Fertilizer Numbers -
What Do They Mean?

Learn How Fertilizer Numbers Can Help You
Get Better Value.

Pick up a bag of fertilizer on sale. Good job. You saved some money!
Or did you…
How can you tell if you got your money’s worth? There is such a broad range of prices when you look at the fertilizer options.
What exactly did you buy, and what is its best use?

You want a good dollar value. You also want the product that will produce the best results. Understand the significance of fertilizer numbers, or N-P-K ratios, and you can have both.

A bag of fertilizer on the garden department shelf is a marketing masterpiece. Each brand promises to be the perfect match for your lawn.

Contrast that with a bag that the professionals buy. Plain and simple. It gives a formula (the fertilizer numbers) and a list of ingredients. Is that enough to make a decision? Yes… if you know what you’re looking for.

What is N-P-K, And Their Role In Fertilizer?

All fertilizers use a three number rating system, like 15-15-15, or 21-7-14. The first number represents Nitrogen, the second is Phosphorous, the third is Potassium (or Potash). These numbers are percentages of the total ingredients in the bag. (In case you forget the order, the words are alphabetical!)

Want to sound like a pro? The chemical notations of these elements are N-P-K, a very official designation for fertilizer numbers. Use this term when you shop and impress someone!

Triple 15 (15-15-15) has equal amounts of each of these nutrients, or 15% of each. (Triple 16 is almost identical.)

This is how fertilizer ratios work. As an example, use fertilizer numbers with an NPK rating of 21-7-14. Nitrogen has the highest concentration. It is 3 times the amount of phosphorous. Phosphorous is only half of the Potassium amount. Technically, this would be called a 3-1-2 ratio.

Look at any set of fertilizer ratios on a label. Notice how the numbers relate to each other by size:

• 21-7-14 is Big - Small - Medium
• 21-3-3 is Big - Small - Small
• 6-20-20 is Small - Big - Big
• 21-0-0 is Big - Zero - Zero

The N-P-K rating you want for grass typically is
Big - Small - Small. If you follow this, you see that grass needs a lot of Nitrogen and a little bit of the other stuff. That’s the essential part. If you’re willing to learn about what these nutrients do, you can vary your selection based on your specific lawn situation.

Why Do The Fertilizer Numbers Matter?

A brief description of each major nutrient will show why grass has its own preference for fertilizer.

Nitrogen:

• essential for growth of foliage;
• produces lush, tender, green leaves (or grass blades);
• deficiency results in a yellow-green color (chlorosis) and little or no growth;
• is easily flushed through the soil.

• stimulates root growth;
• hastens the maturity of plants;
• promotes development of flowers, fruits, seeds;
• deficiency can result in slow or stunted growth and purplish discoloration on leaves;
• remains in the soil quite well.

• gives vigor to tolerate changing weather conditions;
• helps resist disease;
• assists in the food manufacturing process;
• strengthens cell wall structure for strong stems;
• deficiency can cause week stems and slow growth;
• leaches from the soil, not so fast as nitrogen.

How Can This Save You Money?

Look at the current status of your lawn.

An established lawn needs primarily nitrogen, since it is not producing any fruit or flowers = Big - Small - Small. Nitrogen is the least expensive of the nutrients. Don’t buy a formula that has high phosphorous or potassium when you don’t need them.

A new lawn needs more phosphorous to produce roots. Don’t use high nitrogen because the roots are not ready to handle top growth. So use Small - Big - Big. Phosphorous and potassium are more costly, so you would not use this all the time.

Investing in the more expensive list of ingredients for a new or damaged lawn is wise. Be safe when you fertilize new sod! You’ll save the expense of doing it over if you kill the young lawn with the wrong blend. Avoid stress and later expensive problems by helping the grass mature properly.

A stressed lawn (preparing for winter or extreme heat) needs more potassium. (Big - Small - Medium) or (Big - Small - Big)
When the grass has stressful conditions to deal with, applying the more expensive potassium can prevent problems from developing or continuing. Pay now and don’t pay later.

If the fertilizer with higher numbers in the ratio is on sale, you can apply it, knowing you got a good deal. Be cautious though, about applying high phosphorous continually to a lawn area. Mature grass doesn’t need it, and the environment doesn’t either.

(Side-note: Some states restrict or eliminate the application of phosphorous on lawns, to decrease pollution from run-off. A new lawn may be exempt from restriction. Check with your local agencies.)

Is The Fertilizer Number Ratio All That Matters?

The nutrient ratio is easy to look at. It can help you decide quickly to consider or eliminate certain fertilizer blends. It is not the only factor deserving of your attention. Consider learning about the ingredient source that provides the NPK.

The way the fertilizer pellets are made can also affect the value and life-span of the nutrients. Sound too technical? This simple overview could save you time and trouble by applying the food less frequently. Follow the link to get the details: Learn About Chemical Fertilizer

Go to Top of page

Go to Lawn Fertilizer general information page

Go to Home Page from this Fertilizer Numbers page