Ca and Mg

[megladon]

Hello,
Im struggling to find any info on ratios/quantities of calcium and magnesium required by plants generally. I use RO and use CaSO4 and MgSO4 for GH buffers, using test kits have worked out a combination to make 4dgh but would like to know if theres a more benefitial ratio rather than the random mix i have made.
Any info would be appriciated.
Many thanks

 

[dw1305]

Hi all,

would like to know if theres a more benefitial ratio rather than the random mix i have made.

You can ignore ratio, unless you have a huge excess of calcium (Ca++) ions. Freshwater often has about 3:1 Ca:Mg, but the total amounts of the elements vary through several orders of magnitude.

If you wanted a more scientific approach, you can't really use test kits to work out when you've got to 4dGH, but you can use the RMM of the compounds, and percentage of each element (via their RAM, 24.3 for Mg, 40.1 for Ca), to work it out.

You also need to take into account the "water of crystallisation" of each of the compounds. When you use "Epsom Salts" it will be the heptahydrate, MgSO4.7H2O and ~10% Mg, whatever it says on the label. The calcium sulphate is the dihydrate, ~ CaSO4.2H2O

Also dGH is a bit of a strange measurement , it is defined in terms of calcium oxide (CaO) as : "1dGH = 10 mg litre-1 CaO". I won't put the maths in, but 7.14 mg litre-1 of Ca++ ions is equal to 1 dGH. Even more strangely magnesium is also expressed in terms of CaO, in this case "4.33 mg litre-1 of Mg++ ions = 1dGH".

Personally I'm not going to weigh out the right amount of each compound on a scientific balance, I'm going to use the figures at the excellent <"James' Planted Tank">.

So if you mixed 2g of CaSO4.2H2O and 2g of MgOS4.7H2O for every ~25 litres of RO water, you should be about right for 4dGH, and if you added a teaspoon of each that would weigh about 6g and would treat 75 litres of water.

cheers Darrel

 

[ceg4048]

Im struggling to find any info on ratios/quantities of calcium and magnesium required by plants generally.

Hello,
For all intents and purposes, ratios are irrelevant. Even if you could measure the ratios in the water column - which you really can't - it would mean nothing because what is in the water column and substrate is not related to what is in the plant tissue.

Therefore, the only thing we have to worry about is whether there is a deficiency in any nutrient, NOT whether there is "X" amount of one nutrient versus "Y" amount of another.

Have a look at the online version of the water report for your area and scan the tables to see what the Ca and Mg content for the region is. If the values are anything higher than zero then you do not need to worry or to add any more.

Ca and Mg are micronutrients and plants only need a very small amounts of each. If your water report does not clearly show the contents or if you are using RO then any remineralizing powder can be used to add these, or you can use the number listed above in Darrel's post.

In any case, your random mix is fine and I would not consider it a struggle at all. Just use what you have and forget about it. There are more important things to struggle over than Ca/Mg.

Cheers,