Answered

Why did early farmers use slash-and-burn agriculture? A. to get rid of pests B. to get rid of old crops C. to keep the soil fertile D. to keep undergrowth from drying out the earth

Answer :

ampritchard

C. To keep the soil fertile

the ashes somehow replenishes the soil thus providing nutrients for future crops. If they didn't do this, the soil would be depleted. They also rotated crops so that they didn't deplete the soil. 

Hopefully this helps.