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I use a mineral based fertilizer in powder form which - according to the label - should not contain any fiber and is fully soluble in water (I dissolved 2 times the recommended dosage and found no solids remaining in the solution):

NPK fertilizer label

The NPK + Mg + trace elements add up to about 60%.

What constitutes the rest?

CuriousIndeed
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    Note that the fertilizer does not actually contain chemical compounds like $\ce{P2O5}$, $\ce{K2O}$, and $\ce{MgO}$. These are just the reference compounds for the calculation of results of the elemental analysis. –  Nov 02 '19 at 15:41
  • I'm not sure if I can follow. From what I have read so far if chemical compounds are listed after the percentage this does indeed indicate that the fertilizer contains these compounds in the indicated mass percentage. To calculate the percentage of one element one has to calculate the the percentage of that element in the compound using atomic weight. For example with K2O. K = 39u, O = 16u. Total atomic weight = 239 + 16. Percentage of potassium is 239 / (2*39 + 16) ~ 0.83 ~ 83% of K2O is potassium – CuriousIndeed Nov 02 '19 at 16:12
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    You got it wrong. The listed compounds are simply not there, as blunt as that. – Ivan Neretin Nov 02 '19 at 16:27
  • If the compounds are not there why do we have to calculate the elemental percentage in the compounds as described in the wikipedia article? – CuriousIndeed Nov 02 '19 at 16:36
  • E.g. K2O is very probably present as K2SO4. So 10% of K2O is equivalent to 30 . 172/94 = 54.9% K2SO4. Etc for the other components. By other words, there is so much potassium sulphate equivalent to 30% of potassium oxide. – Poutnik Nov 02 '19 at 16:51
  • I called the company a while ago and asked why there is no sulphur in their fertilizer. They told me that sulphur is not necessary for plants, although there is clearly evidence to the contrary. So I doubt that in this specific case K2SO4 is present.. – CuriousIndeed Nov 02 '19 at 17:01
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    @CuriousIndeed You still do not get it. The listed fertilizer composition is NOT it's chemical composition. It is list of selected content, expressed in equivalent amount of respective, historically chosen reference compounds, mostly oxides. It does not mean these oxides are there chemically present. – Poutnik Nov 02 '19 at 17:40
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    It means if nitrogen/phosphorus/potassium/magnesium compounds were chemically converted to N2/P2O5/K2O/MgO, the mass of N2/P2O5/K2O/MgO would be 10/20/30/2.6% of the mass of the fertilizer. – Poutnik Nov 02 '19 at 17:47
  • About K2SO4, sulphur is not mentioned, as it is not the major nutrition element, but is present in nearly any combined fertilizer. – Poutnik Nov 02 '19 at 18:12
  • @Poutnik How could I determine if sulfur is indeed present without sophisticated analysis? I'm really fond of this fertilizer but have discouraged / limited its use because of the supposed lack of sulfur.. – CuriousIndeed Nov 03 '19 at 13:25
  • You cannot. You can use a crystal ball or ask provider. How much sulphur is to be present to be the needed amount ? Sulphur is seldom a deficient element in plant nutrition. Plants like garlic and onions, containing explicitly large amount of sulphur compounds, often use specialized fertilizers. One of common fertilizers is ammonium sulphate. – Poutnik Nov 03 '19 at 14:03
  • At least in Europe sulfur deficiency seems to be a problem. "In particular, incidence of S deficiency has increasingly been reported in Brassica and cereal crops in Western Europe over the last decade, mainly as a consequence of a massive decrease of atmospheric S inputs." (https://www.sciencedirect.com/science/article/pii/S0733521098902417?via%3Dihub) and this was 20 years ago. However thanks for your information. I will probably order a lab analysis for this specific fertilizer. – CuriousIndeed Nov 03 '19 at 14:10
  • Also related is https://chemistry.stackexchange.com/questions/122230/what-is-the-mass-of-ozone-present-in-10-6-g-of-na2co3 in particular look at the end part of my answer. – Alchimista Nov 04 '19 at 08:47

2 Answers2

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There is a specific convention how the nitrogen, potassium and phosphate concentration is given, and it might be surprising because it mentions substances that are not in the fertilizer, namely nitrogen atoms, $\ce{P2O5}$, $\ce{K2O}$ and $\ce{MgO}$. You have to imagine that all of the nitrogen, phosphorous, potassium and magnesium in your sample reacts to form these species, and then you express the masses of those fictitious products as percentage of the original mass.

$$\text{fertilizer}\ce{-> x N + y P2O5 + z K2O + q MgO}$$

For an example, look at this answer: https://chemistry.stackexchange.com/a/64282 which links to this reference https://en.m.wikipedia.org/wiki/Labeling_of_fertilizer

