Why are ashes used after a forest plot is burned for cultivation?

In the context of slash-and-burn agriculture, ashes are used because they serve as a natural source of fertilization for the soil.1

When vegetation is cleared and burned, the resulting ash enriches the soil with nutrients, which is essential for supporting crop growth in environments where long-term, continuous cultivation would otherwise deplete the land's fertility. Additionally, this process helps clear the plot, leaving it relatively free of weeds for the initial planting period.21

While this method is an adaptation to tropical soil conditions, it is important to note that modern, large-scale applications of this practice contribute to deforestation, biodiversity loss, and significant carbon dioxide emissions, which negatively impact the global climate.1