My cat had crystals in its urine and the vet prescribed food but?

My cat had crystals that blocked his urinary track, now that he is home the vet put him on purina UR. I read the ingredients and this is not an impressive food basically filers with the stuff he needs added. It will be good for his urine but reek havoc on the rest of his body. His coat has already gone to heck in the five days hes on it. I normally feed Solid gold and they make a supplement to add for urinary crystalsbut only if its Struvite crystals and not Calcium Oxolate. I called the vet to ask what type he had and they did not send them in and that the food fixed it either way. i dont get how one food could both lower the ph or raise it depending what is needed for the cat. Has anyone ever heard of this or have any other ideas. Thanks