Fertilizer Calculations for Attractive, Healthy Lawns

Source(s): Clint Waltz, Extension Turfgrass Specialist, The University of Georgia

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Applying the correct amount and type of fertilizer is essential in order to produce an attractive, healthy lawn.

Lawn fertilizer recommendations are generally made in pounds per 1,000 square feet of lawn area.

The square footage of a lawn area can be easily calculated. Simply walk off the length and then the width of the yard, taking 3-foot steps. Multiply the total number of steps in each dimension by 3. Then multiply the length times the width of the yard. Remember to deduct areas such as driveways and home area.

Fertilizer Calculations

If a recommendation requires a complete fertilizer, any complete fertilizer of the same ratio can be used. Example:

• 10-10-10 (1-1-1 ratio) can be used for 8-8-8.
• 5-10-15 (1-2-3 ratio) can be used for 7-14-21.

A common recommendation for turfgrasses is to apply 1 pound of nitrogen (N) per 1,000 square feet. To determine how many pounds of fertilizer it would take to supply 1 pound of N, divide the percent nitrogen of the fertilizer into 100. (Note: This is only true when working on a 1,000 square foot basis.)

Examples: How many pounds of (1) 10-10-10; (2) 12-4-8; (3) 5-10-15; and (4) 16-4-8 are needed to apply 1 pound of N per 1,000 square feet.

(1)	100	=	10 pounds of 10-10-10
–	10
(2)	100	=	8.3 pounds of 12-4-8
–	12
(3)	100	=	20 pounds of 5-10-15
–	5
(4)	100	=	6.3 pounds of 16-4-8
–	16

In cases where you are substituting complete fertilizers of different ratios, the application rate should be based on the amount of that fertilizer needed to supply the recommended amount of nitrogen. Thus, all of the above fertilizers would supply 1 pound of nitrogen at those calculated rates.

This same calculation can be applied to other fertilizer materials.

Examples:

1)	Ammonium nitrate (33% N)
–	100	=	3 lbs. of ammonium nitrate will supply 1 lb. N
–	33
(2)	20% superphosphate (P2O5)
–	100	=	5 lbs. of superphosphate will supply 1 lb. P2O5.
–	20

These rates can be converted to a per-acre basis by multiplying by 43.5. Per-acre rates can likewise be converted to per-1,000 square feet by dividing by 43.5.

• 10 pounds of 10-10-10 per 1,000 square feet equals 10 x 43.5 = 435 pounds per acre.
• 435 pounds of 10-10-10 per acre equals 435 divided by 43.5 = 10 pounds per 1,000 square feet.

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Resource(s): Lawns in Georgia

Center Publication Number: 138