What is the main disadvantage of the slash and burn method of growing crops?

There is a high amount of waste when this method is used ( there is always a lot of waste in agriculture but still)

Magma forms by partial melting of upper mantle and crust. partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. the partial melt rises because of its lower density and ascends through he crust. this is so conclusive mean the answer would be magma igneous rocks or balistic magma.

Do you have a picture for this problem?