Magnesium sulfate, if you use LED's it's a nice supplement to add since LED's are known to cause Cal Mag issues due to cannabis needing certain light wavelengths for specific biological processes and if there not present, Cal/Mag with substitute, or STEP in so to speak to complete the task at hand. Anyway, I have older budget LED's, and I've seen it a few times. The plant doesn't skip a beat, but you are left with UGLY leaves(depending on when you catch the Deficiency, usually rust looking spots, or crispy leaves.. probably both). I never worry because it won't affect the end result, but IT is kind of annoying.