Meaning of the Three Fertilizer Numbers - How to Calculate it.


The Three Fertilizer Numbers N-P-K, and how to calculate them. 



So what do those numbers a bag of fertilizer really mean? If you’re like most people who try to fertilize a lawn, you've most likely been to your local garden store, bought a bag of fertilizer, dumped it in the green spreader, picked a number that sounded right, and just started pushing away.

Well, those three numbers on a bag of fertilizer actually mean something. The numbers on the bag usually looks something like 18-24-12 or 16-4-8. 

Three numbers stand for N-P-K, so, for example 18 (N)-24(P)-12 (K).  Lets discuss all three numbers, what they do, and how to calculate the amount you need.

The first number in the sequence represents Nitrogen (N). 


Nitrogen is basically what helps make your lawn green. It does this by helping the lawn make chlorophyll. Grass (just like other plants) needs energy in order to grow. They do this through photosynthesis, which is what plants do to make the energy they need to grow using the sunlight they get, along with water and carbon dioxide.  Chlorophyll is a key part of the photosynthesis process, and Nitrogen is a key part in helping make chlorophyll.  So again, we come full circle to the shorter version of the story. Nitrogen turns your grass green.

However, just like anything else, too much of a good thing can be a bad thing. Too much nitrogen will surely burn the lawn. Fast.

So how much Nitrogen is too much and how do I know that I am putting the right amount. Its easy. Kinda.

We recommend putting out about 1 lb of nitrogen for every 1000 sqft, 2-3 times a year.  On most grasses (Carpetgrass and Centipede only need about ½ lb of nitrogen for the whole year!) To figure out how much nitrogen you need to put out, take 100, and divide it by the first the number in the bag (N),  and that is how many lbs of the fertilizer product you need to put out to cover the amount of Nitrogen you want to put out (1 lb) on the grass every 1000 sq ft.

Nitrogen Calculation Example:

So lets take our 18-24-12 starter fertilizer.  100/18 (N=18, our first number of the three on the bag) =  5.55 lbs. That’s how many lbs of our starter fertilizer we will need to cover the amount of Nitrogen we desire per 1000 sqft.

Most bags of fertilizer come in 50 lb bags. So using the example above, the 50 lb bag of starter fertilizer will cover approx. 9,000 square feet of grass (50lbs/rate of 5.55 lbs of nitrogen).

Different grass types and their recommeded nitrogen rates:

Tall Fescue, Bermuda Grass, Zoysia Grass, and St.Augustine, should all get about 1 lb of nitrogen per 1000 square feet, about 2-3 times a year. So 2-3 lbs of Nitogen for the whole year.

Carpetgrass and Centipede only need about ½ pound of nitrogen for the whole year.

The second number is Phosphorus (P) 


Phosphorus helps stimulate root growth and development. It also helps the grass get through the long, hard winters and makes it less susceptible to all kinds of diseases and stress. Hence, this why you put down a higher middle number (P), when you are over seeding or “starting” a lawn. It helps the roots get going. Hence the name, “starter fertilizer” . You really only need to have a high (p) number when you are doing a renovation or new lawn. Otherwise, stick with a typical 16-4-8 or 12-4-8 fertilizer.

The third and final number is the (K), which stands for Potassium.


Potassium helps strengthen the actual blades of grass. This helps with the stresses that grass might come in contact with, like foot traffic. These stresses will have a lesser effect on the grass since the blades are a little stronger if they have proper Potassium. Potassium also helps the grass hang on to water that it might lose through the leaves. Basically proper potassium results in a tougher blade.


Now You Know

Hopefully this gives you a little better idea of what you are putting out when you actually put fertilizer in the spreader and start waltzing around the lawn. 

If it all seems to much for you, and you would rather turn the job over to one of the Lawn Care Professionals we work with, give us a call at 866.228.5324 or click this link- Lawn Care and select your region. There is also a direct region link at the top left of this page. As always, we'll pick up the phone and help you out!