N, P₂O₅ and K₂O: Do Not Compare Different Label Bases as Equal Masses; illustrative quantitative diagram
Make a controlled comparison
Fertilizer labels can state phosphorus and potassium using oxide conventions. An elemental quantity should not be divided by an oxide percentage as though they were the same substance. Establish the target’s basis first, and use an appropriate conversion when the source information uses another convention.
Fertilizer Calculator makes the stated relationship reproducible. It cannot supply missing product information, identify a representative sample or establish a suitable site-specific management decision.
Change one input at a time
The table varies Product percentage for that same nutrient (%) around the illustrative starting value while retaining the other assumptions. These are sensitivity scenarios, not a measured range or recommended settings.
| Product percentage for that same nutrient (%) | Selected calculated output | Unit |
|---|---|---|
| 16 | 312.5 | kg product |
| 20 | 250 | kg product |
| 24 | 208.333 | kg product |
Browse Biology calculators and its Gardening and crops calculators section when another measurement is needed. A related tool may use a different denominator or physical basis.
Keep the interpretation within scope
Keep Product percentage for that same nutrient (%) with the selected output in the comparison record. The three alternatives do not identify which input applies to a particular site or product. A justified new observation should be recorded as new evidence, rather than choosing an input merely because its result looks convenient.
The target must come from an appropriate site plan; no application rate is recommended.
A related quantity
Water Soluble Fertilizer Calculator addresses a related part of the task. Check its own input definition before carrying a number across.
Continue with Fertilizer Percentages: Convert a Nutrient Target into Product Mass for the other part of this measurement question.
Return to GreenCalc for other subjects. Keep raw observations, chosen assumptions and calculated quantities identifiable so a later revision does not overwrite the basis of the original record.
By GreenCalc · Updated 2026-11-06