I live on a big sea of limestone also and have a green thumb. bad combo. I dig the biggest hole I can 1st. which usually isnt that deep. then I take the garden hose with a sprayer attachment and remove as much dirt from the root ball as possible. this helps pot bound specimins and greatly decreases the size hole you need.then I backfill my wimpy little hole a little with a sandy loam/compost mixture that I purchase by the truckload. Its like $28 a yard delivered.(cheap!!) and grows lants like a mo fo.then I place the basically bareroot specimen spreading the roots out then bring in the motherload of dirt and raise up the entire area and blend as best you can with the surrounding area. this leaves a little hill which blends in as time goes byand elliminates any real backbreaking work.