Understanding Fertilizer Ingredients and N-P-K Numbers3/26/2014

nderstanding Fertilizer Ingredients and N-P-K Numbers - courtesy of Greenview.

Guaranteed analysis

The label on all fertilizer bags is required to show the percentage by weight of nitrogen, available phosphate and soluble potash. This is called the guaranteed anaylsis of the fertilizer.

  • The first number is Nitrogen (N), which promotes overall grass shoot growth.
  • The second number is available Phosphate (Pâ‚‚Oâ‚…), which promotes strong root growth.
  • The third number is soluble Postash (Kâ‚‚O), which helps grass withstand stress, drought or disease.

For example, a 24-2-8 fertilizer has 24% nitrogen, 2% available phosphate and 8% soluble potash. Nitrogen, phosphate and potash are also sometimes referred to as N-P-K.

To understand how much of each nutrient is being applied to your lawn, you must multiply the weight of the fertilizer bag by the percentage of each nutrient. For example, a 30 lb. bag of fertilizer rated 24-2-8 contains:

N: 24% x 30 lbs = 7.2 lbs. Nitrogen
P: 2% x 30 lbs = 0.6 lbs. Available Phosphate
K: 8% x 30 lbs = 2.4 lbs. Soluble Potash


Selecting a fertilizer for nitrogen content

When it comes to assessing the nitrogen content of a lawn fertilizer, what is more important than the quantity of nitrogen is the “quality” of the nitrogen, or the form in which it is present. Most important is the amount of slow release nitrogen contained in the fertilizer. 

Choosing a fertilizer with phosphate

Some soils are high in phosphorus. Therefore, additional phosphate is not always needed. If you believe your soil is high in phosphorus, you should have your soil analyzed by your local state extension agency. Additionally, some counties mandate the use of products without phosphate. Check your local regulations to make sure that phosphate in a fertilizer product is permitted.

To find your local state extension agency, look in the yellow pages, contact your local state university. .