hau Gordon,
Certain trees don't need soil to grow (most of the conifers can be found growing out of what looks like solid rock in many places in the Sierra Nevada Mountains, likewise in the Rockies.
What is needed for an already growing tree is a big enough pocket for the root ball to get growing and fissures (fractured rock) for the roots to grow into, like any tree, shrub or bush planting you need a hole at least twice the diameter of the root ball to get the tree established.
On our land we have many places with bed rock within just a few inches of the surface and I've planted Mulberry trees in a few of these, those trees started out as whips (one year old trees that are mostly just a stick with roots) and are now 7 inches in diameter and giving off lots of fruit.
To plant in these conditions I use a Hammer Drill with a chisel point (my hammer drill is a "professional demolition model"), it takes me about a half hour to break up enough of the bed rock to make a good planting hole, clean it out and then create fractures all over the bottom and sides of the hole.
Next I mix 3 parts soil from my land with one part
compost, one part rotting wood chips and dust and one part potting soil to do the planting with.
I try to make my holes deep enough that I can have about 4 inches of this soil mix under the root ball (I make a cone of soil for bare root trees to sit on) I tease out some of the roots then set the tree and back fill.
Next comes watering in with a root stimulant mixed with water, this settles the soil and I make sure I soak the fill a minimum of three times at planting.
I water the new tree once a day for the next 5 days, this lets water get down into the newly fractured bed rock so the roots will follow that water as they grow over time.
From that point on it is just a matter of watching for leaf wilt on the trees for the first couple of years.
After that, you probably will find that you don't need to water unless you go about a month between rain events.
Redhawk