Understanding the NPK Ratio of Fertilizers (With Examples)

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Note: The NPK ratio is the fertilizer analysis that tells you information about three key nutrients: nitrogen, phosphorus, and potassium. It is usually written in large bold letters on the packaging.

Q1. In package whose NPK ratio is 33-10-5, what percentage of the product contains nitrogen, P2O5, and K2O? If the package weighs 40 lbs, how much of each nutrient does it contain by weight.

Note: If you're asked for the percentage of elemental P and K, the calculation changes.

• Firstly, the molecular weight of P2O5 is 142 grams/mol. The portion of the molecule that is P only is 62 grams/mol. Therefore, P represents 62/142=0.4366 or 0.44 (44%) of the molecule.
• Similarly, K2O has a molecular weight of 94 grams/mol. The portion of the molecule that is potassium is 78 grams/mol. Hence, potassium only represents 78/94=0.8298 or 0.83 (83%) of the molecule.

Q2.

a) For the 50 lb. bag of fertilizer depicted below, state the percent of:

N = ___%
P2O5 = ___%
P = ___%
K2O = ___%
K = ___%
Filler = ___%
Total = ___%
 
b) Calculate the weight of each nutrient in pounds (lb.):
 
N = ___ lb.
P2O5 = ___ lb.
P = ___ lb. 
K2O = ___ lb.
K = ___ lb.
Filler = ___ lb.
Total = ___ lb.

[DattyAlly]

as student lrealy understand, l can go infront of my fellow members and do like the way you teach me

[darshdarsh]

Hello, I have a query, what I can understand you are not calculating N value in molecular weight as it is already in N form, but, what if I have a fertilizer which has nitrogen in Nitrate (NO3-) or Ammonia (NH3) form, do I need to convert the N value then by the same method you have shown in this video? e.x. for Ammonia to get correct N value do I need to deduct molecular weight of 4 Hydrogen atoms from NH3 to get correct number of N value in Ammonia?

[DattyAlly]

what are the methods of fertilizer application

[happydays]

The thing that is super confusing to me is why do they call it an npk ratio when it's not a ratio but a percentage? Can you explain this to me. Some people further add to my confusion when they actually use it as a ratio, for example a product with npk value of 24, 6, 6 has an npk ratio of 4:1:1
Is it a ratio or percentage?

[kennycampbelljr]

What if the numbers are given in decimal format like 0.21-0.11-0.16 is that the same as 21-11-16?

[MdAlamgir]

Conversion of K2O to K also seems wrong. If K2O is 7.5 lb, K should be 6.2257 lb not 1.8675 lb.

[MdAlamgir]

In Q.1 the term fertilizer grade should be used instead of fertilizer ratio.