The answer is a decrease in the usability of soil for agriculture.
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Black codes existed during the Reconstruction Era, which lasted from 1865 to 1877.
Slavery developed in southern colonies due to large populations of native americans died due to the diseases the colonists brought over and then the indentured servants from england did not have the skill set necessary for cropping/harvesting tobacco but they realized people from africa were used to the labor intensive work and were able to work long hours in the sun which led to the spread of slavery through triangular trade.