Why is primitive farming called slash-and-burn farming?

Because the farming method at that time was to "cut down trees with stone knives and axes, dry them and set them on fire, and then turn over the soil with a stone plow to sow." After a year or two of land reclamation, soil fertility declined, and people had to migrate frequently in order to find fertile land. "It means cutting, drying and burning trees with stone knives and axes. It is a' cutting and burning method', which aims to increase the necessary' fertilizer' for crop growth by burning ground vegetation; However, with the cultivation of the land, "fertility declines". In this case, human beings need to find "fertile land" to survive and maintain their livelihood, so they have to migrate frequently, which is manifested as "wandering farming system"