Fertiliser Rate Calculator
The bag says P₂O₅. Your soil test may not.
Work out Fertiliser Rate. The bag says P₂O₅. Your soil test may not. Names the misconception directly.
Application rate
500 kg/ha
Nitrogen sets the rate · supplies 100N 50P 50K
Bag analysis is N, P₂O₅ and K₂O — oxides, not elements. Elemental phosphorus is P₂O₅ × 0.4364 and elemental potassium is K₂O × 0.8301, and confusing the two is the commonest arithmetic error in the subject. Treating a soil test in elemental P as if it were oxide under-applies by more than half. With a compound product one nutrient always sets the rate and the others land where they land. That is unavoidable, and it is why straights are used where the required ratio is far from anything on a bag. The targets above are yours to enter. Nitrogen rates in particular are regulated in most jurisdictions, tied to a nutrient management plan and a soil test, and this page deliberately does not recommend one.
How the Fertiliser Rate Calculator works
Product rate from a bag analysis and a nutrient target, with the nutrient that sets the rate identified and the over- or under-supply of the others reported. Handles targets stated in oxide or elemental form.
Also known as: how much fertiliser per acre · npk application rate · p2o5 to elemental phosphorus · how many bags of fertiliser do i need
Frequently asked questions
How do I work out a fertiliser rate?
Divide the nutrient target by the percentage in the product and multiply by 100. With a compound product, whichever nutrient needs the most product sets the rate for all of them.
What is the difference between P and P₂O₅?
P₂O₅ is the oxide form used on fertiliser bags; P is the element. P = P₂O₅ × 0.4364, so treating a soil test in elemental P as if it were oxide under-applies by well over half.
Why does one nutrient end up over-applied?
Because a compound product has a fixed ratio and your soil rarely wants that ratio. Whichever nutrient needs the most product wins, and the others land where they land.
When should I use straights instead of a compound?
When the required ratio is far from anything available in a bag. Straights cost more per unit of nutrient and waste far less of it.
What nitrogen rate should I apply?
This page does not recommend one. Nitrogen rates are regulated in most jurisdictions and tied to a soil test and a nutrient management plan — the target above is yours to enter.
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