I have about 20 acres south of Riggins I would like to plant a food plot on about 1/2 acre of it. Is that legal to do in Idaho?

Planting a food plot is legal in Idaho, just like gardening or farming.  You may want to consider your objectives and what is possible to grow there before deciding what to plant. It would be okay to hunt over if it left alone and completely unharvested, or if it has been completely harvested using normal farming practices. Otherwise, that may be construed as baiting.