Work out how much fertilizer to apply per acre or per hectare, from a set product rate or from a nutrient target and the product’s NPK analysis.
Enter the area and a product rate to get the total fertilizer needed. Switch to nutrient mode to size the product from a target like kg of N per hectare and the product’s analysis.
By product rate: total = area × rate. By nutrient target: product rate = nutrient target ÷ (analysis ÷ 100). For example, 100 kg N/ha from urea (46% N) needs about 217 kg/ha of urea. NPK supplied is the product applied × each percentage.
How to Calculate Fertilizer Rate per Acre
Divide the nutrient target by the nutrient’s percentage in the product. If your soil test calls for 100 lb of nitrogen per acre and you use urea (46-0-0), you need 100 ÷ 0.46 = 217 lb of urea per acre. For a 10-acre field that is about 2,174 lb.
- Find the target from your soil test: 100 lb N per acre.
- Read the bag: urea is 46% N, or 0.46.
- Divide: 100 ÷ 0.46 = 217.4 lb of product per acre.
- Multiply by your area. For 1/2 acre: 217.4 × 0.5 = about 109 lb.
| Target | Product | Product needed |
|---|---|---|
| 100 lb N per acre | Urea 46-0-0 | 217 lb per acre |
| 50 lb N per acre | 10-10-10 | 500 lb per acre |
| 120 kg N per hectare | Urea 46-0-0 | 261 kg per hectare |
| 1 lb N per 1,000 sq ft | 20-10-10 | 5 lb per 1,000 sq ft |
Reading the Fertilizer Label
The three numbers on the bag are the percentage by weight of nitrogen (N), phosphate and potash. By convention the second and third are expressed as P2O5 and K2O, not pure phosphorus and potassium, so a 20-10-10 product contains 20% N, 10% P2O5 and 10% K2O. Some products also list sulphur, magnesium or trace elements.
If a soil lab states phosphorus as elemental P, multiply by about 2.29 to get P2O5; for potassium, multiply elemental K by about 1.2 to get K2O. Mixing the two forms is one of the most common reasons a rate comes out wrong.
Base the target on a soil test, since soil reserves, the previous crop and any manure all change what the crop needs. Excess wastes money and can wash into streams and groundwater.
Turning the Total into a Field Job
- Calibrate your spreader. The setting chart is a starting point; weighing what comes out over a measured strip shows the real rate.
- Split nitrogen where practical. Several smaller applications timed to crop growth usually lose less than one large dose.
- Check local rules. Some regions limit nitrogen or phosphorus rates, timing or spreading near water.
- Round up to whole bags only after the calculation, and store leftovers somewhere dry.
Straight products such as urea supply one nutrient, so separate N, P and K targets may need a blend or more than one product. Run each product in turn and add up the nutrients supplied. The fertilizer labeling article on Wikipedia explains the NPK convention in more detail.