Leave a couple of inches of concrete cover for all your reinforcing (any embeds really). Don't push the bars into the dirt and pour concrete around them. Rebar exposed to the moist dirt will begin to corrode, which will then wick up into the concrete, further corroding reinforcing, making the steel swell and crack the concrete, etc. It might take a long time, but keep to the 2" cover rule and it will likely last foreve.