Originally Posted by redchigh
@ Byron: I thought Calcium and Mg were Micros, not Macros. Aren't NPK the Macros?
No. Macro nutrients are calcium, carbon, magnesium, nitrogen, oxygen, phosphorus, potassium and sulphur. These are used in the greatest quantity by plants and without them plants would not be able to grow, repair or maintain healthy tissue.
Micro nutrients are boron, chloride, copper, iron, nickel, manganese, molybdenum and zinc. These are required in extremely small quantities, usually referred to as trace elements. These are important to many functions and processes that sustain life and are vital for plant health.
I realize you (redchigh) understand the roles, but for the benefit of other readers like the OP I inclulded the brief descriptions.
I'm not sure which nutrients might be affected by too much calcium, but I would suggest that in most cases this limit is not too likely to be reached. Of course, staying with "hard water plants" in hard water [=more calcium and magnesium] will avoid the problems even more. And that reminds me, the Onion plant mentioned initially does like harder water, so it should do well here.