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Calculations Used to Determine the Amount of Fertilizer Needed to Treat Turf

A fundamental problem in turfgrass fertilization involves determining how much fertilizer is needed to supply a specified amount of nitrogen (or any other nutrient) per 1000 sq ft. Use the following examples to learn how to solve this type of problem.

Example:

You have a 50-lb bag of 26-5-10 fertilizer that you want to apply to a lawn at a rate of 1.0 lb nitrogen per 1000 sq ft. How much of the 26-5-10 fertilizer will you need to apply per 1000 sq ft?

The quickest way to solve this problem is to ignore the weight of the fertilizer bag and simply divide the amount of nitrogen desired (1.0 lb nitrogen per 1000 sq ft) by the percentage of nitrogen in the bag (26%). When using percentages in calculations, convert the number to its decimal form (for example, 26% = 0.26; 5% = 0.05).

(1.0 lb nitrogen per 1000 sq ft) ÷ 0.26 = 3.8 lb of a 26-5-10 fertilizer is needed to supply 1.0 lb nitrogen per 1000 sq ft.

Example:

Find out how much phosphate and potash you are applying to the turf when you apply 3.8 lb of the 26-5-10 fertilizer per 1000 sq ft.

Multiply the amount of fertilizer you are applying (3.8 lb per 1000 sq ft) by the percentage of phosphate in the bag (5%). Do the same for potash (10%). Remember to convert the percentages of phosphate and potash to their decimal forms.

(3.8 lb fertilizer per 1000 sq ft) × 0.05 phosphate = 0.19 lb phosphate per 1000 sq ft

(3.8 lb fertilizer per 1000 sq ft) × 0.10 potash = 0.38 lb potash per 1000 sq ft

Another common problem involves determining the area that a bag of fertilizer can cover and how many bags are needed to cover large sites.

Example:

How much area can be covered with a 50-lb bag of 26-5-10 at the rate of 1.0 lb nitrogen per 1000 sq ft? Now that you know 3.8 lb of 26-5-10 fertilizer will cover 1000 sq ft, determine how many times 3.8 lb goes into 50 lb. 50 lb ÷ 3.8 lb = 13.2. Now multiply 13.2 by 1000 sq ft: 13.2 × 1000 sq ft = 13,200 sq ft.

Thus, a 50-lb bag of 26-5-10 covers 13,200 sq ft at a rate of 1.0 lb nitrogen per 1000 sq ft.

Example:

How many 50-lb bags of 26-5-10 will you need to fertilize a 30,000 sq ft lawn at 1.0 lb nitrogen per 1000 sq ft? If a 50-lb bag of 26-5-10 fertilizer covers 13,200 sq ft at 1.0 lb nitrogen per 1000 sq ft, determine how many times 13,200 goes into 30,000. 30,000 ÷ 13,200 = 2.3 bags of 26-5-10 will cover 30,000 sq ft.

Occasionally, fertilizer recommendations given as lb nitrogen per 1000 sq ft must be converted to lb fertilizer per acre.

Example:

You are treating a large sports turf complex and would like to determine how many lb of a 16-8-8 fertilizer should be applied per acre if the recommendation calls for 0.75 lb nitrogen per 1000 sq ft.

First: Find out how much fertilizer will be needed per 1000 sq ft (see previous examples).
(0.75 lb nitrogen per 1000 sq ft) ÷ 0.16 = 4.7 lb fertilizer per 1000 sq ft

Second: Since there are 43,560 sq ft in an acre, multiply the amount of fertilizer needed per 1000 sq ft by 43,560, then divide by 1000. (4.7 lb fertilizer × 43,560) ÷ 1000 = 205 lb of a 16-8-8 fertilizer will be needed per acre