I thought I would add my 2 pennies (or cents) worth…

As I understand it, sulfur is indeed in the SOM. Just as we need an optimal C:N ratio of 30 (or 40):1 to make good compost. We also need an optimal N:S ratio of 10:1. I guess this reflects the ration of C:N:S in the final bodies of the microbes and stable organic matter. If the C and N are there, but S is missing, it will be quickly snapped up by biology.