Assume that the amount of fertilizer recommended for the plot is 8.5 kg of pure nitrogen per mu, P2O54.5 kg, and K2O 6.5 kg. Single fertilization. The calculation method is: Recommended Fertilizer Quantity ÷ The effective content of chemical fertilizer = the quantity that should be fertilized. The following results can be obtained: Urea application (urea nitrogen content is generally 46%) is 8.5 ÷ 46% = 18.5 (kg).
The application of potassium sulfate (K2O in potassium sulfate is generally 50%) is 6.5 ÷ 50% = 13 (kg) application of compound fertilizer. If compound fertilizer is used, the amount should be calculated based on the fertilizer with the least amount of fertilizer recommended on the formula fertilization table, and then the other two kinds of fertilizers should be added. If the content of nitrogen, phosphorus, and potassium marked on a compound fertilizer bag is 15:15:15, the compound fertilizer should be applied to the plot: 4.8÷15%=32 (kg). In this way, the phosphorus in the soil has already satisfied the nutrition that the crop needs. Due to the fixed ratio of compound fertilizers, it is difficult to meet the demand of different nutrients for different crops in different soils. Therefore, it is necessary to add unit fertilizers to supplement, and the formula is: (recommended fertilizer amount - applied fertilizer amount) 有效 effective content of chemical fertilizers to be applied to the plant = supplementary fertilizer quantity. The plot has already applied 32 kg of compound fertilizer with NPK content of 15%, which is equivalent to pure nitrogen in the applied soil of 32 × 15% = 4.8 (kg) P2O5 and K2O are also 4.8 kg each. According to the table 8.5 kg of pure nitrogen fertilizer recommendation, K2O6.5 kg, further enriching needed: urea (8.5-4.8) ÷ 46% = 8 (kg), potassium (6.5-4.8) ÷ 50% = 3.4 (kg).