Lee Poulsen wpoulsen@pacbell.net
Wed, 07 May 2008 16:01:21 PDT
I think we had this discussion once before quite a while ago. Once 
again, the U.S. seems to be sticking to a standard that is completely 
different from (and less logical than) what the rest of the world uses. 
The Australians clued me into this by pointing out that our 3-number 
description of common fertilizer strengths seemed oddly out of 
proportion to what they were accustomed to. Then I recalled that the 
label on my bag of Apex time-release fertilizer always had a split 
label, kind of like a bilingual label, only both sides were in English. 
One side is the typical American style numbers, for example: 12-6-12. 
The other side is subtitled "Elemental and Metric" and the three numbers 
are: 12-2.5-9.9.

Down near the bottom in the "Guaranteed Analysis" sections, on the 
American side it says,
Total Nitrogen (N)...........12.00%
Available Phosphate (P2O5)....6.00%
Soluble Potash (K2O).........12.00%

On the "metric" side it says,
Total Nitrogen (N)...........12.00%
Total Phosphorus (P)..........2.50%
Total Potassium (K)...........9.90%

And as you noted, the sources of these three are:
Urea, Ammonium phosphate, and Potassium nitrate, and there is no K2O.

I don't know any reasons other than historical for the U.S. way of doing 
this. And I can't see any reason why we continue using it when no one 
else does, and we, even in the U.S., want to know what the N-P-K 
analysis is, not the N-P2O5-K2O analysis (which is what we get in this 

--Lee Poulsen
Pasadena, California, USDA Zone 10a

Jim McKenney wrote:
> In addition to that, there is this. I did a little bit of homework before I
> went potassium shopping. I discovered what to me is a bizarre anomaly:
> retail gardening products are sold on the basis of potassium oxide
> percentage. What makes that bizarre to me is that such products do not
> contain potassium oxide (or do they?). Is the idea that you are buying
> something which is the equivalent of a raw potassium source which would
> yield so much potassium oxide upon burning? Curiously (to my sensibilities
> at least) the potassium oxide "content" on the retail products is in big
> print; you have to read the fine print to find out what the real source of
> the potassium is.    
> Help me out here!
> Jim McKenney

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