Understanding NPK Numbers on Your Fertilizer Label

NPK 20-20-20+TE+Mineral potassium fulvic acid

Yes, the 3 numbers on your fertilizer bag are important! These numbers represent nutrients NPK and they play important roles in your lawn. 

When most people shop for a bag of fertilizer they usually rely on the marketing descriptions printed on the front of the bag rather than reading the small print at the bottom.

The truth is, that boring small print next to the bar code is actually the most important part.  This is usually where the ratios of NPK are.

What Do the Numbers on Fertilizer Mean?

When you look at a bag of fertilizer you will most likely see 3 numbers. For example 32-0-4. These numbers are simply a ratio.  They represent the percent of Nitrogen, Phosphorus, and Potassium in the bag.

The first number tells you the percent of Nitrogen (N). The middle number tells you the percent of Phosphorous (P), and the last number tells you the percent of Potassium (K).

If you add up the percentages from using are example above, you’ll realize that 32-0-4 only adds to a total of 36%. The remainder of the bag is filler material which is usually sand, sawdust, granular limestone, ground corn hubs, peat moss, or dirt. 

NPK Fertilizer Uses

Nitrogen, Phosphorus, and Potassium are the 3 main macro nutrients your lawn needs to be healthy.  Each play an important role:

Nitrogen (N)– Nitrogen is the most important nutrient in your lawn. It is most responsible for promoting green, lush blade growth. This green foliage allows the grass to photosynthesize and provide energy and strength to your grass. 

Phosphorus (P)– Phosphorous is responsible for promoting root growth. It’s seen in higher percentages in starter fertilizers since it helps aid in seed germination and root development.

Potassium (K)– Potassium helps the internal processes of the plants cells. It helps with photosynthesis, absorption, respiration, and protein production. This helps the lawn adapt to harsh conditions. Potassium helps protect your lawn when it heads into periods of stress such as the heat of the summer or cold winter months. It’s the most important nutrient to apply to your lawn before a period of dormancy. 

How to Know Your NPK Levels?

Test your soil! A soil test is the only real way to test for these macro nutrients.

Most do it yourselfers are intimidated by soil tests but the truth is it doesn’t have to be complicated.

A really simple, but accurate soil test is the Yard Mastery Test. This is a kit where you mail in your soil sample and it gets analyzed in a lab. You get a detailed analysis of your soil including the macro nutrients, micro nutrients, and pH.

How to Use NPK Ratios In Your Lawn Care Program?

For established, healthy lawns…

Stay balanced by applying NPK at a ratio of 4 : 1 : 2. These ratios are based on the entire season. I’m not saying every application should be in a ratio of 4 : 1 : 2. In fact, sometimes throughout the season I’ll apply mostly nitrogen. Other times I might apply mostly potassium with little nitrogen. It all balances out in the end. And it doesn’t have to be an exact science, just use these numbers as a rough gauge.

Currently, my 3 most used fertilizers throughout the season are Milorganite 6-4-0, Flagship 24-0-6, and Stress Blend 7-0-20. This combination helps me stay balanced.

For Stressed Lawns…

Look for a fertilizer with a higher Potassium level. This will help your lawn resist against disease which is important since stressed lawns are more susceptible to disease.

A product like Stress Blend 7-0-20 is great to apply for times of the year when your lawn is likely to be stressed. It is slow release and contains Potassium. This is my go to fertilizer when I’m heading into stressed lawn season, which for me in the Northeast is summer.

If your lawn is severely stressed than get to the root of the problem before applying any type of fertilizer. Adding fertilizer to a severely stressed lawn can make things worse.

For New Lawns…

You want a high middle number which is Phosphorus. Phosphorus will help your lawn build a healthy root system so it can get established. A starter fertilizer is a good choice since it contains higher Phosphorus levels.

Overall, being aware of your lawns nutritional needs can save you a lot of money.  Applying unnecessary nutrients to your lawn is not only a waste of money but it is also has negative impacts on your lawns health.

So next time your in the store picking out your fertilizer, don’t just pick out the bag with the highest 3 numbers. Instead, focus on staying balanced and applying the right fertilizer at the right time to give your lawn the nutrients it needs most during that specific time.


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