What do the numbers & letters on a fertilizer bag mean?

graphic of fertilizer bag
Illustration by Connie Oswald Stofko

by Connie Oswald Stofko

The numbers and letters on a bag of fertilizer can be confusing. Let’s take a moment to learn what all this means to our Western New York gardens.

Letters on fertilizer bag

The letters on the fertilizer bag represent primary nutrients: nitrogen, phosphorus and potassium. The abbreviations come from the periodic table of elements: N is nitrogen, P is phosphorus and K is potassium.

The standard label displays the nutrients in the same order, according to this great explanation from the North Carolina Department of Agriculture and Consumer Services. Even if you don’t see the letters, the numbers you see will represent nitrogen, phosphorus and potassium, in that order.

The ingredient that’s actually in the fertilizer might be a compound rather than an element. For example, instead of just the element phosphorus (P), the fertilizer may contain the compound phosphate (P2O5).

Numbers on fertilizer bag

The numbers on a fertilizer bag are percentages. A label with 10-10-10 means that the bag contains 10 percent nitrogen, 10 percent phosphate and 10 percent potash.

That adds up to 30 percent. The other 70 percent is filler, usually sand or granular limestone, according to the page from North Carolina Department of Agriculture and Consumer Services.

What fertilizer does your garden need?

How do you know what your garden soil needs? Do a soil test. If your soil needs a nutrient, add just that nutrient.

For your lawn, there are some standard recommendations for nitrogen, but you should test your soil to see if you need phosphorus or potassium. It’s especially important to avoid phosphorus on your lawn. Don’t apply any lawn fertilizer between Dec. 1 and April 1; the grass is dormant then and doesn’t need fertilizer.

1 Comment on “What do the numbers & letters on a fertilizer bag mean?

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