Poor Man's Pfertz Dosing (PMPD)
By Dan Enright
By Dan Enright
This piece of writing is intended as an interactive thread as much as it is an article. It is intended to be an examination of the pfertz line from top to bottom, with an emphasis on accurate reproduction and the cost of nutrients. I intend to update it for accuracy, correct any technical issues as they arise, and partake in any dialogue on the subject. I hope this to be the first of a series of discussions deconstructing various fertilizer brands, and their implications within the planted tank.
As a brief disclaimer of sorts, I can definitely say that this is all being written for the DIY inclined. Whether pfertz, ADA, Seachem, etc. every company understands that there will be those, both hobbyist and competitor, looking to deconstruct and recreate their products themselves. Their products are for those who do not feel comfortable or are otherwise not inclined to mix their own fertilizers. I encourage those who do not feel that they can work responsibly with any of the compounds mentioned within this article to continue buying premixed fertilizers.
The Short Version:
This section is for anyone who wants Pfertz fast, no questions asked. What follows after is the longer explanation that allows for some of the technical details. Any of these numbers can be halved, quartered, doubled etc. to meet your needs.
The ingredients for each bottle are listed below; simply add the listed weight to half the volume of distilled deionized water, then top off to 1L and stir. It may take some time stirring (or some gentle heat; warm not boiling) to get things right. For pfertz [N] it may be advisable to get the urea and potassium nitrate as a solution in separate volumes of water, then combine and top off.
KNO3 (Potassium Nitrate): 61.27523782g/L
CH4N2O (Urea): 19.3193426g/L
Pfertz [M] is not covered here as it is far more difficult to reproduce. The reasons for this are covered later on.
On Pfertz and its Content Labeling:
Pfertz outlines its nutrient concentrations as a guaranteed analysis. By law these numbers have to be accurate. The derivatives for each of these nutrients are not required listing as far as I have been able to determine. At a glance, the analysis does not contradict the ingredients listed. This lack of contradiction does not negate the possible use of other compounds to achieve the same guaranteed analysis. I suspect as discussion develops, better insight as to regulations and labeling will surface.
At this time, everything I am writing is based on the assertions pfertz makes about its own product, from its own site. In the future there may be some sort of analysis done, but for now the scope is restricted to statements made by the company. None of what I am posting is more than information derived from what pfertz has been required, or chosen, to release about its own products.
The math behind this project was very interesting. I started with the presumption that all measurements had been given with correction for specific gravity. It turns out that the pfertz site did not make this consideration, and simply calculated the presence of their nutrients as if everything being added was the same density as water. Other deviations from the possibility of actually achieving the analysis given with the listed compounds are noted in the respective sections. For any of my own calculations, I am more than happy to share them. I would include all of them here, but it would result in an article several times this length.
I am starting here with the macronutrient analysis for the [N] [P] and [K] bottles. These will be the products most easily examined and reproduced. Mixing comparable micronutrients is a task far more difficult and expensive, one being on the scale of more commercial than private aspirations. This being an article for the hobbyist, I will leave micros as a topic of brand comparison and dosing.
It should also be noted that there was incongruence between the listed K+ on the site and the K+ that is forcibly added through the above levels. This issue with K+ shows up in every product involving potassium as its base, aside from pfertz [K]. These contradictions sometimes blatantly violate what is possible given the atomic weight of the compounds listed. As such, the topic of K+ is a bit hit and miss. It should not effect any other outcomes outside of the KNO3:urea ratio in pfertz [N], however it does leave a good number of questions unanswered. Hopefully some of it can be answered in the following thread discussion.
For all of the information on the contents of pfertz, the ingredient labels can be found at: http://www.pfertz.com/analysis.html
Pfertz [N] has a label for its product listed as follows:
Nitrogen [n] (500 mL and refills): 1.75 - 0 - 5.0
Total Nitrogen (N): 1.75%
Available Phosphate (P2O5): 0.00%
Soluble Potash (K2O): 5.00%
Derived from: Potassium Nitrate, Urea
While the nitrogen is most accurately given as N, by the time the product is broken down, a more accurate and hobbyist friendly analysis would be:
Total Nitrogen: 1.75%
(N) KNO3: 0.848899026%
(N) CH4N2O: 0.901100974%
Total N Expressed As NO3: 7.746907734
From the above, these numbers were derived as concentrations for:
KNO3 (Potassium Nitrate): 61.27523782g/L
CH4N2O (Urea): 41.50563694g/L
Mixing up a batch of your own may be a little tricky on this one. I am not sure as to the compatibility of KNO3 and urea in stock solutions. A good method would be to dissolve both thoroughly in separate measures of DI H2O, and then combine them. Hearing from someone with urea around that can test the stability of this compound would be helpful. Alternatively, the same dosing level of N can be reached through a weight of 126.3185171g/L KNO3.
The phosphate portion of pfertz dosing was significantly easier to figure out than the [N] product. While the derived K+ from necessary KH2PO4 was off according to both my own and APC’s Fertilator calculations, it will not effect the conclusions I have reached about the required KH2PO4.
The given analysis on the pfertz site for this product is:
Phosphorus [p] (500 mL and refills) 0 - 0.35 - 0.25
Total Nitrogen (N): 0.00%
Available Phosphate (P2O5): 0.35%
Soluble Potash (K2O): 0.25%
Derived from: Monopotassium Phosphate
In a more helpful format, it would be listed as:
Total Nitrogen (N): 0.00%
The necessary KH2PO4 to achieve the given level of PO4 would be 19.17446678g/L. Mixing this product would be as simple as adding the compound to a little DI H2O and topping off to 1L. Naturally this level can be adjusted for batch sizes and rounded as scale accuracy permits.
This product was by far the easiest to work with of the pfertz line. Plain K2SO4, and a K2O listing of 5.1% (4.233752% K+) made it all straight forward. After dealing with the last two, it really made me wonder whether the K+ was listed correctly, but the given information is all we have to work with. The necessary level of K2SO4 works out to 42.337523/L, and mixing it can be done precisely the same way as mentioned in the phosphate section.
So what sort of nutrients does this mean for your tank? The pfertz site doesn’t really tell you what sort of nutrient levels are achieved through their method of dosing. Their analysis also leaves confusion as to what levels of K+ are achieved. As such, both implied levels (DIY and true pfertz) of potassium will be shown, as well as the rest of the results of dosing the pfertz line.
Given the instructions for the product, we will be looking at a high tech dosing routine at 7x a week, 1 pump per 10 gallons per product at 1.2ml/pump. The extra pump per day will be presumed to be as part of the 7 day a week schedule as the 7th day, allowing for a 6 day dosing plan with the typical resting day before water change.
In accordance with the above schedule, and nutrients based off both the site and my own calculations:
Pfertz Line K+: 16.34168352ppm
PMPD K+: 17.79787ppm
So despite some small deviations in K+ and N source ratios, the PMPD macro solutions should be very reasonably close to the brand version.
Nutrient-wise, these levels of dosing aren’t too bad. These levels are comparable to a lot of current dosing recommendations, and are not likely to become limiting too quickly. Pfertz macronutrients have shown to work in a number of planted tanks, and the nutrients reflect as such.
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