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Hi
I have done experiments with Ca and Mg levels and ratios. Generally plants don’t care about ratios as long as there are some levels present. Ca is immobile element and therefore must be present constantly.
Not the same can be said about mobile Mg. Mg can be dosed in very small quantities and still be sufficient. Additionally, higher levels of Mg harm some plant species, example is Rotala Wallichii.

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Edward
 

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Hi All

The Rotala wallichii been growing well at 20 ppm Ca and 2 ppm Mg. However, it stops growing when Mg changes to 10 ppm. When conditions change back to 2 ppm Mg the plant starts growing again. Please see the picture.



This test I have repeated several times. Please take in consideration that the Mg is from MgSO4, not MgCO3. The Ca is from CaSO4 and CaCl2, not CaCO3 nor CaNO3.

Thank you
Edward
 

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Hi Dennis
let's not confuse Total Dissolved Solids TDS with concentration in ppm.

A TDS meter measures electrical conductivity or resistance of a solution, whichever you understand better.

1 / resistance = conductivity
1 / conductivity = resistance

Pure water = low conductivity and high resistance.
Impure water = high conductivity and low resistance

Resistance in Ohm
Conductivity in Siemens

We are still talking electricity here. So why is your tester calibrated in ppm? This is where the confusion comes from. TDS testers calibrated in ppm instead of Siemens. Someone had this idea to use NaCl or kitchen salt solution to display in ppm. It may work better for some applications but not for aquariums and water works.

How to get out of this mess? We can get approximate result back to conductivity value:
uS = 2 x ppm
ppm = uS / 2

MicroSiemens [uS] is the right way to talk about water TDS, however, every element conducts electricity in a different rate so there is no way to tell what elements are in the solution.

Edward
 

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Elkmore,

Consistency and Time.

Changing NO3 from 30 ppm to 20 ppm doesn’t affect plants. But changing Mg and Ca does. You need to maintain stable levels for longer period of time and allow plants to adjust to the new conditions.
It is easy for Ca. Dose once to 20 ppm and re check after a month or so. Mg needs to be dosed daily in rather smaller quantities not allowing it to accumulate. Plants will grow well even if Mg reading is at zero.


Edward
 
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