# 8.5: Activity 4 - Fertilizer Calculations

- Page ID
- 14648

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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 P
_{2}O_{5}× 0.44 - lb P
_{2}O_{5}= lb P × 2.29 - lb K = lb K
_{2}O × 0.83 - lb K
_{2}O = lb K × 1.21 - 1 ton = 2000 lb
- 1 ac = 43,560 ft
^{2}

## 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 P_{2}O_{5}, and K as K_{2}O. This fertilizer contains 15% by weight N, 10% by weight P_{2}O_{5}, and 10% by weight K_{2}O. We want to know the following:

- 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 P
_{2}O_{5}are in the bag? How many lbs of K_{2}O? - How many lb of P are in the bag?
- How many lb of K are in the bag?

### Calculate:

- How much N is in a 50 lb bag of 12-22-18?
- How much P
_{2}O_{5}is in a 40 lb bag of 0-46-0? - How much K
_{2}O is in a 70 lb bag of 12-22-18? - How much N is in a 30 lb bag of 0-46-0?
- How much N is in a ton of DAP (18-46-0)?
- 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, P_{2}O_{5}, or K_{2}O.

When we calculate the amount of fertilizer needed to supply a certain amount of N, P_{2}O_{5}, or K_{2}O we need to know:

- How much N, P
_{2}O_{5}, or K_{2}O is needed per unit area (e.g., 1,000 square feet, acre, etc.) - The analysis (grade) of the fertilizer (e.g., 10-10-10, 0-46-0, etc.)
- How much area we need to fertilize.
- 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):

### Calculate:

- 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!)