The efficiency and productiveness of many large cotton plantations found in the south during the first half of the 19th century made most plantation owners economically dependent on

Large amounts of labor. This labor was acquired by purchasing slaves to work the plantations. This made most plantation owners economically dependent on the slave trade.

Labors (slave)

Explanation:

The Southern states in the first half of the 19th century depended on the labors to perform work in the fields of cotton plantations. The labors were the African American who were brought from Africa and put into auctions to sell. For the plantation owners, slavery was the backbone of the economy of the South. Slaves were forced to work in the fields of cotton from sun up to fall. They helped the South in getting rich and prosper.        

from the conditions of the working class in england by friedrich engels,the main idea is :

it increased the power of the king of england.