Lately, I've been wondering if the problems I had with my soil wasn't due to excess calcium.
I was watering with hard water in the 300 ppm range. The thing that sticks in my mind is what happened at the very start of the whole breakdown. I had a gorgeous vegging plant, almost ready for bloom, and it yellowed and went south right after an Energy feed - fast. It struck me a classic toxicity from the feeding, but it was just a mild dose of calcium nitrate - definitely not a hot feeding in any sense.
How could that produce a toxicity? And it stayed that way, despite time and flushings, etc - just never recovered much. Then the plants coming after that one started to go bad too.
Since then, I've occasionally run across plants with the same look, and the issue was often pH/calcium related. I wonder if some of the magdef we see isn't actually calcium toxicity. We've already crammed about as much calcium into this soil mix as we can, right? So, if the soil critters that break down the calcium start slacking, pH and mineral ratios can drift. Something to contemplate ... maybe there's a way to stimulate the individual microbe populations?