I have been asked to compile a list of the ingredients in the substrate I use for my millipedes.
I vary the mixture by species, and I base my choice in components on how much of each I have available. It is better that a substrate not have a component than wait for the acquisition of the component while the millipedes starve without any substrate. My usual starting ingredient is fermented aspen wood chips. When I was first endeavouring to rear detritivores as pets, such as rhinoceros beetle larvae, I prepared this massive vat of aspen wood for fermenting. For those who are unfamiliar with the purpose of fermenting wood, the beetles I mentioned, in addition to most species of millipedes, require decomposed wood on which to feed. The fermentation is a way to artificially accelerate the natural process of decomposition in a controlled manner to yield a steadier, more predictable food supply when rearing these detritivores. I have since made other batches of this fermented substrate out of oak, which is preferred by many beetles. Regardless, I am still left with a massive bin of these perfectly usable wood flakes for the millipedes. My next ingredient is a bit of the aforereferenced fermented oak sawdust, but since I need this for the stag beetles I raise and never have excesses of this material, this component makes up a small proportion of the substrate. Dead hardwood leaf litter is an essential component of the diets of most detritivores, so the next ingredient is a decent pile of dry, shredded oak leaves. By a decent pile, I mean that the leaf litter would be at least ten percent of the substrate volume, before the substrate is mixed and inevitably compressed.
The remainder of the ingredients are added on a whim based on which ingredients I have a surplus of. Some of my tanks get hardwood mulch from Home Depot, which despite its mass-produced nature, seems to be a good food source for the millipedes. (Coincidentally, this mulch is quite useful for culturing bioluminescent fungi, on which a post is necessary [once I figure out how to capture the faint glow].) I sometimes add a bit of fresher wood to decompose and offer a longer term food supply for the millipedes. A couple of my more recent batches have included long-fiber organic sphagnum moss as other hobbyists have incorporated it successfully.
I also use the calcium sand sold for reptile substrate as a calcium supplement by mixing it into the substrate. I have seen some people use cuttlebone for millipedes and isopods, but the important constituent, calcium carbonate, is chemically identical in both of these materials. The cuttlebone seems to have some differences in its molecular structure, but I doubt the millipedes and isopods are affected by such a minute difference. Since millipedes take such small bites, my logic is that they would prefer little granules of a calcium-containing mineral to a large block. I mix in about a quarter of a cup of substrate per gallon of substrate volume.
I have recently have had quite a bit of success with my diplopods and isopods, and while part of this is inevitably due to rehousing into larger enclosures, improving the bioavailability of the nutrients in my substrate with mixtures outlined above must have also been a boon for my colonies.