When your soil test comes back with a target N-P-K ratio, translating that into an actual blended fertilizer product and quantity requires matching your target ratio against the grade of fertilizer available to you. This calculator handles that conversion and gives you the total product weight and cost for your field.
Fertilizer Needed per Nutrient = Target Nutrient (kg/ha) ÷ (Nutrient % in Fertilizer Grade ÷ 100) × Field Area, calculated separately for N, P, and K based on the grade you select (e.g., a 20-20-0 blend supplies 20% N and 20% P2O5 per 100 kg). Total cost scales with your area and the price per 50 kg bag.
| Grade (N-P-K) | Typical Use |
|---|---|
| 20-20-0 | General basal dressing |
| 15-15-15 | Balanced maintenance fertilizer |
| 12-12-24 | Potassium-demanding crops (potato, fruit) |
| 46-0-0 (Urea) | Pure nitrogen top-dressing |
For 3 hectares with a target of 100-60-40 kg/ha (N-P-K) using a 15-15-15 balanced grade: to meet the phosphorus target (the usual limiting nutrient), you'd need 60 ÷ 0.15 = 400 kg/ha × 3 ha = 1,200 kg of 15-15-15, which also happens to oversupply nitrogen slightly (180 kg vs. the 300 kg target) — meaning you'd top up the remaining nitrogen separately with urea, a common real-world blending approach.
Because commercial blends come in fixed ratios, matching one nutrient exactly (usually phosphorus, since it's needed in smaller amounts) typically over- or under-supplies the other two — which is why farmers often combine a balanced blend with straight nitrogen or potash top-ups to fine-tune the final dose.
Phosphorus is usually the nutrient to match precisely, since it's needed in a fixed early-season dose and doesn't respond well to correction later. Nitrogen is more forgiving since it can be top-dressed in additional applications during the season.
The three numbers in a grade like 15-15-15 represent the percentage by weight of nitrogen (N), phosphorus as P2O5, and potassium as K2O respectively — a 50 kg bag of 15-15-15 contains 7.5 kg of each nutrient.
No — soil test results often show one nutrient (commonly potassium in older, intensively-cropped soils) is already adequate while another is deficient. Applying a balanced blend in that case wastes money on the nutrient you don't need.