I’ve read many threads in which someone posts a photo of a plant and someone else diagnoses a calcium or magnesium deficiency. But I haven’t found anything – here or elsewhere – that tells me how much of those minerals should be provided.
The closest I’ve found is the table below, taken from this website.
The tabled ideal values for calcium and magnesium are given as percentages of the soil’s cation-exchange capacity (CEC), which few if any of us have the means to determine. The ideal ratio of calcium to magnesium ranges from 3:1 to 7:1.
The local water department tells me our water contains 70 ppm calcium and 30 ppm magnesium. I use water from a hose bib (to bypass our water softener, which removes both minerals) and use a pH-down product to reduce its pH from 7.9 to 6.5. The water’s EC is 0.4 (i.e., 200 ppm).
I use the Foxfarm nutrient trio and their Ocean Forest soil. They tell me that Big Bloom contains neither mineral. Grow Big contains 0.6% magnesium, Tiger Bloom contains 0.5%, and neither contains calcium. Ocean Forest contains 1% calcium and no magnesium.
So I know my water’s calcium/magnesium ratio is too low, but I’ve no idea how much of either mineral I should add, if any.
Does anyone know the appropriate amounts?
I feed 150 pms of cal mag when I’m not using Jacks, it has it in there. Use 250 pms to treat deficiency.
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What’s the cal/mag ratio in the product you use? Foxfarm’s version contains 3% cal and 0.9% mag (3.3 : 1 ).
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I have 2-0-0 and 4-0-0 both. Not sure on percentages .
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I wonder what happens if I say @Caligurl’s name 3 times? 
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A knot will magically appear on your head….
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Have fun reading this LOL… but then see the very end for my recommendations
Cation exchange capacity (CEC) is how you measure soil’s ability to hold and release various elements and compounds needed for plant nutrition. CEC has a lot to do with the nature of the various elements found in soil particles and the electrical charges they carry.
Summation of exchangeable bases and exchangeable acidity.
Exchangeable bases from cation determination:
K+ meq/100g = (ppm K) ÷ 390
Ca++ meq/100g = (ppm Ca) ÷ 200
Mg++ meq/100g = (ppm Mg) ÷ 120
Na+ meq/100g = (ppm Na) ÷ 230
milliequivalents per 100 grams (meq/100g) of soil
Example CEC Summation Calculation
120 ppm K ÷ 390 = 0.31 meq K/100g
975 ppm Ca ÷ 200 = 4.88 meq Ca/100g
210 ppm Mg ÷ 120 = 1.75 meq Mg/100g
(7.0 - 6.8 pHb) x 12 = 2.40 meq H/100g
CEC = 9.34 meq /100g
Relationship Between Soil Ca:Mg
Ratio and Crop Yield
Ca:Mg ratio range where greenhouse and field research has shown no detrimental yield effects
0.5 Ca : 1 Mg to 50 Ca : 1 Mg
AKA (2 times more Mg to 50 times more Ca) on a meq basis
Sooooo… ALL that being said (oh and there’s more to it than this… just use the recommended amount from the nutrient maker. It is not (really) possible to overdo Ca/Mg as long as you follow the instructions. If you still see deficiencies, use the “instructions for Heavy Feeding”.
I also encourage you to add Epsom. While it is also a source of Mg, it contains sulphur. Sulphur is one of the building blocks of the plant’s immune system. ane it works together with magnesium to increase the uptake of NPK.
Chlorophyll also requires sulphur.
When you are measuring ppm, you are measuring the OVERALL ppm of both input and output to help you determine if oyu are feeding enough or too much. It takes int consideration ALL of the starting ppm of the water and a combination of the nutrients added.
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I think that’s a Lostgirl hazard. In this case, I’m risking only a few scratches and play bites. 
I did. If you recall the Star Trek episode in which Scotty’s idea of fun was brushing up on his tech manuals instead of beaming down to the R&R planet like everyone else, that could be me. Except I’d smoke a joint while reading.
So if the water dept can tell me how much nitrogen and potassium are in our tap water, I can calculate its CEC. That would enable me to compute the desired ppm range for calcium and magnesium, using the table I posted above
But that would deny me the fun of calculating the amounts myself! 
(So I can choose a cal-mag product that best matches my numbers.)
What’s pHb, and why is nitrogen omitted from the sum?
That’s a much larger range than the table recommends, but I suppose we’d expect the range that has no detrimental effects to be wider than the recommended range.
How much per gallon?
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I would do 1 tablespoon per 5 litres of water
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Thanks! I think you missed this, though:
