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Reduced solubility of the APFUK-recipe macro mix

Jack B

Jack B

Member
Joined
3 Jan 2020
Messages
180
Location
London
Hi all

My macro mix dissolves initially, but then comes out of solution after a day or two - and won't then re-dissolve, even if I warm the bottle in hot water and shake it

Not sure why this has started to happen: never used to be an issue. Seemed to begin when I moved on from the original APFUK bags, and bought new bags of Epsom salt and potassium nitrate. I'm 99% sure they are the right stuff. Bittersalz and Salt Petre?

Does anyone have any tips / ideas? Should I just add a bit of vinegar? I've not used added acid before.
 
Hi all,
Jack B said:
My macro mix dissolves initially, but then comes out of solution after a day or two - and won't then re-dissolve, even if I warm the bottle in hot water and shake it
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That sounds like an <"insoluble carbonate, or phosphate">, has formed.
Jack B said:
Bittersalz and Salt Petre?
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What does it say on the bags in terms of purity? The "Bittersalz" may have magnesium carbonate (MgCO3) present as an impurity. "Saltpetre" can be either KNO3 or NaNO3 or a mixture, but they are all monovalent ions and form soluble salts.
Jack B said:
Should I just add a bit of vinegar?
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Yes, it can't do any harm.

cheers Darrel
 
_Maq_ said:
If there are calcium chloride - perfectly soluble - and magnesium sulfate - also perfectly soluble - put together, calcium sulfate will precipitate.
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Hi @_Maq_ I am trying to decipher this one... Are you saying mixing MgSO4 and CaSO4 is a bad idea?

Cheers,
Michael
 
MichaelJ said:
Hi @_Maq_ I am trying to de hicipher this one... Are you saying mixing MgSO4 and CaSO4 is a bad idea?

Cheers,
Michael
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No, MgSO4 and CACL is a bad idea. Because calcium is more reactive than magnesium. It reacts with SO42- ions to form CaSO4 which is very sparingly soluble. So if you say have 30ppm Ca then you could not target that with CaSO4 it will simply not dissolve.
 
plantnoobdude said:
No, MgSO4 and CACL is a bad idea. Because calcium is more reactive than magnesium. It reacts with SO42- ions to form CaSO4 which is very sparingly soluble.
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Got it.

plantnoobdude said:
So if you say have 30ppm Ca then you could not target that with CaSO4 it will simply not dissolve.
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I have done that. The solubility limit of Calcium sulfate dihydrate (CaSO4.2H2O) is ~0.26 g/100ml at 25 C if I am not mistaken that limit correspond to a whopping 600 ppm of Ca... What am I missing?

Cheers,
Michael
 
_Maq_ said:
In the tank, solubility of calcium sulfate is sufficient. But stock solutions are by orders of magnitude more concentrated.
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Sure thing. I was aware of that. I was just responding to Noob's statement about the 30 ppm.... and thought I might have missed something.

Anyway, we often talk about solubility of various Calcium compounds.... and I completely agree it might be a concern when making stock solutions, but otherwise it's not. Even almost insoluble CaCO3 is actually quite soluble in slightly acidic water - increasing somewhere around two orders of magnitude per 1 pH drop from neutral (7 pH). I use CaCO3 to dial in my KH and partially of my GH.

Cheers,
Michael
 
There's no calcium in the macro mix:

  • KNO3
  • KH2PO4
  • MgSO4.7H2O

(I've checked the purity: these are definitely what I'm using)

But I have been using London tap water...so I'll make up a fresh batch of macro mix using dehumidifier water and see how that goes. Thanks all 👍
 
I would boil the humid water also, dehumidifiers can be a source of bactria fungi etc due to moist conditions. I quick boil will disinfect the water so less likely to get mould etc in your fert mix - esp if making batch to last some time. Best to make batches that are used up in 6-8weeks, but we can all be lazy at times. If making larger batch to last longer, split the bacth up in different bottles and just freeze them is best, or chill in fridge, at least store in cool dark place 😉
 
Zeus. said:
I would boil the humid water also, dehumidifiers can be a source of bactria fungi etc due to moist conditions. I quick boil will disinfect the water so less likely to get mould etc in your fert mix - esp if making batch to last some time. Best to make batches that are used up in 6-8weeks, but we can all be lazy at times. If making larger batch to last longer, split the bacth up in different bottles and just freeze them is best, or chill in fridge, at least store in cool dark place 😉
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I agree. I would not bother about trying to use waste water from a humidifier when you can buy a gallon of pure distilled water for a couple of bucks (quid) at the grocery store.

Cheers,
Michael
 
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