The Planted Tank Forum - View Single Post - magnesium and potassium dosing
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post #2 of (permalink) Old 11-22-2004, 10:16 PM
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I think this is how chuck's calculator works.
We know 1mg in 1L is 1ppm. So if we dissolve 1g of KH2PO4 in 1L, we are mixing in 698mg of PO4 and 302mg of KH2, in 1000ml of water. Lets take only PO4, ie 698mg -> 1000ml => 698ppm. Now we take 1ml of that solution -> .698ppm and dissolve in say 10g of water -> 38L. Therefore, we now have 0.0183ppm of PO4 which is about 0.02ppm of PO4. Now we can do the same thing for KH2, and we get:
1g KH2PO4 in 1000ml => 302mgKH2 in 1000ml
Then, 1ml has 0.302ppm of KH2 which in 38L gives 0.0078ppm of PO4. Which is approximately equal to the amount of K in ppm got by dissolving 1ml from a stock solution of 1g in 1000ml of KNO3 (0.01ppm). Therefore I think you can treat KH2PO4 as about the same as KNO3 as far as adding K is concerned.
Anyway I think the formula simplifies to the following form:

ppm increase by adding 1ml of stock = (grams of compound dissolved)x(amount of substance in 1g of the compound) / (Tank volume in Ltrs)x(amount of water in stock solution in L)x1000

so assuming we mix 10g of k2so4 to 1L of water and add 1ml of the stock to 18g tank, plugging in:

(10g)x(449)/68x1000 = 0.066ppm of K.
The same in terms of SO4 gives:
(10g)x(551)/68x1000 = 0.08ppm of sulphate. Now to dose k2 to 20ppm, we also raise sulphate to 24ppm. Dont mean to hijack the thread but what effect does excess SO4 have? I noticed the leaves of some plants (L glandulosa), crumpled when I was dosing K ad got back to normal when I cut back dosing of k2so4.
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