What do the numbers on the bag of fertilizer mean?

By Allyn Paul, filed under Lawn Tips.

Did you ever wonder what the 3 numbers on that bag of fertilizer represent? Yes, they are there by law, but knowing what they mean may help you better decide for yourself what’s best for your turf.
Sometimes that bag labeled “Step 1″ might actually be better suited as a “Step 4″ for your particular situation. Sure, the company that markets that bag of fertilizer would like you to think they know what’s best; but then why do they sell the exact same products in stores from New Jersey to Wyoming and down to Georgia? Aren’t soil conditions different around the country? It’s time you learned to think for yourself DIY’er!

Those 3 numbers are called the fertilizer’s “analysis” and represent the percentage of nitrogen, phosphorus and potassium that is available from the bag. (Actually, it represents the percentage of N, P2O2 and K2O that is available, but for our purposes, nitrogen, phosphorus and potassium will do.) Using the fertilizer “20-10-10″ as an example, the bag with this analysis will contain 20% nitrogen, 10% phosphorus and 10% potassium.fertilizer analysis

Now, you’re probably thinking that 20 plus 10 plus 10 only adds to 40%, so what’s in the other 60% of the bag? Depending on the brand of fertilizer, the rest of the bag may contain micro-nutrients like iron to turn KY Bluegrass, Blue and other filler material (like Urea). The filler material (in which the nutrients are suspended) allows you to evenly apply the nutrients across the area you want to fertilize. If the filler material were not a part of the mix, you could potentially over-or-under-fertilize certain areas creating un-even coloring or burning. The filler material allows for a uniform application of the nutrients.
Understanding the fertilizer analysis is essential to purchasing and applying it correctly. Let’s next explore what each element does in your lawn.

  • Nitrogen – Of the 3 major elements grass plants need, Nitrogen is the staple. This is what gives grass its rich green color and makes it grow hardily and reproduce itself by thickening up. A healthy, thick lawn will naturally resist weeds and other pests and require fewer pesticides.
  • Phosphorus – does most of its work in the root system. Strong, healthy and deep roots are able to support more blades on the top, so don’t overlook this element.
  • Potassium – works right along with the nitrogen to help toughen the roots and foliage, which enhances your lawn’s ability to resist drought, wear and tear, disease and extremes in temperature.

Typically, I will NOT use fertilizers with analysis’ like 33-3-3 or 35-0-2 (we are talking about synthetic fert here). In my opinion, these rates create way too much top growth (way too much nitrogen in this fertilizer analysis) and offer little else for overall turf health, although your lawn will be very green if you use them. I like to spend the extra money and use ratings that look like this: 20-20-10 or 20-10-10. Some companies refer to these analysis’ as “starter fertilizer,” and while they are great for new seed, they are even better for existing lawns. Sometimes, what’s healthy for the baby is also healthy for adults.

The final consideration is in knowing how much fertilizer to apply to your lawn (the ‘rate’). Most products can be applied at a 1-pound per thousand rate, which means that one pound of fertilizer is applied to a 1,000 square foot area. This is pretty tough for you to figure out at home, spreaderso I recommend you experiment a little with your broadcast spreader. Put exactly 1 pound of play sand in your spreader, measure out a 1,000 square foot area, and go spread the sand. Adjust your spreader accordingly until you can completely cover that area with the single pound of sand (each pass with the spreader overlaps the previous one back to the wheel marks). If you can do it with sand, you should be ready to actually apply the fertilizer and get pretty close to the 1-pound per thousand rate.
If you find the 20-10-10 product at 1-pound rate does not give you the green-up you desire, then up your rate to 1.75 pounds per thousand on the next go-round. After that, it’s just a matter of fertilizing on an “as needed” basis. If you irrigate your turf regularly, and you live in the Midwest like me, you may put down 5 or 6 or even 7 applications per year. If you use slow-realease organic fertilizers, then 4 or 5 will be sufficient.
On a final note, you will find that analysis in natural fertilizer products is much lower. (something like 8-4-7) This means that if you want to use a natural or organic product to fertilize your lawn, you will nee to put down as much as 17 pounds per thousand like with Milorganite (my favorite organic fertilizer) to achieve the same results that the synthetic rates will at 1-pound per thousand.


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8 Responses to “What do the numbers on the bag of fertilizer mean?”

  1. New Organics Information » What do the numbers on the bag of fertilizer mean? Says:
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    [...] Allyn Paul put an intriguing blog post on What do the numbers on the bag of fertilizer mean? [...]

  2. Andy Says:
    4Avatars v0.3.1 v0.3.1

    OK fine, so I DID learn something today even if I didn’t want to.

  3. Lawn Tips: 4 things you must do this Spring | Life and Lawns Says:
    4Avatars v0.3.1 v0.3.1

    [...] (like Scott’s or K-grow) you need to grab the 20-20-10 or 20-10-10. (more about these fertilizer numbers can be found here). If you want to use a more natural option this season, I’d recommend a hybrid formulation [...]

  4. Too Much Nitrogen will Starve Your Lawn and Ruin Your Soil | Life and Lawns Says:
    4Avatars v0.3.1 v0.3.1

    [...] Purchase products with some potassium in them too and not just a bunch of nitrogen. Look for ratings of 20-20-10 or 15-15-15 instead of ratings of 33-3-3 or 28-2-5. (more info on the numbers on the fertilizer bag here) [...]

  5. The Fool Proof Way to Fertilize Your Lawn | Life and Lawns Says:
    4Avatars v0.3.1 v0.3.1

    [...] first need to decide which kind of fertilizer you are going to use. I prefer a 20-20-10 blend for the first application of the year to help promote the main elements that support turf’s [...]

  6. Lawn Moths, Sod Webworms, and How to Control or Eliminate Them | Life and Lawns Says:
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    [...] Milorganite, and irrigate your lawn regularly to help it grow out. In the Fall, I’d recommend a granular fertilizer with a rating of 20-20-10 to help the root structure recover prior to [...]

  7. How To Rejuvenate Your Lawn This Fall, It's Easy! | Life and Lawns Says:
    4Avatars v0.3.1 v0.3.1

    [...] some 20-20-10 or 10-10-10 fertilizer to jolt the lawn’s root system. You can learn about the numbers on the bag of fertilizer here. You can learn how to apply fertilizer [...]

  8. Lawn Treatment Information for DIYers | Life and Lawns Says:
    4Avatars v0.3.1 v0.3.1

    [...] are a few more helpful articles for you: What do the numbers on the bag of fertilizer mean? Milorganite organic lawn care fertilizer information Iron applications for lawns How to apply [...]

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