How did swidden agriculture maintain soil fertility in the Americas?

In the Americas, swidden agriculture—also known as slash-and-burn agriculture—maintained soil fertility primarily through the strategic clearing and burning of vegetation. By cutting down trees and shrubs and burning them, cultivators added essential nutrients to the soil in the form of ash.1234

This system functioned as a form of shifting agriculture, where plots were cultivated for a limited period until soil nutrients were depleted or weeds became unmanageable. Once productivity declined, the land was abandoned, allowing it to lie fallow and revert to natural vegetation or secondary forest, which enabled the soil to regenerate over time.34

Beyond the immediate fertilization provided by ash, these systems were effective because they mimicked the high ecological diversity of tropical environments. By maintaining this diversity, swidden practices provided a range of resources and prolonged the usefulness of fields and gardens, offering a sustainable approach that did not require outside inputs like synthetic fertilizers.124