I have a client that has 20-25 4-6' Foster Holly trees. (I didn't plant) They are at best ok down to almost dead. They have had almost NO gowth since planting 4-5 YEARS ago. He is concerned that the wash off from the concrete slab directly in from of them is affecting them. Apparently there is a chemical in concrete that is bad?????? He has since installed slate over the concrete. Here are my questions.. could concrete have hurt them?? If so, can it be undone?? HOW??? IF not, what can I do to try to heal them....ie. fertilizer, deep root fert, WHAT????? THANKS FOR ALL YOUR HELP IN ADVANCE!!!!!!!!!!!!!!!