I’ve done some reading on the topic. The worms will produce or transform whatever they eat. So if they eat egg shells then calcium would be added. If they eat banana peels then it adds phosphorous. The reason a lot of people believe worms only produce nitrogen is because they’re only feeding them cardboard and paper, which will in fact produce nothing but nitrogen. So feeding them a more diverse diet, including rock powders and lots of food scraps, makes for more nutritionally diverse castings. It isn’t being wasted, in fact it becomes more bioavailable for plants as it gets digested and covered in worm juices.