magnesium and potassium dosing - The Planted Tank Forum
Old 11-22-2004, 07:13 PM Thread Starter
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Join Date: Feb 2004
Posts: 148
magnesium and potassium dosing

Hi all,

right, i am trying to knock myself up some cheap ferts, as i dont like forking out for the commercial stuff (too many \$\$\$)

i think i understand the basics, having read some of chuck's site and downloaded his dosing calculator. i have also looked at Tom Barrs estimative index and will be dosing potassium nitrates and pottasium phosphates

but i have a couple of questions:

Am i right in thinking potash of sulphur is the same as pottasium sulphate? how much pottasium will my pottasium phosphate add? negligable or can i treat it like pottasium in potasium nitrate?

as for dosing magnesium, i am using a 7 trace element mix (Mg, Mn, Fe, B, Mo, Zn, Cu)... will this provide me with sufficient levels of Mg, or should i suppliemnt with epsom salts as per chucks calculator. if so, do i ignore the Mg in the trace mix?

kingborris is offline

Old 11-22-2004, 10:16 PM
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Join Date: Sep 2004
Location: Salt Lake City, Utah
Posts: 250
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.
baj is offline
Old 11-22-2004, 10:48 PM
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Join Date: Dec 2003
Location: USA
Posts: 1,740
Yes, Potash of Sulfur is Potassium Sulfate, K2SO4.

The amount of K that comes from Mono Potassium Phosphate, KH2PO4, is negligible. The added amount doesn't compare to the levels we usually dose. On the other hand the K from Potassium Nitrate can be sufficient, especially if you need to dose nitrates frequently. Some depend solely on KNO3 for K.

You may not even need to dose additional Mg from epsom salt. Only if your GH is very low, or you are observing Mg deficiency then it should be done.

Baj - Your problem doesn't seem to be sulfate or sulfur. Crumbled leaves is a classic sign of Ca deficiency. Providing you have enough Ca, your problem might be high K. It is believed that high K levels can limit the plant's ability to uptake Ca, but this is not a solid concept. There can be other factors.
Rolo is offline

Old 11-23-2004, 09:37 AM Thread Starter
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Join Date: Feb 2004
Posts: 148
thanks for the input guys. i have ordered the above now, and will digest the info while i wait for it to arrive.

cheers!
kingborris is offline

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