- Fertilizer Numbers -
What They Really Mean

Myth #5: The fertilizer numbers on bags of fertilizer are not that important. Besides, most people have no idea what they actually mean anyway, so it couldn't matter much.

Accepted Truth Vs Common Philosophy

The truth is that the numbers on a bag of fertilizer are very important to understand. Knowing how to use them properly is the first step that separates the experienced from the inexperienced. Nitrogen management is where the rubber meets the road for the serious applicator.

Without understanding these numbers, the homeowner or professional turf managers cannot apply the correct amount of fertilizer that is needed for a particular application. Simply putting down "what feels right" without having any idea of the affect can be disastrous to your lawn.

I have come to the aid of many homeowners who had unintentionally injured or killed their lawns by applying way too much or when applied at the wrong time of year. Understanding what the numbers mean and how to use them are easier than you think.

Sadly, about the only thing many people know is that fertilizer makes grass green. The thinking is that a little fertilizer makes it green and a lot of fertilizer makes it even greener. While this flawed philosophy may actually produce a temporarily green lawn, the damage may last much longer. Applying too much can promote diseases, attract turf-feeding insects, produce growth too quickly that harms grass structure, and can actually deplete stored nutrients severely stressing the grass.

What the Fertilizer Numbers Mean

All fertilizer bags will have three numbers separated by a dash (-) usually located across the top. Most bags do not tell you what the numbers stand for in terms of nutrients. The manufacturers assume the user knows, but 80% of people do not.

The three fertilizer numbers represent the percentage of Nitrogen (N)- Phosphorus (P)- Potassium (K), in that order. These numbers will always be listed on the bag in bold writing near the top and they will always be listed in N-P-K order. 

Many people believe the numbers represent the amount of those elements in the bag. This is a partial misconception. The numbers actually represent the percentage of nutrients by the weight of the bag. An example is 24-4-12. Therefore, a 50 lb bag of 24-4-12 will contain 12 lbs of Nitrogen.

People commonly think the bag should contain 24 lbs of Nitrogen, but that is wrong. Why? Because 24% of 100 lbs is 24 lbs, but 24% of 50 lbs is 12 lbs. 

I know this is confusing so I will explain in more detail. 

Examples of Using Fertilizer Numbers

Howard Johnson FertilizerHoward Johnson Commercial fertilizer

The photo shows a bag of Howard Johnson's Commercial Fertilizer with an analysis of 28-3-10. This is one of the products I use in the fall for cool season grasses. The bag in the photo contains 28% nitrogen, 3% phosphorus, and 10% potassium.

The easiest way to show the importance of fertilizer numbers is to give examples in how they are used. Professionals and informed homeowners use these numbers to develop a good fertility plan. It is a lot easier than you might think.

Examples of numbers may look like 25-5-12 or 13-13-13 or 0-0-20 or any number of variations depending on the fertilizer formulation.

The Power of Simple Math (made easy)

Through simple math formulas and the numbers on the bag we can figure out several important things. (Don't worry, Lawn Care Academy gives you all these formulas, so it is easy as pie.) Here are two examples of what you can learn:

  • How much nitrogen and other nutrients are actually in the bag.
  • How much fertilizer to apply to put down the correct amount of nitrogen.

Here is how simple it is. using the analysis from the bag of Howard Johnson fertilizer in the photo, a 50 lb. bag that contains 28% Nitrogen actually has 14 lbs of Nitrogen.

50 lb bag X 0.28% of N = 14 lbs of nitrogen (N) in the 50 lb. bag of fertilizer.

Now get this: If a person wants to put down one pound of nitrogen per 1000 sq. ft., he now knows the 50 lb. bag will cover 14,000 sq. ft.

The Importance of the Fertilizer Numbers in Applying the Correct Amount

You can also look at it in other ways. Most lawn fertility programs will ask you to apply a certain amount of nitrogen per 1000 sq. ft. If you need to apply one pound of nitrogen (1 lbs) per 1000 sq. ft., but only have a 3000 sq. ft. lawn, then here is how you do it. It simply requires another math equation. No sweat at all.

1 lb. of N รท 28% N in bag X 100 = 3.57 lbs (rounded to 3.6 lbs) of fertilizer needed to apply 1 lb. of nitrogen (N) over 1000 sq. ft.

So, for a 3000 sq. ft. lawn you need to apply 10.8 lbs of fertilizer. You can round it to 11 pounds for 3000 sq. ft.

What If The Nitrogen In Your Bag is a Different Number?

Replace the 28% in the equation above with the percentage on your bag of fertilizer. Using the same formula you can determine the amount you want to apply. See, I told you it was easy.

As you can see, the numbers are important to anyone seeking to apply a specific amount of fertilizer.

For a complete look at how to calculate fertilizer rates, click on the link provided: Simple Formulas for Calculating Fertilizer Rates

What About the Numbers on Homemade Fertilizer

Homemade fertilizer, including raw or aged manure, have average nutrient contents that can be easily found on the internet. An example may be homemade chicken manure collected from poultry farms may have about 4% nitrogen. These and other livestock manures can be used on gardens or in lawns in the absence of small children for health reasons. Aged manure is somewhat safer that raw manure.

Processed and composted manure is much safer and easier to use since it has been heat treated to kill harmful bacteria.

What the Numbers Do Not Tell You

The fertilizer numbers alone do not tell you the type of nitrogen that is in the bag, but is just as important to know. The specific type of nitrogen is listed on the ingredients label found at the bottom of the bag or on the back.

Understanding the type of nitrogen and how it will be released is important. The bag may contain "All Mineral", meaning it is a quick release nitrogen, or it may contain a percentage of quick release and slow release nitrogen.

Slow release nitrogen will feed your lawn gradually over two to three months and is the type most recommended for home lawns. Check to see if the bag contains between 25% and 50% slow release. Slow release nitrogen can include organic forms of nitrogen and the many forms of urea nitrogen.

Quick release forms of nitrogen feeds your lawn the instant it receives moisture and may continue for one to three weeks. These often contain heavy amounts of salt that may burn your lawn if enough moisture is not received. It needs to receive a significant amount of water to not burn.

Whatever is not absorbed into the roots may leach below the root zone. More nutrients are wasted when all the nutrients are quick release fertilizers. These fertilizers have a purpose in the professional industry and many professional turf managers possess the skills needed to use them effectively.

Other helpful pages:

How to Develop a Lawn Fertility Program

Simple Formulas for Calculating Fertilizer Rates

Nitrogen Sources for a Green Lawn

Spreader Calibration Made Easy

These pages above and the others throughout Lawn Care Academy will help you get the most from your fertilizer and achieve the best results.

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