I'm currently working/thinking through how to get some nutrients into my springtails. I'm worried about the myth that springtails are high in calcium. That is only true if the springtails themselves are living in a calcium rich soil. The soil in our tanks has high calcium content because of the weekly dusting that gets poured in. However, a charcoal substrate and feeding white rice leaves springtails that in a few generations should have no calcium.
I'm toying with the ideas of adding calcium to the charcoal. I'll let you know if that has any affects on production.
I've seen springtails raised in a charcoal + plaster of paris medium at universities, and I don't think it had anything to do with trying to get nutritious springtails. Rather, I think it was supposedly the medium that enabled them to produce springtails most efficiently/effectively.
I'm not sure what plaster of paris consists of, but I'd be surprised if it wasn't calcium based. Whether that calcium is useable, I don't know either. I keep meaning to try mixing some of this into a few of my cultures, but never get around to it.
One theory about why springtails are high in calcium is that calcium rich soil particles stick to their body. So I don't think that springtails reared on artificial substrates can be counted on to be calcium rich. The things that have already been suggested are probably the best we can do.
I feed my springtails lots of different stuff...but the best (from my experience) jump-start comes from ground rice...it seems to give a higher yield when I add a little repcal to the rice...around an 1/8 teaspoon calcium to 6 tablespoons rice.
so it is (mybe) a possibility to feed them directly with minerals and other healthy goods. Mybe Someone (i need to get some springtail cultures so i can too) can try possibly in a very small pan boil a little amount of water and make a paste with vitamins than let dry in alittle tablets and add to the springtail cultures and see if they eat it or climb on it or whatever.