How were southern plantations able to meet the demand for more cotton in the 1800s

I believe the answer is: by increasing the number of enslaved workers and using the cotton gin

The northern states in America were the one who adopt the innovations created during industrial revolution, which eliminate their needs to use slaves for their labour. The southern states on the other hand, did not embrace the revolution after the civil war was over. Because of this, they still relied on slaves.

b. by enlisting in the military to fight

after the civil war, and with the termination of slavery, the south was faced with a big problem; what was supposed to happen and how they were supposed to deal with the freedom and the rights that black people and freed slaves had gained as a result of the north defeating the south and forcing the end of slavery. first, most, if not all, of the workforce of the south came from slaves and they did not have an option to replace the loss of said workforce. second, particularly in the south, people were convinced that black people were lesser, less intelligent, less capable and less smart than whites. this posed a huge problem for them. also, southerners were faced with another issue; freed slaves now had a say in how their lives would be lived and the right to politically choose the way they would be governed. people in the south did not want to accept this, none of this and this opposition and the conflicts that arose from it, not to mention the afereffects of the war, forced the congress of the time to enact and enforce the reconstruction plans that had been proposed by president andrew johnson.