Sciencemadness Discussion Board

Fertilizer's NPK rating - how is this calculated?

RogueRose - 4-10-2017 at 05:26

I was trying to figure out how various fertilizers calculate the mineral values for the specifics like N-P-K. I looked at ammonium nitrate and calcium nitrate which are 34-0-0 and 15.5-0-0 respectively.

I thought it was the percentage of nitrogen for the weight of the product and when I calculated the amount of N in cal nitrate (164.1g anhydrous & 236g tetrahydrate) I get a value far different as well as with ammonium nitrate.

Ca(NO3)2 has 2 nitrogen's for a weight of 28g/mole which comes out to about 11.86% for the tetrahydrate and 17% for the anhydrous, which I really don't think the fertilizer is, but maybe it is I guess?? does this mean the rating is low b/c they may take into account atmospheric absorption of moisture?

How is this supposed to be calculated?

Tsjerk - 4-10-2017 at 06:07

ammonium nitrate anhydrous: 80 g/mol, two nitrogens is 28 g/mol. 28/80*100 = 35.0%

calcium nitrate monohydrate : 186 g/mol, two nitrogens is 28 g/mol.
28/186*100 = 15.0%

pretty close I would say

RogueRose - 4-10-2017 at 06:25

Quote: Originally posted by Tsjerk  
ammonium nitrate anhydrous: 80 g/mol, two nitrogens is 28 g/mol. 28/80*100 = 35.0%

calcium nitrate monohydrate : 186 g/mol, two nitrogens is 28 g/mol.
28/186*100 = 15.0%

pretty close I would say


I'm not sure there is a monohydrate. when I weighed the diff when dehydrated, I think it lost about 40% water.

woelen - 4-10-2017 at 06:36

As far as I know, the percentages are computed as if the elements are present as their oxides:
- Nitrogen as if it were present as N2
- Phosphorus as if it were present as P2O5
- Potassium as if it were present as K2O

This weight is related to the real weight of the material you have.

An example:

KCl fertilizer is sold as N : P : K = 0 : 0 : 60

100 grams KCl contains 52.44 grams of K (and 47.56 grams of Cl).

Now imagine as if all K were present as K2O. Then the total weight of K2O would be MW(K2O)/(2*MW(K))*52.44 = 63.17 grams. Here MW() stands for the molar weight of the compound or atom between ().

Real KCl-based fertilizer always contains a little MgCl2 and some other impurities as well and hence it is sold as K=60 instead of K=63.


Another example:

Pure ammonium nitrate is sold as N : P : K = 35:0:0. If we take 100 grams of NH4NO3, this contains 35 grams of N. The common fertilizer KAS (Kalk Ammon Salpeter) is appr. 80% NH4NO3, the remainder being a mix of CaCO3 and MgCO3. This is sold as 27:0:0, which is close to what would be expected from 35% N in the pure compound.


The compound potassium ammonium hydrogen phosphate, K(NH4)HPO4, also is available as a high grade fertilizer in some shops (aquarium shops IIRC). 100 grams of this contains

9.148 grams N --> 9.148 if it were N2
20.229 grams P --> 46.352 grams if it were present as P2O5
25.535 grams K --> 30.760 grams if it were present as K2O

So, the N : P : K rating is 9:46:31


[Edited on 4-10-17 by woelen]

Tsjerk - 4-10-2017 at 07:01

Quote:
I'm not sure there is a monohydrate. when I weighed the diff when dehydrated, I think it lost about 40% water.


How did you dehydrate? In an oven or in a flame? The nitrate decomposes to CaO at 600 degrees Celcius

The monohydrate would be a mixture of anhydrous and the tetrahydrate.

UC235 - 4-10-2017 at 18:50

Calcium Ammonium Nitrate: 5Ca(NO3)2*NH4NO3*10H2O

m.w. 1080.6

12 Nitrogens @ 14.01 each is 15.6% nitrogen.

for DOT transportation purposes, CAN is not an oxidizer, unlike ammonium nitrate.

RogueRose - 4-10-2017 at 19:11

Quote: Originally posted by UC235  
Calcium Ammonium Nitrate: 5Ca(NO3)2*NH4NO3*10H2O

m.w. 1080.6

12 Nitrogens @ 14.01 each is 15.6% nitrogen.

for DOT transportation purposes, CAN is not an oxidizer, unlike ammonium nitrate.


This would align much better with my experiences with "Cal nit" as when heated a strong ammonia smell was emitted, and also when KOH or NaOH was added - it was so strong I couldn't breath after 1-2 seconds and it heated to 200
+ degrees after about 4-5 seconds with very minimal mixing to the dry substances. I was told there was no ammonia in the mix though..

[Edited on 5-10-2017 by RogueRose]

feacetech - 5-10-2017 at 17:55

Quote: Originally posted by woelen  
As far as I know, the percentages are computed as if the elements are present as their oxides:
- Nitrogen as if it were present as N2
- Phosphorus as if it were present as P2O5
- Potassium as if it were present as K2O

This weight is related to the real weight of the material you have.

An example:

KCl fertilizer is sold as N : P : K = 0 : 0 : 60

100 grams KCl contains 52.44 grams of K (and 47.56 grams of Cl).

Now imagine as if all K were present as K2O. Then the total weight of K2O would be MW(K2O)/(2*MW(K))*52.44 = 63.17 grams. Here MW() stands for the molar weight of the compound or atom between ().

Real KCl-based fertilizer always contains a little MgCl2 and some other impurities as well and hence it is sold as K=60 instead of K=63.


Another example:

Pure ammonium nitrate is sold as N : P : K = 35:0:0. If we take 100 grams of NH4NO3, this contains 35 grams of N. The common fertilizer KAS (Kalk Ammon Salpeter) is appr. 80% NH4NO3, the remainder being a mix of CaCO3 and MgCO3. This is sold as 27:0:0, which is close to what would be expected from 35% N in the pure compound.


The compound potassium ammonium hydrogen phosphate, K(NH4)HPO4, also is available as a high grade fertilizer in some shops (aquarium shops IIRC). 100 grams of this contains

9.148 grams N --> 9.148 if it were N2
20.229 grams P --> 46.352 grams if it were present as P2O5
25.535 grams K --> 30.760 grams if it were present as K2O

So, the N : P : K rating is 9:46:31


[Edited on 4-10-17 by woelen]


it depends on which country you are from, some use the archaic oxide forms for P and K

In NZ we use elemental % so Murate of Potash (KCl) (yah for more archaic fert industry terminology) in NZ is 0-0-50

52.45% T/K @ 95% purity is ~ 50 (49.8), impurities are usually NaCl and mositure

Im not sure about N, I though it was % T/N every where not % N2.

it is %N in NZ anyway. So Urea would typically be 46-0-0 with 46.64 being 100% purity (depedning how many sig figs you use for molar mass) which it isnt normally as it will have moisture, formaldehyde and some % of Biuret (usually <1% if it is in spec) but this has N conetnt also

so Urea is CH4N2O = 60.06 g /mol
N =14.01

since ther are two Ns in urea (not because it is %N2) it is (14.01*2)/60.06

[Edited on 6-10-2017 by feacetech]

woelen - 5-10-2017 at 22:52

it does not matter whether N is counted as N2, N, N3 or Nx. The relevant thing is that the total weight of nitrogen atoms counts. In our country, urea fertilizer also is 46:0:0.

feacetech - 8-10-2017 at 11:51

Yea thats as i thought every where express N as elemental in a NPK, but P and K can vary depending on country, i was confused for a second around N and N2, of course they are the same they have no other elements present.

A rational explanation why some countries still use oxide forms is because it looks like more. The conversion factor for T/P (P) -> T/P(P2O5) is ~ *2.29136

Its the same as if you have a T/Ca result as % El.Ca you can express it any which way you like (%Ca, %CaO, %CaCO3, %CaF) but it must be converted to the desired form these can be worked out as woelen explained above

it is a good skill to have as it can be used in multiple areas of chemistry, running a TKN Nitrogen standard (Say 2-Amino-2-(hydroxymethyl)-1,3-propanediol or Proline) you need to work out percent nitrogen for theroretical 100% recovery

Have elemntal analysis for Slag and the F component is at 110% as NaF so there must other F forms presnt as well

Simplified conversion factors are readily avaliable for common conversions esp fertiliser based ones

[Edited on 8-10-2017 by feacetech]