There is a lot of literature devoted to the actual act of digging a proper hole for plants, from annual plants to trees. I especially love the descriptions that have pictures showing the perfect hole, excavated just so, with vertical sides and horizontal bottoms. The soil is also amended to compensate for whatever its deficiencies are. In a perfect world this would be exactly how it is done each and every time. Because it is not a perfect world and because there are instances where compromises have to be made, the perfect hole is an ideal and not always practical for the myriad of exceptions in the real world.
To start, a perfect planting hole is twice as wide as the ball/pot of the plant and at least as deep as the depth of the same ball. The theory is to loosen the soil so the new roots have space to grow. It is especially important if the soil has recently been compacted by construction machinery driving over the soil due to recent construction on the site. Compaction is problematic especially if there is a heavy clay component. Clay particles, unlike sand, are very small and can be squished together forcing out air spaces in the soil profile. The soil profile extends the depth of the root ball and adding a soil amendment, like peat moss or leaf compost, etc., to the depth of the profile is, basically, consistent with adding these pore spaces back in. It is also an opportunity to add nutrients directly to the area where roots are growing. Too much soil amendment can be counter productive though because the backfill needs to be tamped down to provide support for the plant and prevent it from tipping over. Too much amendment will make the backfill composition too spongy and difficult to tamp sufficiently.
A less than perfect hole needs to be at least as deep as most of the depth of the root ball and wide enough to allow for rotating the root ball to find the best side of the plant. Will the plant die if the hole is not twice as wide as the ball of the plant? Will the plant die if the soil does not have enough organic material? Will the plant die if the hole is not deep enough? Read on.
The literature on planting practices is all about what is recommended but it is not the final word. If you can't dig the perfect planting hole, don't sweat it. Plants really have a stronger ability to survive less than perfect conditions than you might think. I have seen trees growing through cracks in concrete and along the face of a cliff. I have seen shrubs ripped out of landscapes only to be replanted somewhere else after sitting on the back of a truck in the hot sun all day and survived, in fact, thrived. Granted, some plants are more sensitive than others but there would be a LOT more dead trees and shrubs if they had no tolerance of less-than-ideal conditions.
Every plant goes into a period of “shock” after transplanting, which is simply an adjustment period where there is no apparent growth. There are multiple biological processes activated to protect the plant after transplantation that are not obvious to the naked eye but knowing this makes treating plants with exceptionally good post transplant practices a logical and prudent next step.
So to get started: