How to calculate nutrient needs before applying amendments

Calculating nutrient needs is a crucial step to avoid over-fertilizing and ensure your plants get exactly what they require. The process relies on a soil test report.

1. Get a Soil Test: This is the most important step. A soil test report from an agricultural lab will tell you the current levels of Nitrogen (N), Phosphorus (P), and Potassium (K) in your soil and provide a specific recommendation (e.g., in kg/hectare) for the crop you plan to grow.
2. Understand the Recommendation: The report will recommend a specific N-P-K application rate. For example, it might recommend 120-60-40, which means you need to apply 120 kg of Nitrogen, 60 kg of Phosphorus (as P₂O₅), and 40 kg of Potassium (as K₂O) per hectare.

3. Calculate the Fertilizer Amount: Look at the nutrient percentage on your fertilizer bag.

   • Urea contains 46% N.
   • DAP contains 18% N and 46% P₂O₅.
   • MOP contains 60% K₂O.

   Example Calculation for 1 hectare:

   • Potassium (K): You need 40 kg of K₂O. The amount of MOP needed is (40 kg / 60%) = 66.7 kg of MOP.
   • Phosphorus (P): You need 60 kg of P₂O₅. The amount of DAP needed is (60 kg / 46%) = 130.4 kg of DAP.
   • Nitrogen (N): You need 120 kg of N. The DAP already supplied (130.4 kg * 18%) = 23.5 kg of N. So, you still need 120 - 23.5 = 96.5 kg of N. The amount of Urea needed is (96.5 kg / 46%) = 210 kg of Urea.

Therefore, for one hectare, you would apply 66.7 kg of MOP, 130.4 kg of DAP, and 210 kg of Urea.