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Calling Chemistry and Math Pros

Discussion in 'Aquatic Plant Fertilization' started by csmith, Jun 18, 2010.

  1. csmith

    csmith Guest

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    I'm in need of assistance with reverse math. To follow is my equation used to figure liquid dosing. My PO4 level is a bit low (about 2 ppm) so for my next batch I'm upping it a bit. I'm in need of assistance with the reverse math of this.


    Equation:
    [(atomic mass of compound / mass of desired nutrient) * desired ppm] / (x) mL dosed for each 1 L of water column * 1000 (to convert the liter bottle to mL) / 1000 (to convert mg to grams)

    K-H2-P-O4
    K - 39.0983
    H - 1.00794 *2
    P - 30.973762
    O - 15.9994 *4
    0.5 mL dosed for every L of water column
    Desired ppm of PO4 is 4

    So...

    {[39.0983 + (1.00794 * 2) + 30.973762 + (15.994 * 4)] / [30.973762 + (15.9994 * 4)] * 4} / 0.5 (removed the *1000 / 1000 for obvious reasons)=
    {[39.0983 + 2.01588 + 30.973762 + 63.976] / [30.973762 + 63.976] * 4} / 0.5 =
    [(136.063942 / 94.949762) * 4] / 0.5 =
    11.46407861454145

    So roughly 11.46 grams of KH2PO4 gives me 4 ppms of PO4 per 0.5 mL dosed in each L of water column from a 1000 mL stock. I've got that part. What I need to know is how to figure what 11.46 grams of KH2PO4 gives me as it concerns K. The following equation is as far as I can get. Desired ppm replaced with an (x) as it's now the variable. I've also changed the mass of desired nutrient to that of K.

    {[39.0983 + (1.00794 * 2) + 30.973762 + (15.994 * 4)] / [39.0983] * (x)} / 0.5 = 11.46407861454145

    Can anyone help me out here? I'm lost.
     
  2. Darkblade48

    Darkblade48 Guru Class Expert

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    Do you not simply solve for x?

    For each 0.5 mL of your KH2PO4 stock solution that you add to 1 L of water, you will be increasing your potassium levels by 1.64 ppm.

    By the way, why do you bother with how much potassium you are adding to your water column? Are you just ensuring that it is not lacking?
     
    #2 Darkblade48, Jun 19, 2010
    Last edited by a moderator: Jun 19, 2010
  3. csmith

    csmith Guest

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    You do solve for (x) I presume, I'm just not sure how to do it. Two answers I came up with were completely different and didn't make any sense to me. I want to figure how much K is in my KH2PO4 so I know how much K2SO4 to add as well.
     
  4. Tom Barr

    Tom Barr Founder
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    [​IMG]

    Note range C?
    See how wide it is? Vs say A or B ranges?

    This is a huge target, anywhere from 5-10ppm to 100ppm or more for K+.

    While it might be interesting for some reason, the question will not help much in terms of management, most assume that the K+ from K2HPO4 is neglieble.
    Quite a few folks simply use GH booster, which is about 50% K2SO4.

    You get enough K from KNO3 anyway.
    Lots of math and pain for not much gain.

    Regards,
    Tom Barr
     
  5. csmith

    csmith Guest

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    I didn't know those things. So it would appear if your NO3 and PO4 are spot on then by default you're almost guaranteed of good K.
     
  6. Darkblade48

    Darkblade48 Guru Class Expert

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    Are you following your order of calculations correctly? :D

    {[39.0983 + (1.00794 * 2) + 30.973762 + (15.994 * 4)] / [39.0983] * (x)} / 0.5 = 11.46407861454145

    Multiply both sides by 0.5

    {[39.0983 + (1.00794 * 2) + 30.973762 + (15.994 * 4)] / [39.0983] * (x)} = 5.73 (I'm cutting down the number of digits)

    Simplify the L.H.

    139x/39 = 5.73

    Solve for x

    x = 1.64

    Pretty much. Some people want to be extra sure there is no potassium deficiency, so they dose K2SO4 to compensate.
     
  7. Tom Barr

    Tom Barr Founder
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    Yep

    Regards,
    Tom Barr
     
  8. Tug

    Tug Lifetime Charter Member
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    I was waiting for you to get some replies from people that know how to answer your math question. I admire you for trying to understand it all. I finally gave in to the available calculators I am finding on the Web. Here are just a few, but don't let this stop you from learning to do it the hard way. ;)

    http://www.theplantedtank.co.uk/calculator.htm
    http://wet.biggiantnerds.com/fe_calc.pl
    http://www.fishfriend.com/fertfriend.html
    http://reef.diesyst.com/chemcalc/chemcalc.html
    http://wet.biggiantnerds.com/ei/con_v_time.pl
     
  9. csmith

    csmith Guest

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    Everybody pick on the guy with the Tennessee public school education. :p Thanks for the links Tug. I tried fertilator but it didn't really have what I needed. I know you don't have to know this stuff, but I kind of like the learning process involved.
     
