How this calculator works
Fertilizer grade is the percentage of nitrogen, phosphate equivalent and potash equivalent by mass. Divide the required mass of the controlling nutrient by its label fraction to obtain product mass. The other nutrients are reported so you can compare the same application with the full soil-test recommendation. The tool does not determine an agronomic rate or balance multiple fertilizer products.
Formula
Nutrient mass (kg) = rate (kg/ha) × area (m2)/10000. Fertilizer mass = nutrient mass/label fraction. Bags = ceiling(fertilizer mass/bag mass).
Worked example
Treating 1000 m2 at 100 kg N/ha requires 10 kg N. A 20-10-10 fertilizer supplies this in 50 kg of product: two 25 kg bags. It also supplies 5 kg each of P2O5 and K2O.
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Frequently asked questions
Does a 20-10-10 label mean elemental phosphorus?
No. The second and third values are phosphate (P2O5) and potash (K2O) equivalents. Match the nutrient basis to the recommendation.
Where should the target rate come from?
Use a current soil test, crop requirement and local extension guidance. More fertilizer does not necessarily improve growth.
What if the selected nutrient is zero on the label?
That product cannot supply a positive target for that nutrient. Choose another nutrient or fertilizer.