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Making Sense Of Fertilizer Labels & NPK Numbers

Posted By Tipnut On April 11, 2012 @ 9:11 am In Outdoor Gardening | No Comments

When looking at the various commercial fertilizers on the shelf in any home & garden center, you’ll notice they have three digits on their labels (something like 20-15-5). Unless you can properly interpret those numbers, how can you know which one is the best to use?

Potting ShedHere’s how to demystify them:

  • The first number (N) represents the percentage of Nitrogen content in the product.
  • The second number (P) represents the percentage of Phosphorus compound present.
  • The third number (K) represents the percentage of Potassium compound present.

Why the letters N-P-K? Although the letters are not typically found on the label, these are the chemical symbols for each and generally referred to by gardeners.

You may come across recommendations to use a specific “ratio” (rather than percentage) of nutrients and the NPK numbers still apply. For example, a balanced fertilizer of 5-5-5 (5% for each of the 3 nutrients) is also considered a ratio of 1-1-1 (equal parts). If the NPK was 15-5-5, the percentages would be 15%, 5%, 5% and the ratio would be 3-1-1.

What’s the difference between a 10-10-10 and a 5-5-5, aren’t they both a ratio of 1-1-1? Yes but the 10-10-10 is stronger (since the fertilizer contains a higher amount of each nutrient) so you wouldn’t need to use as much of it as a 5-5-5.

If the numbers represent percentages, why don’t they total 100%? Because these three nutrients aren’t the only ingredients, there are fillers and other minerals present as well. Filler content (such as sand) is used to dilute the strength of the fertilizer (otherwise burned plants will be the result).

Which plant benefits from what:

Leafy: (such as spinach, lettuce, cabbage) appreciates a boost of nitrogen (N). Encourages foliage growth and green color.

Fruiting: (such as tomatoes, melons, squash) appreciates a boost of phosphorus (P). Believed to also promote root and bloom growth.

Root: (such as garlic, carrots, radishes) appreciates a boost of potassium (K). Helps strengthen cellular growth (ie. strong branches) and build stress resistance (from disease, drought, etc.).

Aside from purchasing commercial fertilizers, there are also plant and animal matter you can use instead, here’s a sample:

  • Nitrogen: blood meal, cotton seed meal, fish meal, soy bean meal, grass clippings
  • Phosphorus: bone meal (steamed), rock phosphate
  • Potassium: kelp meal, wood ash, granite dust, greensand
  • Legumes: (such as beans, peas) will put nitrogen back into the soil.

When to use what? Nitrogen (N) content is beneficial at the start of the growing season, Phosphorus (P) is beneficial once the plant begins fruiting or blossoming and Potassium (K) is good to use in the Fall to help plants get ready for winter.

That’s a lot of letters, numbers, percentages and ratios to wade through, hope this made sense and you’ll be confident the next time you pick up a bag of fertilizer! :)


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