Can I use a soil drench on my hollies under a pecan tree?

You can, but you will need to make sure the product you are using is labeled for hollies and pecans. This will control the scale on the hollies but will not make the pecans inedible.

Related Videos:
Scale
Systemic Insecticides

Related Resources:
Armored (Hard) Scale - Trees and Shrubs
Soft Scales
Armored Scale Insects & Control
What is a systemic insecticide?
An Introduction to Integrated Pest Management of Landscapes