What invention led to an increase in the number of slaves needed on Southern plantations?

Answer 1

I would assume the cotton gin also. It was a machine that took the cotton off the stalk--separated it from the rest of the stem, etc. Since the slaves picked cotton, once the cotton gin was invented, they could process the cotton much faster, and sell tons of it all over the world, instead of just pounds of it. This meant that the plantation owners planted thousands of acres of cotton and got very rich. They needed hundreds of slaves to plant and pick cotton, until the land was nearly ruined and the bottom began to fall out of the cotton market

Answer 2

This probably means the cotton gin.