P & K Oxide Converter & Element ↔ Oxide
Converts P → P₂O₅
Convert phosphorus and potassium between the element (P, K) and the oxide (P₂O₅, K₂O) used on fertiliser grades — P × 2.29 = P₂O₅, K × 1.20 = K₂O — and back, with the factor shown.
P/K ↔ oxide converter
Next: multiply 100 by 2.29 to read the result as 229 kg P₂O₅.
Factors are fixed from atomic masses: P→P₂O₅ ×2.29, P₂O₅→P ×0.44, K→K₂O ×1.20, K₂O→K ×0.83. Fertiliser labels usually quote oxide form (P₂O₅, K₂O); soil/plant analyses often quote elemental P, K.
Oxide conversion — key facts
- Soil tests use
- element (P, K)
- Fertiliser grades use
- oxide (P₂O₅, K₂O)
- P → P₂O₅
- × 2.29
- P₂O₅ → P
- × 0.4364
- K → K₂O
- × 1.20
- K₂O → K
- × 0.8301
- Nitrogen
- no conversion (N is N)
- Privacy
- Runs in your browser; nothing uploaded
Same nutrient, two number systems — don't mix them up
Phosphorus and potassium get reported two different ways and it trips up a lot of fertiliser maths. Soil tests and crop-removal tables give them as the element — plain P and K. Fertiliser bags and grades give them as the oxide — P₂O₅ and K₂O. The factors are P × 2.29 = P₂O₅ and K × 1.20 = K₂O, with the reverse being 0.4364 and 0.8301. Because phosphorus more than doubles, treating one form as the other can wreck a dose.
This tool converts a value either way and shows the converted figure and the exact conversion factor it used, so you can line up a soil test against a fertiliser grade with confidence. Use it whenever you're moving between crop-removal data, soil reports and bag labels — and pair it with the NPK-from-Grade and Crop Nutrient Removal calculators where the same P₂O₅/K₂O convention shows up.
Match the forms
Line up soil tests with fertiliser grades.
Both directions
Element to oxide and oxide back to element.
Avoid dosing errors
Stop confusing P with P₂O₅ or K with K₂O.
See the factor
Transparent maths you can re-check by hand.
Frequently Asked Questions
What does the P & K oxide converter do?+
It converts phosphorus and potassium between two forms: the element (P, K) that soil tests and crop-removal data use, and the oxide (P₂O₅, K₂O) that fertiliser grades use. Enter a value, pick the direction, and it returns the converted figure plus the exact conversion factor applied.
Why are there two ways to report P and K?+
Soil tests and crop-nutrient-removal data usually report phosphorus and potassium as the element — P and K. Fertiliser bags and grades, by long-standing convention, report them as the oxide — P₂O₅ and K₂O. The same nutrient, two different number systems, which is exactly why a converter is needed.
What are the conversion factors?+
For phosphorus, P × 2.29 = P₂O₅ (and P₂O₅ × 0.4364 = P). For potassium, K × 1.20 = K₂O (and K₂O × 0.8301 = K). These come from the ratio of the molecular weight of the oxide to the element it contains, so they're fixed constants — the tool applies the right one for your chosen direction.
How do I convert P to P₂O₅?+
Multiply the phosphorus value by 2.29. For example 10 kg of P equals about 22.9 kg of P₂O₅. To go the other way — a fertiliser grade's P₂O₅ back to elemental P — multiply by 0.4364, so 22.9 kg P₂O₅ is about 10 kg P.
How do I convert K to K₂O?+
Multiply the potassium value by 1.20. So 10 kg of K equals about 12 kg of K₂O. Reversing it, K₂O × 0.8301 gives elemental K, so 12 kg K₂O is about 10 kg K.
Why does mixing the two up cause errors?+
Because the factors are well above 1, treating a P₂O₅ figure as if it were elemental P (or vice versa) can over- or under-state the nutrient by more than a factor of two for phosphorus and 20% for potassium. That turns into big dosing errors — wasted fertiliser or a starved crop — so always check which form your numbers are in.
Which form should I use for a soil test?+
Soil tests typically report available P and K as the element. If you're comparing a soil test or a crop-removal figure against a fertiliser grade, convert one of them so both are in the same form — usually convert the element to the oxide to match the fertiliser bag, or the oxide back to the element to match the soil test.
What does the calculator output?+
The converted value in the target form and the conversion factor it used. Showing the factor makes the maths transparent so you can sanity-check the result and apply the same factor by hand elsewhere if you need to.
Does it convert nitrogen too?+
No — nitrogen is reported as elemental N on both soil tests and fertiliser grades, so there's no element-versus-oxide conversion to do for N. This tool covers only phosphorus and potassium, the two nutrients with the P₂O₅/K₂O oxide convention.
Are the conversions exact?+
The factors are exact constants from molecular weights, so the conversion itself is precise. Any real-world inaccuracy comes from the input value — how the lab measured it or how the fertiliser grade is rounded — not from the calculation. Use consistent units in and out and the result is exact.