1. Preparation of the Planting Hole

The size and depth of the planting hole are critical factors in the success of planting and establishing a new tree.

Holes should be dug with a diameter no less than three times the diameter of the root ball.
This is particularly important in compacted soils.

The hole should have sloping sides similar to a wok. This provides a greater volume of loose cultivated soil in which rapid root initiation can occur.
Research has shown that plants benefit from larger planting holes.
(Figure 1 & Figure 10)

figure1 wFigure 1. Cross section of a wok hole showing sloping sides and top of root ball level with ground.

A larger hole means a greater volume of loose cultivated soil that is not compacted but aerated which allows good water penetration for rapid healthy root growth. 
(The majority of the roots on the newly planted tree will develop in the top 30-40cm of soil) 

The depth of the hole should be equivalent to the depth of the root ball, do not excavate more than the depth of the root ball. If you do, firm down the soil immediately to prevent root ball sinking. 

Planting too deep is a common mistake that leads to symptoms of poor vigor and slow growth that can often lead to plant death and or diseases for example cellar rot.