(sorry, title should be deducing Magnesium levels from GH)
I'm trying to figure out if might have a calcium deficiency problem (curling/cupping in new growth, mostly on hygros).
I don't have any tests kits for Magnesium or Calcium, but I do have our tap water analysis, which gives me:
Can I therefore deduce that if the calcium is 102, then the other 350ppm must be magnesium?
That would therefore put me in a 3-4: 1 ratio which could be causing my plants to not be able to absorb calcium efficiently?
You can make a rough estimation of the magnesium content. However, all divalent metals add to GH. Generally, those other elements contribute very little to GH and are rather insignificant. This is why a "ballpark" magnesium level can be obtained. Unfortunately, there are a couple other things you need to understand.
The numbers listed in your water report are different measures than we use in our tanks. The 102ppm of Calcium for example is a "calcium carbonate equivalent".
When hobbyists say add 10ppm of calcium they are referring to elemental calcium. Unfortunately, general harness is typically reported in CaCO3 equivalents or more specifically how much CaCO3 it would take to administer the same amount of elemental calcium.
Let's look at the weights of CaCO3 and Ca.
CaCO3 = 100.0869 g/mol
Ca = 40.0780 g/mol
To calculate a CaCO3 equivalent we need the ratio from the two. We also do the same with magnesium, iron, manganese...
100.0869 / 40.0780 = 2.497302759618743
Now we can calculate the CaCO3 equivalent using the following formula,
CaCO3 Equiv. = 2.497 * Ca
So let's say we have 15 ppm of Ca...
CaCO3 Equiv. = 2.497 * 15
CaCO3 Equiv. =37.455
Let's add magnesium to the equation,
CaCO3 = 100.0869
Mg = 24.30500
100.0869 / 24.30500 = 4.118
CaCO3 Equiv. = (2.497 * Ca) + (4.118 * Mg)
So let's use 15ppm of Ca and 40ppm of Mg...
CaCO3 Equiv. = (2.497 * 15) + (4.118 * 40)
CaCO3 Equiv. = 203ppm
Now that you see how to calculate CaCO3 equivalents algebra solves your problem,
You have a total harness of 450 and Ca of 102, both being reported as CaCO3 equvalents...
450 = 102 + (4.118 * X)
450 = 102 + 4.118x
4.118x + 102 = 450
4.118x + 102 − 102 = 450 − 102
4.118x = 348
4.118X / 4.118 = 348 / 4.118
X = 84.51
So you have an elemental Magnesium level of no more than 84.51ppm. The calcium is about 40ppm of elemental calcium.
Sorry, I know that's a lot of information to digest but one really does need to understand the basics when making these types of calculations. Does any of this make sense?
The ratio of Ca to Mg is fine. To see antagonistic events you would need vastly different ratios.