Lawn Fertilizer Calculations

"The University of Nebraska- Lincoln Extension recommends that 1.0 lb of actual Nitrogen per 1,000 sq. ft. be applied to Kentucky bluegrass lawns in spring, between April 20-May 10." Have you seen this recommendation in a NebGuide, heard it on Backyard Farmer or been told this by an Extension Educator? Did you realize that 1.0 lb of a fertilizer product does not equal 1 lb of actual Nitrogen? Have you ever wondered how to determine the amount of fertilizer needed to equal 1.0 lb of Nitrogen? It's actually a simple bit of arithmetic.

For example: Apply 1.0 lb. of Nitrogen per 1,000 sq.ft. to a 6,000 sq.ft. lawn using a 28-3-6 fertilizer product. How much fertilizer should be applied?

First, determine the percentage of nitrogen in the fertilizer product. All fertilizers express the amount of nitrogen, phosphorus and potassium they contain with three numbers listed on the front of the bag, such as 28-3-6 or 12-3-6. This expression is called the fertilizer ratio. The fertilizer's contents are always listed in the same order, i.e. nitrogen, phosphorus and potassium. So the amount of nitrogen in your fertilizer product is the first number of the fertilizer ratio. This number represents the percentage of nitrogen per pound of fertilizer. So a fertilizer with a ratio of 28-3-6 contains 28% nitrogen per pound of product.

Next, determine how many pounds of fertilizer should be applied per 1,000 sq.ft. To do this, divide the pounds of nitrogen required per 1,000 sq.ft. by the percentage of nitrogen in your fertilizer product, expressed as a decimal. So, for this example, divide 1.0 lb by 0.28. The answer is 3.57 lbs. of fertilizer are required to apply 1.0 lb of Nitrogen per 1,000 sq.ft. when using a 28-3-6 fertilizer ratio.

Finally, to determine how many pounds of product are required to treat a 6,000 sq.ft. lawn, multiply 3.57 lb. by 6. Our final answer is 21.42 lbs of fertilizer product are required to apply 1 lb of Nitrogen to 6,000 sq.ft. when using a 28-3-6 fertilizer ratio.

Do you know the square footage of your lawn? If not, then with the help of a friend or spouse, divide your property up into squares, rectangles, trapezoids, triangles or circles and measure the length and width of each. (Remember- the radius of a circle equals half it's width.) Add together the square footage for each section of your lawn to determine the total square footage. A few simple equations will help:

  • Area of a square or rectangle (sq.ft.) = width x length
  • Area of a trapezoid (sq.ft.) = height x (Length of side A + length of side B)/2
  • Area of a triangle (sq.ft.) = width x height /2
  • Area of a circle (sq.ft.) = 3.142 x radius x radius or 3.142 x R2

So remember, one pound of a fertilizer product does not equal 1 lb of actual Nitrogen. Base your fertilizer aplications on the percentage of Nitrogen in the fertilizer product you buy.