How To Calculate NPK, Application Rate & Fertilizer Requirement

If you are willing to calculate NPK based on the ratio and the formulae, then you can use these methods or the NPK calculator to calculate the amount of nutrients, fertilizer requirements and application rate.

NPK stands for nitrogen (N), phosphorus (P) and potassium (K). These three essential nutrients play an important role in the overall growth of the plant. NPK fertilizers are labeled with three numbers that indicate the percentage of each nutrient in the product.

For example, if you see NPK 10-20-10 written on a fertilizer bag, then it means that the fertilizer inside the bag contains 10% nitrogen, 20% phosphorus, and 10% potassium by weight.

To calculate the exact amount of each nutrients in the bag, you should follow these steps:

Understand the Label

The numbers on the fertilizer bag represent the percentage of each nutrient in the bag. For example, in a 50 pounds bag of 10-20-10 NPK fertilizer, there are 5 pounds of nitrogen (10% of 50 pounds), 10 pounds of phosphorus (20% of 50 pounds), and 5 pounds of potassium (10% of 50 pounds).

NPK Calculation Formulae

Use this simple formulae for calculating the NPK values present inside the fertilizer bag.

Amount of Nutrient (in pounds)= (Percentage of Nutrient/100) × (Weight of the Fertilizer in pounds)

For the 10-20-10 fertilizer example:

Nitrogen (N) content = (10/100) ×50 pounds= 5 pounds

Phosphorus (P) content = (20/100) × 50 pounds= 10 pounds

Potassium (K) content = (10/100) × 50 pounds= 5 pounds

How To Calculate Fertilizer Application Rate?

Use this simple formulae to calculate fertilizer application rate.

Fertilizer Needed (in pounds)= Nutrient Requirement (in pounds per acre)×100/Percentage of nutrient in the fertilizer

For example, if your nutrient requirement is 50 pounds per acre of nitrogen and you are using a fertilizer with 10% nitrogen, the calculation would be:

50×100/10 = 500 pounds of fertilizer per acre

NPK Calculator

You can use this NPK calculator to calculate amount of NPK and application rate.

Conversion Units:

1. Pounds to Kilograms: To convert pounds to kilograms, multiply the weight in pounds by 0.45359237.
   Kilograms= Pounds×0.45359237

2. Kilograms to Pounds: To convert kilograms to pounds, multiply the weight in kilograms by 2.20462.
   Pounds= Kilograms×2.20462

3. Square Feet to Acre: To convert square feet to acres, divide the area in square feet by 43,560 (the number of square feet in an acre).
   Acre= Square Feet/43,560Acre

3. Acre to Square Feet: To convert acres to square feet, multiply the area in acres by 43,560.
   Square Feet=Acre×43,560

1 hecatre= 10000 square meters = 25 cents = 2.5 acres

Important Points

If the question is asked in terms of Urea, DAP and MOP, SSP or SOP, then remember:

To understand this, read this text carefully,

Urea contains 46% of nitrogen, it means 100 Kilograms of urea contain 46 Kilograms of nitrogen. Likewise, 100 Kilograms of Muriate of potash contains 60 Kilograms of potassium. 100 Kilograms of DAP contains 18 Kilograms of nitrogen and 46 Kilograms of phosphorus.

Using this table, let’s solve a problem on calculating fertilizer requirement to get a clear understanding,

Simple Problem: Calculate the fertilizer requirement of 1 hectare paddy, if the fertilizer recommendation is 100:50:50 NPK through Urea, SSP and MOP?

Solution: 1) Nitrogen requirement through urea:

As we know, Urea contains 46% of nitrogen

Formulae For Fertilizer Requirement= Recommended dose/ % Nutrient content × 100 × Area

Using this formulae,

100/46 × 100 × 1 = 217.4 Kilograms of urea (It means that 217.4 Kilograms of urea can provide 100 Kilograms of nitrogen for 1 hectare paddy field)

2) As we know, SSP contains 16% Phosphorus

Fertilizer Requirement: 50/16 × 100 × 1 = 312.5 Kilograms of SSP (It means that 312.5 Kilograms of SSP can provide 50 Kilograms of Phosphorus for 1 hectare paddy field)

3) As we know, MOP contains 60% of Potassium

Fertilizer Requirement: 50/60 × 100 × 1 = 83.3 Kilograms of MOP ( It means that 83.3 Kilograms of MOP can provide 50 Kilograms of Potassium for 1 hectare paddy field).

After solving this simple problem, let’s solve another problem with complex fertilizer.

Complex Problem: Calculate the fertilizer requirement of 1 hectare paddy, if the fertilizer recommendation is 100:50:50 NPK through Urea, DAP and MOP?

Solution: In this case, urea and MOP are straight fertilizers whereas DAP is a complex fertilizer which consist of nitrogen and phosphorus.

First, let’s solve the fertilizer requirements through the complex fertilizer that is DAP

1) a. Phosphorus requirement through DAP,

Fertilizer Requirement= 50/46 × 100 × 1 = 108.6 Kilograms of DAP.

1) b. 108.6 Kilograms of DAP can provide 50 Kilograms of Phosphorus, but it also contains 18% Nitrogen.

100 Kilograms of DAP can provide 18 Kilograms of Nitrogen

therefore, 108.6 Kilograms of DAP can provide: 18 × 108.6/100 = 19.5 Kilograms of Nitrogen.

The amount of nitrogen that should be applied through urea: 100 – 19.5 = 80.5 Kilograms (Because the reccomended doze is 100 Kilograms of nitrogen)

2) Nitrogen requirement through urea: 80.5/46 × 100 × 1 = 175 Kilograms of Urea

3) Potassium requirement through MOP: 50/60 × 100 × 1 = 83.3 Kilograms of MOP.

I hope you have understood how to calculate NPK content, application rate and fertilizer requirement. If you have any ideas, queries or suggestions, then please comment below.