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How much Potassium Sulphate to add to replace Potassium phosphate

dw1305

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Hi all,
how much potassium sulphate do i need to add to 500ml water to get the same amount of potassium i would get using potassium phosphate?
It would depend on which "potassium phosphate" salt you are using, unfortunately there are two options. Dipotassium phosphate (K2HPO4.xH2O) contains more potassium than monopotassium hydrogen phosphate (KH2PO4). There is a further complication in that dipotassium phosphate can have various numbers of "xH2O" <"water of crystallization"> attached.

Potassium sulphate (K2SO4) is a bit more straight forward. It has a Relative Molecular Mass (RMM) of 174.3 and potassium has a RAM of 39.1. The potassium percentage then becomes (39.1 * 2)/174.3 = 45% K

Because it has less "unknowns" I'll use KH2PO4, which is 28% potassium. The difference between them, in potassium content, would mean that 10g of monopotassium hydrogen phosphate (KH2PO4) would be equivalent to 6.22g of potassium sulphate (K2SO4).

cheers Darrel
 

MichaelJ

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@john6 If I understand this correctly you want to skip adding Phosphates entirely? also what Potassium levels are you targeting and exactly what potassium phosphate amount are you currently using?

K2SO4 packs a lot of K. If you target say 20 ppm you get about 8 ppm of S.

K2CO3 might also be a viable alternative if you don't mind adding to your KH (if your targeting say 20 ppm with K2CO3 you get about 1.5 KH).

Cheers,
Michael
 

john6

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I dont think i was specific enough.
I dont want to add Phosphate at the moment.
I use the dry ferts you get with the starter kit from APFUK and use their recipe for EI dosing, One of the salts is Potassium phosphate but i dont want any phosphates adding, so i got some potassium sulphate so how much potassium sulphate should i add to get the same amount of potassium i would get if i added potassium phosphate.
Hope this makes sense
 

MichaelJ

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One of the salts is Potassium phosphate but i dont want any phosphates adding, so i got some potassium sulphate so how much potassium sulphate should i add to get the same amount of potassium i would get if i added potassium phosphate.

Alright then:

I assume you are using monopotassium phosphate (KH2PO4) as opposed to Dipotassium phosphate (K2HPO4).

1 gram of KH2PO4 will give you 5.75 ppm of Potassium (K) in 50L

1 gram of K2SO4 will give you 8.97 ppm of Potassium in 50L


5.75/8.97 = 0.64 or 64% the amount.

So , in order words, if you are currently dosing say 2 grams of KH2PO4 to reach 11.5 ppm of K in 50L you would only need to add 0.64 * 2 = 1.28 grams of K2SO4 to reach the same amount of K.

Hope this helps!

Cheers,
Michael
 

Wookii

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Further to Michael’s post above, to relate that to your APFUK 500ml bottle and their dosing recipe - instead of the 1 tsp (6g) of KH2PO4, you would add around 2/3 tsp (4g) K2SO4. To be honest though, the APFUK recipe is a little light on K, so I would just add a whole teaspoon.
 

dw1305

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Hi all,
My Phosphates are very high
You can measure <"PO4--- reasonably easily">, but I wouldn't be too worried about <"what value you have">, plants will reduce its level. If you did want to remove phosphate from your mix and then decided that your plants were now deficient in phosphorus (P), it is <"highly mobile within the plant"> so they show pretty instant response when it becomes available again.
much potassium sulphate should i add to get the same amount of potassium i would get if i added potassium phosphate.
There isn't a simple answer at the moment, it depends on what compound your "potassium phosphate" is.
I assume you are using monopotassium phosphate (KH2PO4) as opposed to Dipotassium phosphate (K2HPO4).

1 gram of KH2PO4 will give you 5.75 ppm of Potassium (K) in 50L

1 gram of K2SO4 will give you 8.97 ppm of Potassium in 50L


5.75/8.97 = 0.64 or 64% the amount.

So , in order words, if you are currently dosing say 2 grams of KH2PO4 to reach 11.5 ppm of K in 50L you would only need to add 0.64 * 2 = 1.28 grams of K2SO4 to reach the same amount of K.
That will do.
instead of the 1 tsp (6g) of KH2PO4, you would add around 2/3 tsp (4g) K2SO4. To be honest though, the APFUK recipe is a little light on K, so I would just add a whole teaspoon.
and so would that.

cheers Darrel
 
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