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How to Calculate Fertilizer Application


These calculations can be used if phosphorous or potassium are the only other fertilizer requirements. For example, a fertilizer description of 0-0-20 contains only potassium or 0-20-0 contains only phosphorous.

Contact your local county agricultural extension office for information on soil testing.


Never add a fertilizer to your soil that contains ingredients other than what is recommended in your soil test. For example, if nitrogen is recommended, a fertilizer with a description of 21-12-15 contains other ingredients that may not be needed and can damage your plants or soil.

Knowing how much fertilizer to add to your garden or lawn is important because over-fertilization is toxic to soil and plants. Most fertilizer application rate recommendations are given in pounds needed per 1,000 square feet. For example, a soil test may recommend that you add 3 pounds of nitrogen per 1,000 square feet in the spring. Since it's difficult to buy pure nitrogen, you usually will find a fertilizer such as nitrogen sulfate is listed as 21-0-0, meaning each pound is 21 percent nitrogen. You must know how to calculate the amount of a 21-0-0 product to fulfill the requirement of 3 pounds of nitrogen per 1,000 square feet.

How to Calculate Fertilizer Application

Read soil test or recommendation to be sure you are reading the recommended rates correctly. Occasionally, recommendations are given in 100-square-foot amounts. Too much fertilizer can damage your garden and lawn.

Calculate the square footage of your lawn or garden area by measuring two sides and multiplying the numbers. For example, if your lawn is 50 feet long and 100 feet wide multiply 50 by 100. In this example the square footage of the lawn is 5,000 square feet.

If lawn is uneven or odd shaped, mark it off in squares, calculate the square footage of each square and add together.

Divide the amount of nitrogen recommended per 1,000 square feet by the amount of nitrogen in the fertilizer mixture. For example, if the amount recommended is 3 pounds per 1,000 square feet and the amount of nitrogen in the fertilizer mixture is 21 percent, then divide 3 by .21 or 3/.21 = 14.28 ( pounds of 21 percent nitrogen fertilizer needed per 1,000 square feet).

Multiply the amount needed per 1,000 square feet by the number of square feet in your garden or lawn. In our example, 14.28 (number of pounds needed per 1,000 sq ft) x 5 (number of 1,000 square feet in 5000 square feet) = 71.42 pounds of fertilizer containing 21 percent nitrogen should be applied.

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