How did farmers who used slash- and- burn agricultural fertilize their land

They would destroy parts of the earth, and get rid of plants already there or parts of their over grown crops, this allows the rich and fertile soil to be uncovered and used again at a later time. This would destroy thousands and thousands of acres of forests as they burned them down to put agricultural crops down.

Explanation:

They used the ash left from burning the plants and trees on the land.

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