I would use it regardless of if it had fertilizer value or not because comfrey is a great herb and if nothing else it would be water for the plants.
BTW, do you boil the roots and make a poultice for cuts & abrasions. I have tried it once and it really works fast. I had a cut on my hand and used Comfrey on it only once and that cut was healed within a day. I guess it's true what they say about Comfrey, "Make sure the wound is clean and sterilized before you use it, otherwise you'll be opening it up again."
Farmers use it on their horses legs for cuts and abrasions. They always have some Comfrey growing near the barn.
