How To Understand Fertilizer Numbers

Garden-ready image by garden, ronnieb/


Commercial fertilizer, whether it is organic or chemical, is required by law to be labeled with the ratio of the component ingredients. The nutrients are always nitrogen (N), phosphorus (P) and potassium, also called potash (K). The ratio is always stated in that order, NPK. All fertilizers consist of the same three basic nutrients. Fertilizer is chemical or organic, depending on the origin of the nutrients.

What the Analysis Ratio Means

Step 1

Find the three numbers on the fertilizer package. For this example, use 10-10-10. Each number represents a percentage of the total weight of the fertilizer. So this bag has 10 percent N, 10 percent P and 10 percent K. The other 70 percent is filler, usually fine sand or limestone granules. Filler makes the fertilizer easier to spread evenly.

Step 2

Understand that the NPK ratio is not always a simple breakdown, and fertilizer comes in differently sized bags. You sometimes need to know exactly how much of each nutrient is in the bag, so you can apply it at the correct rate to meet the needs of your lawn or garden. Use a calculator or pencil and paper to follow this if helpful. This example will use a 50 pound bag of 8-0-24 fertilizer. To figure the exact amount of nitrogen, multiply 50 X .08 (eight percent) which equals four. That represents four pounds of the total bag weight. Phosphorus weighs in at zero. Figure the potash. Multiply 50 X .24 (24 percent), which equals 12. This represents 12 pounds of the total bag weight.

Step 3

Crunch the numbers. Now you know that of the total 50 pounds, four pounds are nitrogen, 0 is phosphorus, and 12 pounds are potash. Only 16 pounds of the 50-pound bag are actually nutrients. Filler makes up the other 34 pounds.

Step 4

Figure a 50 pound bag of 10-10-10 fertilizer. Each nutrient is 50 X .10 (ten percent), which equals five pounds. That means 15 pounds total of actual nutrients and 35 pounds of filler.

Step 5

Figure a 25 pound bag of 10-5-5. Nitrogen is 25 X .10, which equals 2.5 pounds. Phosphorus is 25 X .05, which equals 1.25 pounds. Potash is 25 X .05, which equals 1.25 pounds. The total weight of actual nutrients in the bag is five pounds, and there are 20 pounds of filler.

Step 6

Use this method of figuring to help you compare prices. Remember, you still must have the correct nutrients to meet your soil's needs. A fertilizer ratio of 10-5-5 requires twice as much fertilizer to achieve the same concentration as one with a ratio of 20-10-10. In other words, you would set your spreader to apply the 20-10-10 at half the application rate that you would use for the 10-5-5.

Things You'll Need

  • Fertilizer
  • Calculator or pencil and paper


  • NPK definitions
  • Aspects of NPK
  • NPK explained

Who Can Help

  • Figure fertilizer rates
Keywords: understand garden fertilizer, fertilizer NPK, meaning of NPK ratio

About this Author

Fern Fischer is a freelance writer with more than 35 years' experience. Her work has been published in various print and online publications. She specializes in organic gardening, health, rural lifestyle, home and family articles. Fischer also writes about quilting and sewing, and she professionally restores antique quilts to preserve these historical pieces of women's art.

Photo by: garden, ronnieb/