# Basis for Ca:Mg Ratios



## cS (Jan 27, 2004)

Would someone please provide me with the rationale behind having a Ca:Mg ratio of 3:1 and/or 4:1?

My tap water is <1dGH/dKH with approximately the same [Ca] & [Mg]. I increase hardness via CaCO3 and MgSO4.7H2O. Since mid March, I have been experimenting with various ratios of Ca:Mg (1:1, 3:1, 4:1, 1:3, 1:4) and I have not noticed anything peculiar. Therefore, I wonder if anyone has experienced anything negative that s/he can reasonably attribute to a deviation from the 3:1/4:1 Ca:Mg? Please share.

Last week, I started adding Ca & Mg in a 1:7 ratio just to see what happen so I am anxious to see how it'll turn out in the next few weeks.

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Also, folks in areas with hard water, would you please consult your water report and see what the individual [Ca] and [Mg] are? I would love to know.

Thanks a lot.


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## plantbrain (Jan 23, 2004)

I have not found a ratio to be critical with Ca and Mg.
Generally tap water is high in Ca and the ratio is 10:1 or greater in some cases.
As long as the plants have enough nutrients, the ratios are not an issue. They become issues when the levels get too low to supply the plants with enough of that nutrient. 

The ratios of and in themselves are non critical. 
But you'll waste less by using a ratio

You can try this independently easily.
Use CaCl2 and MgSO4 and then for KH use baking soda
You can easily, quickly and cheaply achieve any KH or GH you want.

Regards 
Tom Barr


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## Roger Miller (Jun 19, 2004)

I'm not sure where those ratios came from but I think the rationale is that the Ca:Mg ratio in freshwater is usually about 3 or 4, depending on who you talk to. In fact, the natural ratio is pretty variable.

If I were to pick one composition to use as "average fresh water" then it would be the world average river water. That is the average composition of freshwater entering the oceans. The Ca:Mg ratio in average river water is 3.66. By contrast, up until a few years ago my tap water had a ratio of 14 and now it's running about 5.

A few months back I pulled together some water analyses for a talk. In those analyses some of the Ca:Mg ratios were:

3.25 headwater mountain stream
2.68 Lake Malawi
2.09 Lake Victoria
1.59 black water stream in Georgia
0.32 Lake Tanganyika
0.31 sea water

I have no reason to believe that plants are particularly sensitive to the ratio. The best guidance I've read in hydroponics literature is that the magnesium concentration should not exceed the calcium concentration. That is the guidance I pass around. The guidance is based on the belief that when the ratio gets too far out of proportion then magnesium can block the uptake of calcium. Similar guidance applies to potassium:calcium ratios.

I've been supplimenting magnesium in my tanks for years now. If I don't suppliment it then I have deficiency problems. I don't think the Ca:Mg ratio has anything to do with the problem. It's more likely because without supplimentation the tanks get less than 1 ppm of magnesium.


Roger Miller


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## MarcinB (Apr 16, 2004)

My tap water contains about 100pm of Ca and 8-9ppm of Mg (analysis was made in a professional lab) so the Ca:Mg ratio is very high, about 12:1. IMO such a high Ca:Mg ratio can induce Mg deficiency in some plants. I used to change water to a mix of a tap and distilled water in a ratio of 3:1. So the concentration of Mg should be around 6ppm. I also dosed 1,5ppm of Mg weekly from my trace mix, so the water should contain enough Mg for plants. At that time I had some problems with Ludwigia repens and cabomba. Addition of MgSO4 solved those problems quick and completely.


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