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post #1 of 5 (permalink) Old 03-03-2014, 08:42 PM Thread Starter
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Commercial fert analysis?

If a fertilizer reports a percentage of an element isn't this by weight?

Example, Seachem Flourish Iron is 1% Fe derived from Ferrous Gluconate. Since 1ml of water weighs 1,000mg there should be 10mg of Ferrous Gluconate. The math does not work out. What am I missing?

If I want 0.5ppm of Fe for 1 liter of water this calculator reports 4.01mg of Ferrous Gluconate or 0.05ml of Flourish iron. I don't get it.
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post #2 of 5 (permalink) Old 03-04-2014, 12:57 AM
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Is the 1% w/v or w/w? That makes a difference.

You are assuming the density is 1L = 1 kg, but this is only true for water with no dissolved solutes.

Also, we don't know what purity of iron gluconate Seachem uses.

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post #3 of 5 (permalink) Old 03-04-2014, 02:35 PM Thread Starter
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Is the 1% w/v or w/w? That makes a difference.
What is w/v and w/w?

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Originally Posted by Darkblade48 View Post
You are assuming the density is 1L = 1 kg, but this is only true for water with no dissolved solutes.
Yes. It's my understanding that commercial liquid fertilizers analysis are based on this. Otherwise every analysis would be different and could not be computed accurately. Similar to the reporting of potassium and phosphorus. These percentages are always based on K2O and P2O5 not the actual chemical added. A conversion has to be done to accurately calculate these. This is where I think I am making the error.

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Also, we don't know what purity of iron gluconate Seachem uses.
The analysis tells us this. When reporting a percentage derived from a chemical the percentage is based on purity. So the assumption should be pure Fe Gluconate.

I'm wondering if the 1% in this case is NOT 1% Fe Gluconate but instead 1% elemental Fe. When calculating elemental quantities for K or P we have to convert the elemental percentage from the molar mass of whatever chemical is used to equal the percentage of K2O or P2O5. I am assuming we don't do this for everything else.

The math works out if I don't utilize the mass of Fe Gluconate. I just want to be sure this is the proper method to use for all commercial liquid fertilizers.

If the percentage is an elemental representation then the calculation to raise Fe 0.5ppm in 10G would be 1.89272ml. This seems accurate based on what Seachem's calculator suggests as well as a couple of others. However, Wets calculator suggests 1ml. Unfortunately, both Wet's and Seachem's calculators round the numbers so a precise comparison can't be made.
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post #4 of 5 (permalink) Old 03-05-2014, 02:50 AM
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What is w/v and w/w?
Mass concentration and mass fraction, respectively. I also forgot to include v/v (volume concentration).

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Originally Posted by Zorfox View Post
Yes. It's my understanding that commercial liquid fertilizers analysis are based on this. Otherwise every analysis would be different and could not be computed accurately. Similar to the reporting of potassium and phosphorus. These percentages are always based on K2O and P2O5 not the actual chemical added. A conversion has to be done to accurately calculate these. This is where I think I am making the error.
From my understanding, you mentioned that it is 1% Fe derived from iron gluconate. To me, this would imply that it is 1% elemental iron, derived from some concentration of iron gluconate (whether it be 12.5% or 25% or XYZ%). I think this is where there is some interesting math going on.


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When reporting a percentage derived from a chemical the percentage is based on purity. So the assumption should be pure Fe Gluconate.
Not necessarily. A final percentage can be reached irrespective of whatever the initial purity was. I can make a 10% solution of something whether the starting concentration is 50% or 99%.

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post #5 of 5 (permalink) Old 03-05-2014, 03:47 PM Thread Starter
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Thanks for your help DB.

I did some more research. It turns out that guaranteed analysis is reported in elemental percentages for commercial fertilizers regardless of the chemical used.

So in this example Flourish Iron is 1% elemental Fe.

Here is the article that explains all of this including the formulas to make the calculations. The calculations are actually much easier than what I had originally thought.

Here is the basic formula presented there.

Amount of fertilizer to make 1 volume of stock solution = Desired ppm x Dilution factor / % of element in fertilizer x C

C are the constants listed below. The dilution factor can be used for making solutions or omitted for direct fertilization.

Ounces per U.S. gallon 75
Pounds per U.S. gallon 1200
Grams per liter 10

Oddly enough this formula seems to work out for premixed solutions as well such as Flourish products etc.

Example, If we want 0.5ppm of Fe in one liter using Flourish Fe

0.5ppm / 1% * 10 = dose
0.5 / 10 = 0.05
dose = 0.05ml/liter

For 10 gallons (37.8544L * 0.05) that's 1.89ml

Now if I can figure out how to calculate oxidized portions such as NO3 versus total nitrogen or P vs PO4 it will make life much easier lol. It appears that you add the oxygen portion of the molar mass to either P or N. However, I’m not sure that works out all of the time.

Edit: Never mind the total nitrogen versus Nitrate question. It's simply the molecular weight differences. Sometimes I miss the obvious lol

Last edited by Zorfox; 03-05-2014 at 05:09 PM. Reason: addition
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