NisiNJ wrote: And the final picture is of healthy peat-started petunia seedlings transplanted up into Promix.
Note, RickCorey, that the one on the right is yours from the Robin.
So could it be too much salt? The seed starting peat mix had no fertilizer either. It is Fafard. My (grown) daughter Kat, who gardens with me, tried to help the pale seedlings by bottom watering in diluted MG. Would that have added more salt, and compounded the problem? (I just came across this thread today and didn't know about too much salt in coir.)