Barefoot Lawn | DuPont and Olympia Lawn Care Blog Lawn Care Determining fertilizer application rates

Determining fertilizer application rates

Fertilizer application rates are determined by the actual Nitrogen applied rather than the total amount of fertilizer product.  A bag of fertilizer will have a three number designation called analysis or grade.  The three numbers represent the primary nutrients Nitrogen-Phosphorus-Potassium (N-P-K). These numbers refer to the content of the bag of fertilizer in terms of percentage by weight.  Here is an example:

A bag of 20-10-10 fertilizer contains 20% Nitrogen, 10% Phosphorus, and 10% Potassium.  The remaining 60% is known as the carrier.  The carrier often contains valuable nutrients such as Sulfur or Calcium.  If the analysis has four numbers, the last number is Sulfur (N-P-K-S).

Fertilizer applications are measured by actual Nitrogen per 1000 sq. feet.  If we have a 50lb bag of (20-10-10) fertilizer, we can multiply the percentage of Nitrogen by the total weight of the fertilizer in the bag.  .20 x 50lb =10lbs actual Nitrogen per 50lb bag.

If we want to apply 1lb of actual Nitrogen per 1000 sq. ft here is the formula:

1lb Nitrogen per 1000 sq. ft.
________________________   = 5 lbs. fertilizer per 1000 sq. ft.
20% Nitrogen in bag

In our example of (20-10-10)

1/.20=5lbs fertilizer per 1000 sq. ft.


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