I have about 10 thornless blackberry plants, given to me by a relative 10 years ago. They have not produced many berries for the past several years. One problem, is that the deer Keep browsing them. But I have never fertilized them or amended the soil. I have pruned them, and just now, watched the video link from your site of the extension agent from the Univ of Kentucky, as a refresher on pruning.
My question is: how do I fertilize them? How often and with what?
You will have to protect from the deer. Consider deer fencing. The plants can start to produce less if they are bothered by insect or disease issues. Here is our page on brambles which includes fertilizing, care, pruning,etc.