Depends on what it looks like. If it's high in clay, you'll need to break it up with something. Peat mulch, perlite, or vermiculite will work. But, these are good for any soil, they help keep it from packing in too tight on the root ball, and they hold moisture. Next, you'll need to look at how it looks. Is it dark, and earthy smelling? Is it grey and powdery, is it red, hard and heavy? I usually make a mix of vermiculite, peat mulch, and regular potting soil. But, If you were trying to keep the bulk down. Just opt for the vermiculite, or perlite, plus bone meal, and dried blood. That will give it something to eat for a couple of months, anyway. Unless the soil is awesome, you'll still have to juice 'em somewhere along the way.