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Understanding Fertilizer Numbers - N-P-K

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James Kilkelly
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PostPosted: Wed May 02, 2007 11:40 pm    Post subject: Understanding Fertilizer Numbers - N-P-K Reply with quote

Understanding Fertilizer Numbers - N-P-K
By Daniel Overcash

Trying to determine what fertilizer to use can be an overwhelming task. Which formulation do you need for your particular situation? Should you get the bag of 7-6-17 or the bag of 0-7-30?

And what the heck do those numbers mean anyway?

It may seem intimidating, but you can figure it out. When selecting a fertilizer, the first question to answer is, "What analysis do I need?" The analysis is actually the three large numbers you see on every fertilizer label - put there by law - such as 10-20-10 or 10-10-10 or 18-46-0. These numbers represent the percentage (by weight) of the three major nutrients required for healthy plant growth, always in the same order: nitrogen-phosphorus-potassium (N-P-K). Each of these nutrients affects plant growth differently, and the formulation you select should depend on your specific gardening needs.

The first number is the percentage of nitrogen in the bag. So a bag of 24-8-4 has 24 percent total nitrogen. Nitrogen provides plants with the ability to produce more chlorophyll, which in turn allows plants to grow quickly. With each additional nitrogen application, plants will grow taller and develop a darker green color. So if you want a dark green lawn, use a fertilizer that's high in nitrogen - but then expect to mow more often.

The second number in the analysis is the percentage of phosphorus in the mix. For example, a bag of 24-8-4 would contain 8 percent phosphorus. Phosphorous aids in root development and increases flowering ability and bloom size. The fertilizer industry smartly markets high phosphorus fertilizer as "Bloom Booster." High-phosphorous fertilizer should be used when plants are being established in your garden - when sowing a new lawn or planting new trees and shrubs, for instance.

The third number represents the percentage of potassium found in the product. A bag of 24-8-4 has 4 percent potassium in the mix. Potassium has many functions: It guards the plant against diseases and aids in drought protection and cold tolerance. It also serves a role in improving root development and helps in the process of photosynthesis. You might consider using a high-potassium fertilizer at the start of winter and summer to protect crops from temperature extremes or when insects and disease have caused damage to your plants.

If you're a left-brainer, you've probably noticed that the sum of the percentages don't equal 100 percent. That's because there are other nutrients and filler product in fertilizer mixtures. This filler helps to apply the nutrients evenly over an area. So no need to double-check the math.

An experienced gardener may recognize a plant's need for fertilizer. For example, plants that are deficient in nitrogen may start turning light green or yellow. Similarly, purple foliage (on an otherwise green plant) is a telltale sign of phosphorus deficiency. The only true way to determine how much fertilizer a crop needs is to have a soil test done. Most areas offer soil samples through their agricultural dept for a small fee). A soil test ensures that the correct amount and type of fertilizer will be used on your plants.

Next time you're in the garden center selecting fertilizer, don't let the numbers intimidate you. Just consider what your plants need and match their needs to the numbers. You, and your garden, will be fine!

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Last edited by James Kilkelly on Sat Apr 24, 2010 7:45 pm; edited 1 time in total
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PostPosted: Thu May 03, 2007 9:19 am    Post subject: Reply with quote


I put down 10-10-20 on my lawn 2 years ago.... boy was I sorry I did... Had a beautiful lush lawn but had to mow every 2 days for at least 3 weeks!!! You could almost hear the grass growing....
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PostPosted: Sat Jul 14, 2007 1:28 am    Post subject: Reply with quote

Percentage by weight, not volume, in our area.

Same in your's?

M. D. Vaden of Oregon

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PostPosted: Mon May 16, 2011 3:37 pm    Post subject: Reply with quote

Good article.
Not trying to nit pick but this one
has potassium as the bloom booster?
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michael brenock
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PostPosted: Mon May 16, 2011 4:22 pm    Post subject: Reply with quote

very good article and well explained. However if the fertilizer is made or packed in U.K. then the second figure usually usually refers to the content of P2O5 (phosphorus pentoxide) the form that the plant takes up its requirements, similarly for Potash, the U.K. uses K2O (Potash as distinct from potassium.
The department of agriculture do not provide a soil testing service, there is a soil testing laboratory FBA in Lismore Co. Waterford. There is no test for Nitrogen as it fluctuates from day to day.
michael brenock horticultural advisor (retired)

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