How do you get rid of tree stumps?
Stumps and tree roots will eventually decay, but it is probably ten years or more before you can work the soil. You can speed up the process by drilling holes in the trunk and pouring saltpeter in the holes.
Alternatively, you can have the trunks ground out, but will still have the roots to contend with. If the trunks are a foot or less in diameter, a bobcat machine can be used to pull them out. This will get most of the roots out also.