Karsten
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  • According to the link on wikipedia, phosphorus and potassium percentages have to be adjusted according to their portion of mass in the chemical compound. However this is not true for nitrogen. What is the rationale behind this convention? – CuriousIndeed Nov 02 '19 at 16:24
  • @CuriousIndeed It’s elemental, my dear Watson ... well, atomic, actually. – Karsten Nov 02 '19 at 16:30
  • But it is listed as nitrate or ammonium..(Nitratstickstoff, Ammoniumstickstoff) – CuriousIndeed Nov 02 '19 at 16:32
  • @curiousindeed yes, of the nitrogen, some is in the form of nitrate and some as ammonium, but percentages are based on mass of nitrogen (adds up correctly by just adding percentages). – Karsten Nov 02 '19 at 17:18
  • Is there a specific reason for this convention? I find it pretty confusing... – CuriousIndeed Nov 02 '19 at 17:24
  • Historical reasons. More consistent would to express all as element content. But it would not give 100% either, as less relevant elements would be omitted as well as they are now. – Poutnik Nov 02 '19 at 17:54
  • @Poutnik So is it correct to say that because P2O5 and K2O are not present in the fertilizer but their mass percentages are only indicative of how much P or K are present, the remaining 40% of the mass percentage is constituted of the remaining elements in the compounds in which P and K are part of?? – CuriousIndeed Nov 03 '19 at 13:32
  • Yes, they are indicative of element content, expressed for historical reason as the element oxide. The missing mass are formed by compounds the listed elements take part in, or possibly by auxiliary content, improving handling the fertilizer. – Poutnik Nov 03 '19 at 13:56
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The rest is probably Oxygen in nitrates, Hydrogen in ammonium, and Sulfur in sulfates. Let us make some calculations !

In 1000 g fertilizer, there is : - 74 g N (or 74/14 = 5,28 mol N) as nitrate ion

  • 26 g N (or 26/14 = 1.85 mol N) as ammonium ion
  • 200 g P2O5, or 200/142 = 1.41 mol P2O5, or 2.82 mol P, or 2.82 mol PO4 ion.
  • 300 g K2O, or 300/94 = 3.13 mol K2O, or 6.26 mole K
  • 26 g MgO, or 26/24.3 = 1.07 mole MgO = 1.07 mol Mg.

There must be the same number of moles of positive ions as of negative ions. To start with negative ions, one must triple the number of phosphate and add the double of magnesium plus the number of moles of K. This total gives : 3·2.82 + 5.28 + 2·1.07 = 14.81 mol.

For positive ions, it is more difficult because some H ions are added to PO4^3- ions. Phosphate ions are too much basic to exist in the ground. In neutral mixture, it must be transformed into a mixture of H2PO4^- and HPO4^2-. So, to get the sum of positive ions, one must add 1.85 mol NH4, 6.26 mol K, 2·1.07 mol Mg, and about 1.5 times the number of P for obtaining the number of H atoms bound to PO4 ion. This makes : 1.85 + 6.26 + 2.14 + 1.5·2.82 = 14.48 mol.

The agreement between positive (14.48 mol) and negative (14.81 mol) charges is extremely good ! It is a success. And it is not necessary to add any sulfate ions !...

We may carry out the same calculations with the masses, to see whether the total is 1000 g.

  • 5.28 mol NO3 weighs 327.4 g
  • 1.85 mol NH4 weighs 33.3 g
  • 2.82 mol PO4 weighs 267.6 g
  • 6.26 mol K weighs 244 g
  • 1.07 mol of Mg weighs 26 g
  • 2·2.82 mol H weighs 5.64 g

The total of these masses is 906 g. It is not 1000 g. It shows that there are some non ionic substances in the fertilizer, like clay or organic substances.

Maurice
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  • I like your approach but aren't Mg and K positive ions? Could it be that you mixed positive with negative? – CuriousIndeed Nov 03 '19 at 18:52
  • Mg and K are elements, occuring in fertilizers in form of ions $\ce{Mg^2+}$ and $\ce{K+}$ – Poutnik Nov 03 '19 at 19:03
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    That's why I'm asking. He wrote "To start with negative ions, one must triple the number of phosphate and add the double of magnesium plus the number of moles of K" – CuriousIndeed Nov 03 '19 at 19:05
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    Note that there are hydrogen or dihydrogen phosphate. One has to calculate charge balance. – Poutnik Nov 04 '19 at 04:15
  • I thought we have to add the positive and negative charges. So why does he add phosphate (PO4 3-) to the positive ions Mg2+ and K+? – CuriousIndeed Nov 04 '19 at 20:24
  • @CuriousIndeed I do not know, but there is no significant amount of $\ce{PO4^3-}$, otherwise the fertilizer solution would smell by ammonia and magnesium would precipitate as $\ce{ NH4MgPO4}$ or $\ce{Mg(OH)2}$ – Poutnik Nov 05 '19 at 03:54
  • My Simple check shows there is about 1.77 of charge per 1 P, so there is ratio HPO4^2-/H2PO4- about 3:1. – Poutnik Nov 05 '19 at 04:35
  • About the mass deficit, there can be some crystal water. – Poutnik Nov 05 '19 at 06:38