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Re: Potassium Sulfate

Gamera is wondering about the contents in Sulfate of Potash:

> In the past, I order K2SO4 from Homegrown Hydroponics.  It was a
> fine, white
> powder.  I just ran out of it recently, and went out looking for
> more.  I
> couldn’t find any at local garden centers (must be out of season), so
> I
> decided to order some online from Light Manufacturing Company (
> www.litemanu.com <http://www.litemanu.com/> ).  My father-in-law told
> me he
> would continue to look for the K2SO4 at stores while he is out and
> about.
> He is semi-retired and has much more time to search for this stuff. 
> Well,
> he found Sulfate of Potash (K2SO4) at a garden store a couple days
> ago and
> paid $5.49 for 5 lbs.  By this time, however, I had already ordered
> some
> Potassium Sulfate from Light Man. Co. for $5 for 1 lbs. Plus $8
> shipping and
> handling.  I know, it’s expensive, but I figured it would hold me
> over until
> my local garden centers started stocking the stuff for spring.  My
> question
> is regarding the appropriate color of K2SO4.  The Sulfate of Potash
> my
> father-in-law bought me is reddish-brown in color and very granular. 
> The
> only ingredients listed on the packaging are K2O (50%) and S (17%). 
> I just
> received the Potassium Sulfate from Light Man. today and it is the
> same kind
> of fine, white powder I had originally order from Homegrown
> Hydroponics.  It
> doesn’t list the element concentrations.  Light Man. shipped the
> powder in a
> plain Ziploc bag with “Potassium Sulfate” written on it in Magic
> Marker.
> The Ziploc bag was then wrapped in a brown paper bag and then shipped
> in a
> large envelope lined with bubble wrap.  Why is there a difference in
> the
> color and texture of the two products?  Is it normal for there to be
> differences between K2SO4 products?  Which one should I use?
> Gamera

This doesn't clear up much for Gamera.  I'm really just adding to the

The Light Manufacturing stuff is the compound chemical Potassium
Sulfate (K2SO4).  So far so good.  The other stuff could be the same,
less refined, or not.  That it gives a measurement for K2O doen't mean
that potassium oxide is in the bag.  So far, I think I'm still right.

I believe that fertilizers must be (or at least by convention are)
reported in terms of equivalent oxides of the chemical.  so the N-P-K 
ratings on fertilizer products are giving the equivalents by weight of
oxides of Nitrogen, phosphorous, potassium.  Seachem's Flourish
Potassium, by way of counterexample, reports the percentage of K not
the percentage by weight of an oxide.

So if a fertilizer says it's 10% K, it means that, if the amount of K
in the container were in the form of K2O, then the K20 would weigh one
tenth what the container contents actually weigh.

Can someone verify or correct me on this?  And if so, explain why this
apparent agricultural goofiness was started in the first place?

Scott H.

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