Fertiliser Requirement
Fertiliser product needed to supply a target nutrient rate.
Formula
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Example
Need 120 kg N/ha over 5 ha, urea 46% N: F ≈ 1304 kg
How it works
This formula converts a target nutrient dose for a field into the amount of fertiliser product needed. First, multiply the desired nutrient rate by the area to find the total nutrient required; then divide by the fertiliser’s nutrient fraction because only part of the product is the nutrient itself. For example, if a fertiliser is 25% nutrient, only 0.25 kg of nutrient is supplied per 1 kg of product, so you need more product than the pure nutrient amount.
When to use it
Use this when planning fertiliser applications for crops, lawns, or trial plots and you know the target nutrient rate per hectare plus the fertiliser analysis. It is especially useful for comparing different products with different nutrient concentrations.
Worked example, step by step
A farmer wants to apply 80 kg P2O5/ha to 12 ha using a fertiliser that is 20% P2O5. Step 1: Write the formula: F = (R × A) / c Step 2: Substitute values: F = (80 × 12) / 0.20 Step 3: Calculate the nutrient needed in total: 80 × 12 = 960 kg P2O5 Step 4: Divide by the nutrient fraction: 960 / 0.20 = 4800 kg Final answer: 4,800 kg of fertiliser is required.
Common mistakes to avoid
- using the percentage as 20 instead of converting it to 0.20
- forgetting to multiply the rate by the total area before dividing by nutrient content
- confusing nutrient rate units, such as mixing kg/ha with kg/m²
Did You Know?
Over-fertilising wastes money and pollutes waterways — precise rates protect both profit and environment.
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