The easiest way to avoid this is never to dig the hole deeper than the root ball. Soft fill should not be left in the bottom of the hole, as the root ball will settle and be planted too deep. In almost all types of soil, the tree should be planted slightly shallow, with the top 2 - 4 inches of the root ball sitting above the surrounding soil grade. Remember, the exposed portion of the root ball will be covered with 2 - 3 inches of mulch by the time you are finished planting. Drainage is also an important consideration in successful planting.
Poor drainage kills more plants than any other cause.
A poorly prepared planting hole may act as a dish and hold water, especially in clay soils. Oxygen levels are low in the bottom of such holes, and not conducive to healthy root growth. The Root Ball Handle roots carefully during planting - small absorbing roots are easily broken. Check balled and burlapped plants to ensure the roots originate near the surface of the ball. When setting the plant in the hole, make sure these roots are no deeper than the soil grade.