Lynn,
I wouldn't be surprised if the extra fine dust from gravel provides minerals small enough to be absorbed by the semps. I use a fine horticultural sand I think may be sourced from a river, maybe that also provides some usable mineral content. I've been concerned about it's use as an additive lately though because it's so fine it may not be helping all that much with drainage, other horticultural sands I've seen have a coarser particle size. It does create a loose texture when used with peat, but when I replaced the peat with bark the soil became much more solid, I think it's the interplay of the fine fibers and granules that make the sand useful, the bark is much coarser than the peat, so if I continue to use it I think switching to a thicker sand particle might mesh better.
Paleo,
I skimmed the article and it does sound like Thiamine might help the rot problem. Toughening the structure and acting as an antioxidant, defending the plant from fungus and bacteria. Sounds like it doesn't act the same in all plants though, so it'd still be an experiment to try it on the semps. If I recall correctly there was something in there about some plants forming dependencies on direct B1 supplements, that'd be something to watch for, wouldn't want plants to grow well under your care just to not be able to defend against external stresses when whoever you give them to doesn't continue to treat them.
-Sol
