Sulphate of Potash

Hans-Werner Hammen
Thu, 15 May 2008 11:06:33 PDT
A high grade fertilizer should not contain "Muriate of Potash" (KCl) and Chlorides at all, respectively except as the counter Anion for some cationic trace elements.

I agree with John Harrison that plants should be nourished exclusively with well balanced complete formula fertilizers.

And, I agree with Veronica Read ("Hippeastrum -The Gardeners' Amaryllis) that a very good formula (for knight star lilies) if not the perfect (for bulbous plants in general) is something like 14-10-27 
that is a relation of N(total of N from Nitrate and Ammonium) : P(Phosphorous as P2O5) : K (as K2O) 
like 1.4 : 1 : 2.7

When I examined a yet older list of established formulas provided for various commercial crop on hydroponic cultivation / cultivation on rockwool…
(from Byk & Sonneveld 1981)

I calculated the following relation of nutrients out of the mean fertilizer ion supplies:

c(N) : c(P2O5) : c(K2O): c(MgO) : c(CaO) : c(S) =
150 : 90 : 268 : 34 : 164 : 34 each in mg/L

and related to phosphorus
m(N) : m(P2O5) : m(K2O):m(MgO) : m(CaO) : m(S)=
1.7 : 1 : 3.0 : 0.4 : 1.8 : 0.4

You see 1.7 : 1 : 3.0 this is close to the suggestion of Veronica :)

Without consideration of trace elements you can easily compose a complete formula from only 4 in words FOUR different salts. This formula is meant to be fully water soluble and therefore cannot contain Calcium salts (these would precipitate with phosphate when the stock solution is prepared. Calcium ions are assumed to be supplied in sufficient amounts with the tap water):
Mix thoroughly 353 g KNO3 (Potassium Nitrate) 170 g KH2PO4 (Potassium Dihydrogenphosphate), 240 g NH4NO3 (Ammonium Nitrate) und 185 g MgSO4x7H2O (Bittersalt=Epsomsalt) and obtain 948 grams of a complete fertilizer formula of
N:P:K[:Mg:S] (more exactly: N : P2O5 : K2O [: MgO : S]) like
14 : 9 : 24 [: 3 : 3] that is, again very close to Veronica's formula AND additionally Magnesium is provided too. Note that tap water is often too low in Magnesium. Furthermore, a fertilizer which is adequately rich in potassium must provide Magnesium too because potassium can effectuate a Magnesium deficiency because of competition of K+ and Mg++ ions for uptake by the plant.

Note that sulphate of potash is NOT included in this formula :) :p

Happy gardening, and mixing your complete formulas (with moderate phosphorus' contents which diametrally oppose certain transatlantic miraculous growth-formulas with astronomic middle numbers) *giggling*

> From: Judy Glattstein 
> Date: May 14, 2008 11:58:45 AM PDT

> Just from curiosity I e-mailed John Harrison at Espoma, asking about
> sulphate of potash. Here is his reply.

> Thank you for your mail.
> Bulbs like all plants are best fed with a complete plant food rather
> than a single ingredient. We produce a product called Bulb-tone that
> will provide all major, minor, and trace nutrients required. A fact
> sheet is attached.
> Sulfate of potash (SOP) is a mined mineral with an analysis of 0-0-50.
> It is generally considered an acceptable input for organic gardening and
> agriculture. It is superior to the muriate because it is lower in salts
> that can harm plants (particularly tender emerging roots of bulbs).
> We no longer produce the SOP as a single ingredient. We do use the
> material as a potassium source in our blended plant foods. We do have
> the muriate, but plan to discontinue that as well.

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