The calcium is in the product, because it aids the uptake of magnesium, at least in some forms, I think. The magnesium products without calcium may be a slightly different type of magnesium. I don't know for sure; that's just a guess. That's usually why calcium is in any particular fertilizer. I doesn't really get absorbed by the plant itself, like other nutrients, or at least the manner in which it does is complex and enigmatic. For example, blossom end rot is displayed as a calcium deficiency, but giving the plant calcium doesn't solve the problem, because the uptake of the nutrient is very difficult.
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