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How to Read a Fertilizer Bag Label

How to Read a Fertilizer Bag Label

10-2-5? 10-20-10? What do all the number on a fertilizer bag label mean?

For years, when Your Humble Writer saw those odd strings of numbers on a fertilizer bag label, she couldn't help but wonder what they meant. Now, after years of working the stuff into her garden soil, she understands and approves. Here's what you need to know about those numbers.

N-P-K

We've all seen numbers displayed on the front of a fertilizer bag, generally in this format: 5-10-15, 10-0-5, 10-20-10, or something of the sort. This invariably refers to the content, in percentages, of chemical compounds containing nitrogen, phosphorus, and potassium, in that order.

These are the three primary plant nutrients necessary for healthy growth. There may be other macro- or micronutrients included, from aluminum down to zinc, but they won't be in the primary number string. They'll be listed elsewhere on the label, though.

The primary number is also known as the NPK rating, because those are the accepted chemical abbreviations for each of the three elements involved.

Please note, incidentally, that the chemical compounds involved may be 100% natural in origin. By definition, a chemical is any combination of natural elements. Water is as much a chemical as trinitrotoluene (a.k.a. TNT).
Yes, But What Does It All Mean?

If you see a number like 10-20-10 on a fertilizer bag, it means that (by weight) 10% of the contents consist of a nitrogen compound, 20% is a phosphorus compound, and 10% is a potassium compound. The remaining contents are other nutrients or fillers.

So if you purchase a 50 pound bag of 10-20-10 fertilizer, it will contain five pounds of a nitrogen compound, 10 pounds of a phosphorus compound, and five pounds of a potassium compound. But don't assume you're getting all these as pure elements.

Why Not?

Nitrogen may be in elemental form as N2 -- so you do get five pounds of that -- but the phosphorus in fertilizer usually comes in the form of phosphorous pentoxide, so the actual weight of phosphorus in a 50 pound bag is 4.4 pounds.

Similarly, the potassium is almost always potassium oxide, so the bag contains 4.15 pounds of potassium by weight.

This might not seem like a big deal, but it's important to know if you need to add a specific weight of each element (rather than just a compound) per unit of garden area. Sometimes this comes in handy.

If your electronic soil tester tells you that you need to add five pounds of phosphorus to your soil per 10 cubic yards, you don't want to add five pounds of a compound that isn't pure phosphorus, or your plants will be shortchanged. You can avoid this problem if you know how to read a fertilizer bag label properly.

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