How to Read an NPK Ratio

Fertilizer that contains nitrogen, phosphorus or potassium must list the percentage of each ingredient by weight on the package. For example, a fertilizer labeled "21-7-14" contains 21 percent nitrogen, 7 percent phosphorus and 14 percent potassium, while a fertilizer labeled "15-0-34" contains 15 percent nitrogen, no phosphorus and 34 percent potassium. The relative amounts of nitrogen, phosphorus and potassium are collectively called the NPK ratio. Reading this ratio enables you to choose the fertilizer best suited for your lawn's needs.

Instructions

1. • 1

     Identify the lowest number in the NPK listing. For example, a NPK listing of 18-6-12 has 6 percent phosphorus as the lowest number.

   • 2

     Divide each of the other two numbers by the lowest number to find the ratio. For example, 18 divided by 6 is 3, and 12 divided by 6 is 2.

   • 3

     Place the resulting numbers back into the NPK list. For example, "18-6-12" simplifies to "3-1-2." NPK fertilizers with ratings of 6-2-4 and 30-10-20 also have a 3-1-2 ratio.