Burning the forest leaves fertile land, but not for long. Most of the nutrients in the Amazon are in the plants. As soon as you burn and start growing crops, you start removing the soil nutrients, Farming does well for a few years but then gets poor. You need to keep on restoring nutrients because there is no "20 feet of topsoil" like in the Midwestern US. It's like sand hydroponic growth medium, with no nutrients in the soil.