Forget the old technique for planting shrubs and trees, the one I learned when I was a kid. We were told at the time to amend the soil removed from the planting hole of a tree, an evergreen or a shrub by mixing in a good dose of compost, organic matter or fertilizer. Some gardeners even went so far as to replace all the soil removed with quality top soil.
However, studies led notably by the American tree guru, Dr. Alex Shigo, have since shown that when the soil in the planting hole is better than the soil all around, the roots tend to stay in that area, circling within that patch of richer, more friable soil. In fact, with time, they can literally girdle the tree, strangling it.
Instead, backfill with the original soil, adding no soil amendments (fertilizer, compost, manure, etc.). Since the soil is now of the same quality everywhere, the roots will do what Mother Nature wanted them to do: extend out in all directions.
You can, however, apply mycorrhizal fungi to the plant’s rootball, especially in soils that have been perturbed. These beneficial fungi improve root growth and, contrary to fertilizers that, when added to the planting hole, encourage the roots to stay nearby, actually tend to spur them to spread out widely.
If you do want to improve the texture or quality of the soil, do so after planting, fertilizing or applying a layer of compost over the entire area, as far as the roots will eventually reach, that is out to and beyond the tree’s eventual drip line. That way, you’ll encourage the roots to spread in all directions in search of the added nutrients … and you’ll also improve the growth of all the plants in that zone.