Final answer:
The main reason for the drop in cotton prices between 1865 and 1890 was the overproduction of cotton on a global scale.
Step-by-step explanation:
The main reason for the drop in cotton prices between 1865 and 1890 was the overproduction of cotton on a global scale. The invention of Eli Whitney's cotton gin in 1793 made it easier and faster to remove the seeds from cotton, leading to increased production. As Americans moved into the Old Southwest, they found the soil well-suited to grow cotton, and with the rising demand and price of cotton in the international market, new land was quickly put into production.