[APD] Why do we dose Magnesium?

["red kelvin" <veryredkelvin at hotmail_com>]

Hi,

Suppose that I have GH=3 - "soft water" - right?
Now if I multiply this by 17.8 then I get ~54 ppm.
If the calcium to magnesium ratio is a typical 1:4 then I could expect that 
I have 10 ppm magnesium. Right?
And this should be enough - correct?
So why dose MgSO4?

Thanks,

VRK.

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