco and rice. Little cotton was grown in the United States, in large part because separating the seeds from the raw cotton by hand was extremely labor-intensive and time-consuming. In 1793, Eli Whitney invented the cotton gin, which greatly simplified the procedure and made growing cotton much more profitable.

In the early 19th century, southern planters moved into areas like the Mississippi River Valley and started growing lots and lots of cotton with slave labor. At the same time, industrialization created a huge demand for cotton by textile factories in the North and in England - thus southern cotton plantations became a key factor in the burgeoning global industrial economy, so much so that the region came to be called the Cotton Kingdom.

The Cotton Kingdom depended on slave labor. Slaves planted, tended, and harvested the cotton. So much of southern planters' money was tied up in land and slaves for cotton production that slaves were often the closest thing they had to liquid capital. Slaves were simultaneously capital, collateral, and the means of production. Historian Walter Johnson puts it in very evocative terms: “The Cotton Kingdom was built out of sun, water, and soil; animal energy, human labor, and mother wit; grain, flesh, and cotton; pain, hunger, and fatigue; blood, milk, semen, and shit.”

Cotton became the symbol of American slavery because cotton grown by slave labor in the American South comprised a large part of the raw material that powered the global economy. By 1860, the South produced 2/3 of the world's cotton.

As the nation edged closer to Civil War, many southerners expected that the demand for cotton would help secure southern independence. It didn't, but that's a story for another time.

Sources: The Economics of Cotton

Walter Johnson, River of Dark Dreams: Slavery and Empire in the Cotton Kingdom