Final answer:
The Spanish relied on African slaves in their American colonies due to the need for controllable labor on cash crops like sugarcane.
Explanation:
The Spanish began relying on African slaves in their American colonies primarily due to the need for a large, controllable workforce to work on cash crops like sugarcane. The introduction of sugarcane as a profitable crop led to the demand for laborers. Additionally, the high death rates of Native Americans from European diseases and the clergy's interventions further drove the Spanish to seek African labor.
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