“Lawn Fertilizer Numbers, What They Mean to You...NPK Fertilizer ”

The fertilizer numbers are prominently displayed on all packages of grass fertilizer. The big numbers represent the ratio nitrogen, phosphorus and and potassium. These are the top three nutrients all lawns require. 

Choosing the right lawn fertilizer for your lawn is important. Selecting the wrong fertilizer for your grass can ruin your lawn, waste your hard earned money and your valuable time. It is important to first know what nutrients your lawn and soil lack, then buy the right grass fertilizer.

All fertilizer labels include the same basic information:

  • Chemical Analysis of the grass fertilizer
  • Outline of the various forms of nitrogen
  • Secondary and micro-nutrients contained in the fertilizer

From Understanding Fertilizer Numbers to Fertilizing Your Lawn

“N-P-K......Understanding the Fertilizer Numbers”

15-5-10 (Example)

This is the chemical analysis of the grass fertilizer.  

Nitrogen (N) - The first number on the package (15)

Nitrogen helps with the overall health and growth of the grass blades and roots. It also impacts the color of the grass blade and the thickness of your lawn. Lastly, it helps your grass survive environmental stresses such as heat and the winter.

Too much and too little nitrogen is bad for your lawn. Excessive amounts of nitrogen can burn your lawn, attract lawn pests and cause lawn diseases. A lack of nitrogen causes your grass to turn a yellowish color and can stunt it’s growth.

Phosphorus (P) - The second number on the package (5)

Phosphorus helps build your lawn’s root system. It also aids in the transfer of energy to the grass plant.

Excessive amounts of phosphorus can impact the ability of your grass to absorb secondary nutrients such as iron and zinc. It is rarely seen, but if the grass plant is phosphorus deficient the grass blades will have a purplish tint.

NOTE: If you live in a lake front community, phosphorus may be banned because it increases algae growth. 

Potassium (K) - The last number on the package (10)

Potassium assists with the healthy growth of your lawn’s root system. It also plays an important role in the grasses ability to withstand stresses such as cold weather, drought, foot traffic and lawn diseases.

Lawn fertilizers contain Potash (K20) which delivers potassium to your soil and lawn.  If your soil is sandy, lawn fertilizers with higher amounts of potassium are recommended.

In our example, 15-5-10 (N-P-K), the nutrients are combined in a ratio of 3 to 1 to 2.  Generally, lawns require higher levels of nitrogen than phosphorus and potassium.

When reading the fertilizer numbers you may notice the words “Guaranteed Analysis” on the package.  This is the manufacturers guarantee that the weight analysis is true.

“Lawn Fertilizer Numbers - Secondary Nutrients”

Calcium, magnesium, and sulfur are considered secondary nutrients.  There are usually sufficient quantities of these nutrients in your soil.  A soil test will help determine if your soil contains the correct levels of these elements.

If your soil’s pH level (alkaline and acid levels) is out of whack, you can use lime to add calcium and magnesium to raise your soil’s pH level. Sulfur can be added to lower the soil’s pH level.

“Lawn Fertilizer Numbers – Micro nutrients”

Iron, zinc, manganese, copper, molybdenum, boron and chlorine are usually in the soil. A soil test will show if your soil is deficient of any of these elements.

If your soil lacks iron it is pretty apparent. An iron deficient lawn will turn yellow.  This usually happens when your soil has a high pH level.  Quality grass fertilizers usually correct this problem.

From  Understanding Fertilizer Numbers to Lawn Care Home Page

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Label of Fertilizer Bag showing Levels of the 3 major nutrients: Nitrogen (N)-Phosphate(P)-Potassium/Potash(K)

Manufacturers guarantee that the levels indicated are correct.

When using any sort of chemical, always read the instructions on the back of the bag/container.