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8.5: Activity 4 - Fertilizer Calculations

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    Additions of fertilizer are important for maintain ideal soil health in agricultural management. Please complete these calculations to determine the amount of N, P and K in fertilizers.

    The following conversion factors and units equivalencies will be useful to complete the fertilizer problems.

    • lb P = lb P2O5 × 0.44
    • lb P2O5 = lb P × 2.29
    • lb K = lb K2O × 0.83
    • lb K2O = lb K × 1.21
    • 1 ton = 2000 lb
    • 1 ac = 43,560 ft2

    Determining the amount of N, P, K in fertilizers

    You have a 50 lb bag of granular fertilizer with a label that reads 15-10-10. The grade is 15-10-10, which is the percentage by weight of N, P as P2O5, and K as K2O. This fertilizer contains 15% by weight N, 10% by weight P2O5, and 10% by weight K2O. We want to know the following:

    1. How many lbs of N are in the bag? Of the 50 lb of fertilizer in the bag, we know that 15% of that is N. How many lbs of P2O5 are in the bag? How many lbs of K2O?

    2. How many lb of P are in the bag?
    3. How many lb of K are in the bag?


    1. How much N is in a 50 lb bag of 12-22-18?
    2. How much P2O5 is in a 40 lb bag of 0-46-0?
    3. How much K2O is in a 70 lb bag of 12-22-18?
    4. How much N is in a 30 lb bag of 0-46-0?
    5. How much N is in a ton of DAP (18-46-0)?
    6. How much P is in a ton of DAP (18-46-0)?

    Calculating the amount of fertilizer needed to supply a certain amount of N, P2O5, or K2O.

    When we calculate the amount of fertilizer needed to supply a certain amount of N, P2O5, or K2O we need to know:

    1. How much N, P2O5, or K2O is needed per unit area (e.g., 1,000 square feet, acre, etc.)

    2. The analysis (grade) of the fertilizer (e.g., 10-10-10, 0-46-0, etc.)
    3. How much area we need to fertilize.
    4. For manure applications, you need to know the amount of N that will be available during the growing season. Rule of thumb is 60%.

    You need to apply 2 lbs of N per 1,000 square feet. You have a 30-0-0 grade fertilizer available for application. The area you need to fertilize is 150,000 square feet. How much fertilizer do you need to apply?

    Step 1. Determine the rate of fertilizer needed.

    Step 2. Determine the fertilizer analysis (grade).

    Step 3. Calculate how much fertilizer you need per 1,000 square feet.

    Divide the fertilizer application rate by 0.30 (30%):

    Step 4. Calculate how much fertilizer is needed to cover the required area.

    We need to fertilize a total of 150,000 square feet (given in the problem), which means that we multiply by the amount of fertilizer needed to apply 2 lb N to 1000 sq ft. by 150. (Note: 150 comes from dividing 150,000 sq ft/1000 sq ft):


    1. You need to apply 2 lbs of N per 1,000 square feet. You have a 15-10-10 fertilizer available, and the total area to be fertilized is 3 acres. How much fertilizer will you need? (Hint: Don’t forget to convert sq. ft. to acres!)

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