Tomato Fertilizer and NPK ratios

Lee Poulsen
Mon, 07 Nov 2005 20:25:45 PST
On Nov 7, 2005, at 5:07 PM, Jim Lykos wrote:
> Hi Mary Sue,
> One aspect of NPK ratios that can be overlooked is the difference  
> in calculating fertilizer amounts based on the country of origin,  
> as   NPK ratios are not internationally compatible. I understand  
> that in the USA, the NPK ratios are calculated by comparing  the  
> percentage of N% against Phosphorus oxide% and Potassium oxide%  ie  
> the minerals of N : P2O5 : KO2  in a fertilizer.  However in  
> Australia and presumably other nations we calculate the NPK ratios  
> by the relative amounts of the elements of N, P and K  that the  
> fertilizer contains and not by the oxides of P and K.
> Hence there is a difference in that the NPK ratios used in the  
> USA,  appear to us (in other countries) to have quite inflated P  
> and K ratios and obviously if someone in the USA picks up a  
> recommendation for using a NPK ratio from a member in another  
> country they will  use the wrong  fertilizer unless they make the  
> necessary calculations for a conversion to the USA system.
> Cheers
> Jim Lykos
> Blue Mountains Australia

I can confirm what Jim wrote. Ever since an extended conversation,  
probably back in the old IBS list days, about controlled release  
(timed or slow release) fertilizers and which ones were actually  
dangerous under the wrong conditions, after finding a local source of  
the most highly recommended one, I've been using Apex brand. It comes  
in an almost bewildering variety of formulations, some with minors  
included, and under standard temperature conditions (70°F) different  
ones last for anywhere from 3 months to 12 months or more. [Each ten  
degrees F colder or warmer makes it last one month longer or shorter  
respectively.] Also it comes in 50 lb bags and the price can't be  
beat compared to anything else I've found (US$ 35). Anyway, the label  
is "bilingual" written for the USA on one half and Australia on the  
other half. The first thing I noticed (since it is printed in huge  
font at the top of both halves of the label) is the NPK ratio.

I chose Apex Tree & Shrub 6-month formulation with an NPK ratio of  
21-5-12/21-2.1-9.9 since I heard that in particular many Australian  
plants and some South African plants do not like, and can be hurt by,  
high amounts of phosphorus. On the Australian side of the label, it  
also says underneath the ratio: "elemental and metric". So once again  
it seems we Americans have to do it a different, and less accurate,  
way than the entire rest of the world. In the analysis section on the  
USA side it says: Total nitrogen (N) is 21.00%, Available phosphate  
(P2O5) is 5.00%, Soluble potash (K2O) is 12.00%. On the Australian/ 
metric side of the label is says: Total nitrogen (N) is 21.00%, Total  
phosphorus (P) is 2.10%, Total potassium (K) is 9.90%.

So there you have it. Thanks, Jim.

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

More information about the pbs mailing list