Backfill is the soil returned to the planting hole after the plant is set. The soil interface is the place where the backfill meets both the soil ball and the sides of the planting hole. If the backfill soil is significantly different from either the soil ball or the surrounding soil in texture, moisture content, aeration, or added amendments, the transplant may be negatively affected. For example, if the backfill is amended to become finer textured than the soil in the root ball, it creates an underground sponge that quickly pulls water away from the root ball and holds it where the plant cannot reach it. If the backfill is more conducive for root growth than the surrounding soil, many root systems will not develop properly. They will stay within the planting hole and not spread wide and deep as they should. Therefore, for most planting situations, the backfilling should either be done using the soil removed from the hole or with a soil that closely matches the original soil in texture and other attributes.
As backfill is added, care should be taken to avoid leaving air pockets within the planting hole. Bare root plants should have soil worked under and into the root mass to eliminate air space. The persons doing the installation should use their feet to tamp down the soil around the soil ball of b & b and container grown materials. Just enough pressure should be applied to eliminate air pockets while avoiding compacting the backfill.