    #9 csmith, Jun 19, 2010
    Last edited: Jun 19, 2010
  10. Tug

    Tug Lifetime Charter Member
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    My GED and $1.50 will get me a bus ride. I would love to know this stuff too. Until then, I cheat. As an old friend from my Army days would say, "If you want something bad enough you are willing to cheat."

    Here is a neat little trick to fool Wet's Fe calculator if you are not dry dosing and use stock solutions,
     
  11. aquabillpers

    aquabillpers Lifetime Charter Member
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    Please excuse this but Judas Priest! Has plant growing come to this?

    Maybe try the Walstad approach?



    Bill
     
  12. Tug

    Tug Lifetime Charter Member
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    What's good for the goose is good for the gander.
    :p
     
    #12 Tug, Jun 19, 2010
    Last edited by a moderator: Jun 19, 2010
  13. aquabillpers

    aquabillpers Lifetime Charter Member
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  14. Darkblade48

    Darkblade48 Guru Class Expert

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    Just to let everyone know, Fertilator is able to verify the original calculations, and that 1.64 ppm is the correct amount of potassium being added.
     
  15. Left C

    Left C Lifetime Members
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    It hope that it is OK to post this question in this thread.

    Does anyone have the Windows desktop version of Chuck Gadds fert dosing calculator that they can send me?
     
  16. Darkblade48

    Darkblade48 Guru Class Expert

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    Please PM me your e-mail!
     
  17. Left C

    Left C Lifetime Members
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    Thanks a bunch!!!!
     
  18. Tom Barr

    Tom Barr Founder
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    I'll repost an old post based on ratios for K+ and N(dating back to 1998-1999 or so):

    If you dose a fair amount of NO3 via KNO3, keep nice tank parameters, don't have much fish load, I'd suspect you don't need to add K separately.
    Steve and I did this for awhile, there were times when folks could not get any K2SO4 around the area, so we did without.

    We never really found much difference. Steve suggested we did not need any extra K as it was being supplied via KNO3.
    If you have 6.4 ppm of so of NO3, then you add 4 ppm of K using KNO3. Some products have K+, like TMG, KH2PO4, and GH booster.
    I find it unlikely you really need MORE K+ to grow plants well and ************it's one less thing that folks need to dose which is my goal here****.
    Looking at a mass balance of N/K+ ratios, __most/many__ plants have about 1.5 :1 ratios of N:K(Epstien 1972). Aquatics are pretty close to this also.
    Now take into account the O3 part of NO3 and to get elemental N and you suddenly find you have plenty of K+ for plant growth.
    So 1.5/4.4= .34 N's for every K+ you add.

    So you should theoretically have 3x as much K as you need relative to N x the 1.5/1 ratio = 4.5x as much K+ relative to N per dose.
    There are other sources of N, plant decomposition, fish waste but some K+ comes from these pools also.
    I really do doubt that adding K2SO4 to such a tank will make any significant difference. you'd have to have over 75% of the N coming from others sources(yes, it's possible, but not that likely)
    Unlike NO3 and denitrifying bacteria NO3=>N2 gas, there is not much to the K+ cycle except for plant uptake/leaching.

    So it certainly is something folks can skip if they add KNO3 as their main source of N.

    Now if you have a good fish load, over feed Discus, slow growth/non CO2 etc, adding K+ from KCl/K2SO4 is advisable since you already will have plenty of NO3.

    But for many, KNO3 as the source of both N and K should do the trick.
    I'd say you simply don't need but 3 things, KH2PO4, KNO3 and traces.
    And then the other two parts: Light and CO2.
    The goal is to reduce down with simpler designs/methods/dosing routines for folks that still work very well.

    Importantly:
    Just remember if you have NO3 issues from the tap/fish load, you will still find a use for K+ dosing from KCl or K2SO4.

    So while I did not address the math or the calculators(I do not use them), using ratios and indirect relationships with KNO3........I know my K+ will be at least well above a non limiting level. I also can say the upper bound is safe also, since Erik and myself went looking for upper K+ issues and low Ca++, and found none at either 60ppm(me) and 100+ppm for Erik for several months on specific pants that where suspects for sensitivity to K+/Ca issues(we both had large healthy groups of A gracilus).




    Regards,
    Tom Barr
     
  19. Tom Barr

    Tom Barr Founder
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    It gets the job done.

    Some wanna climb the North Face, others take the long easy stroll up to the top.

    Regards,
    Tom Barr
     
  20. Tug

    Tug Lifetime Charter Member
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    Maybe I am just Dyslexic.

    Burp!
    I have a small tank, damn well close to the heart of Walstad.
     
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