Black gum--failure to thrive
This one is a challenge, I think.
Three years ago I purchased a black gum tree (Nyssa silvatica) from an excellent tree farm in my area of N. Florida. It was about 8 feet tall in a 50 gal. pot. I followed best practices for its transplantation--digging a planting hole twice the diameter of the rootball, leaving an inch of the root ball above the ground level, and mulching. The extension service said not to enrich the soil, so I did not.
The site is moist but not wet, and I irrigate when the soil gets dry. The soil is sandy, but black with organic material. I fertilize with a slow release fertilizer in the fall. The planting site is also open to the sky and gets what I would call ample slightly filtered sun.
Nonetheless the tree does not thrive. It has put on new twigs and gets leaves every year, but it has not grown substantially since I planted it. Other trees I planted at the same time (live oaks) have almost doubled in size.
I know black gums like acid; should I try to boost the acid content of the soil. Should I mulch deeper. Does the soil need to be wetter?
What would you do to help this tree? Your help is very much appreciated.