I. Basic Concepts

NPK stands for three major macronutrients:

• N (Nitrogen): Promotes leaf and stem growth
• P₂O₅ (Phosphorus Pentoxide): Boosts root development and flowering (phosphorus content labeled on fertilizers is calculated as P₂O₅, not elemental phosphorus)
• K₂O (Potassium Oxide): Supports fruit swelling and stress resistance (potassium content on fertilizer bags is expressed as K₂O, not elemental potassium)

Numbers printed on fertilizer bags such as 15-15-15 mean 15% nitrogen, 15% phosphorus pentoxide, and 15% potassium oxide, with a total nutrient content of 45%.

II. Two Common Calculation Scenarios

Scenario 1: Calculate Dosage of Straight Fertilizers Based on Target Nutrients Per Mu or Per Cubic Meter

Common raw straight fertilizers:

1. Urea: 46% N
2. Superphosphate: 12% P₂O₅; Monoammonium Phosphate (MAP): 11-52-0; Diammonium Phosphate (DAP): 18-46-0
3. Potassium Sulfate: 52% K₂O; Potassium Chloride: 60% K₂O

Core Calculation Formula

Raw Material Weight (kg)=Nutrient Content of the FertilizerTarget Pure Nutrient Weight (kg)​

Practical Calculation Example

Produce 100 kg of compound fertilizer with a target nutrient formula of 16-6-20, meaning the 100 kg finished product contains 16 kg N, 6 kg P₂O₅ and 20 kg K₂O.

1. Nitrogen source: Urea (46% N) Required urea = 16 ÷ 0.46 ≈ 34.78 kg
2. Phosphorus source: DAP (18-46-0); calculate DAP dosage to meet phosphorus demand first Required DAP = 6 ÷ 0.46 ≈ 13.04 kg Nitrogen carried by DAP = 13.04 × 0.18 ≈ 2.35 kg Remaining nitrogen shortage = 16 − 2.35 = 13.65 kg Supplementary urea = 13.65 ÷ 0.46 ≈ 29.67 kg Total urea dosage: 29.67 kg
3. Potassium source: Potassium sulfate (52% K₂O) Required potassium sulfate = 20 ÷ 0.52 ≈ 38.46 kg

Total weight of raw materials: 29.67 kg (urea) + 13.04 kg (DAP) + 38.46 kg (potassium sulfate) = 81.17 kg Weight shortfall to reach 100 kg finished product: 100 − 81.17 = 18.83 kg, which can be filled with fillers (talc powder, bentonite, organic manure powder).

Scenario 2: Specialized Ratio References for Different Crops

1. Leafy Vegetables (Lettuce, Chinese Cabbage, Chives)

High nitrogen, low phosphorus, medium potassium: Recommended ratios 20-8-12 / 18-7-11 Rationale: Nitrogen prioritizes leaf growth; moderate phosphorus stimulates root growth; potassium prevents lodging.

2. Fruiting & Solanaceous Crops (Tomato, Cucumber, Pepper)

Balanced formula for fruit development: Base fertilizer 15-15-15; high-potassium topdressing during fruit swelling stage 12-8-22 / 10-5-25

3. Root & Tuber Crops (Potato, Sweet Potato, Radish, Ginger)

High potassium, low nitrogen: 10-10-20 / 9-6-24; potassium facilitates starch accumulation.

4. Base Fertilizer for Fruit Trees (Apple, Citrus)

Slow-release balanced formula: 14-16-15; high-potassium formula 10-5-22 during fruit expansion.

5. Field Crops (Rice, Wheat)

Base fertilizer: 15-15-15; jointing stage topdressing with high-nitrogen formula 25-10-10

III. Field Application Conversion (Dosage Per Mu)

Example: Target nutrient supply per mu: 8 kg pure N, 4 kg P₂O₅, 10 kg K₂O, using only 15-15-15 compound fertilizer.

1. Required compound fertilizer per mu = Minimum target nutrient ÷ Corresponding nutrient content of fertilizer The limiting nutrient is phosphorus: 4 ÷ 0.15 ≈ 26.67 kg/mu
2. Nutrient verification after application: N supplied = 26.67 × 0.15 = 4 kg, far below the target 8 kg; K supplied = 26.67 × 0.15 = 4 kg, below the target 10 kg.
3. Conclusion: Single balanced compound fertilizer cannot meet demand; supplement with urea and potassium sulfate: Nitrogen deficit: 8 − 4 = 4 kg → Supplementary urea = 4 ÷ 0.46 ≈ 8.7 kg Potassium deficit: 10 − 4 = 6 kg → Supplementary potassium sulfate = 6 ÷ 0.52 ≈ 11.5 kg Final application per mu: 26.67 kg compound fertilizer + 8.7 kg urea + 11.5 kg potassium sulfate

IV. Step-by-Step Practical Blending Procedures

1. Confirm target N-P₂O₅-K₂O values based on crop growth stage;
2. Select nitrogen, phosphorus and potassium raw materials and record their respective nutrient contents;
3. Calculate phosphate fertilizer dosage first, deduct nitrogen inherently carried by phosphate fertilizer, then compute supplementary urea dosage;
4. Calculate potassium fertilizer dosage separately;
5. Sum weights of all raw materials, fill the weight gap with fillers or organic manure to reach the target finished weight;
6. For field application: Apply compound fertilizer as base, then supplement straight fertilizers to offset nutrient shortages.

V. Critical Notes to Avoid Mistakes

1. Phosphorus and potassium content are converted as oxides, do not calculate using elemental phosphorus or potassium directly;
2. MAP and DAP contain both nitrogen and phosphorus. Always deduct their inherent nitrogen to avoid excessive nitrogen and seedling burn;
3. Chloride-sensitive crops (grape, strawberry, potato, tobacco) must not use potassium chloride; only potassium sulfate is allowed;
4. High-nitrogen formulas are suitable for seedling stage; reduce nitrogen and increase potassium during fruit swelling, otherwise excessive vegetative growth will suppress fruit setting.