Did you let the supersoil "cook" long enough after you mixed it? If I had to guess I'd think nute burn but I'm not sure, I gotta leave the diagnosis to the more experienced growers. If you don't give the supersoil enough time to break down before using it, it can definitely burn though
also I'd try not to use tap water. we have super high tds/hardness here, and the chlorine is supposed to be bad for the microbes that are essential to an all-organic formula like ss