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NPK Requirement

Free NPK Requirement calculator with clear step-by-step results.

About NPK Requirement

Compound fertilisers never match a crop's nutrient ratio exactly. Setting the rate to satisfy nitrogen - usually the limiting and most valuable nutrient - then reports what phosphate and potash that rate happens to supply and what remains to be made up with straight products.

Formulas

Compound rate

kg/ha = nitrogen target / nitrogen percentage of the product

Nutrient supplied

Supplied = compound rate x nutrient percentage for each of P and K

Worked example

With the default values below, NPK Requirement returns Compound rate to meet nitrogen of 750.0. Change any input above and every figure here is recalculated in your browser.

Inputs used

Areaminimum 0.01 ha
5 ha
Nitrogen targetminimum 0 kg N/ha
150 kg N/ha
Phosphate targetminimum 0 kg P2O5/ha
45 kg P2O5/ha
Potash targetminimum 0 kg K2O/ha
80 kg K2O/ha
Compound fertiliser gradeone of 20-10-10, 15-15-15, 27-5-5, 10-20-20
20-10-10

Results

Compound rate to meet nitrogen
750.0 kg/ha
Total compound fertiliser
3,750.0 kg
Phosphate still needed
0.0 kg P2O5/ha
Potash still needed
5.0 kg K2O/ha

Step by step

  1. 1Values used
  2. 2Compound rate
  3. 3Nutrient supplied
  4. 4Compound rate to meet nitrogen= 750.0 kg/ha
  5. 5Total compound fertiliser= 3,750.0 kg
  6. 6Phosphate still needed= 0.0 kg P2O5/ha
  7. 7Potash still needed= 5.0 kg K2O/ha

Assumptions and limits

  • The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Nutrient application limits, closed periods and buffer zones near water are legally enforced in many regions. Base rates on a current soil analysis and comply with any nutrient management regulation.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Nutrient application limits, closed periods and buffer zones near water are legally enforced in many regions. Base rates on a current soil analysis and comply with any nutrient management regulation.

Frequently Asked Questions

Why size the rate on nitrogen?

Nitrogen drives yield response most directly and cannot be stored in soil between seasons. Phosphate and potash are buffered by soil reserves, so a small shortfall is less immediately damaging.

What if the compound oversupplies phosphate?

The shortfall shows zero and you are applying more than the target. Repeated over-application of phosphate is an environmental and regulatory concern - pick a lower-P grade.

Related Calculators

NPK Requirement – Calculate Compound rate to meet nitrogen