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Understanding Fertilizer Numbers: N-P-K

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Fertilizer bag with clear numbers
Photo Credit: Daniel Overcash
This bag has 34 percent nitrogen, 0 percent phosphorus and 0 potassium. It’s often used as a lawn fertilizer to promote greening and growth.
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 18-9-9 or the bag of 5-10-5?

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.

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Lawn starter fertilizer
Photo Credit: Daniel Overcash
This fertilizer is marketed as “Lawn Starter” because it has a high percentage of phosphorus to aid in root growth for newly established lawns.
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.

Warnings
  • As is often the case, too much of a good thing may result in something bad. If fertilizer is applied too heavily, it may cause the plant tissue to burn, or even result in plant death. Read product labels carefully and follow directions to avoid toxicity problems.
Tips
  • Some plants, like conifers, don’t respond well to excessive nitrogen. Without a soil test report, it is difficult to determine the exact amount of fertilizer a plant needs. As a general rule, the slower the plants habit of growth, the less fertilizer it needs. Plants that are producing fruit, or plants that that have a fast growth habit, generally need more fertilizer.
Facts
  • Denitrification can occur if temperature and humidity are high. Denitrification is the process where nitrogen is converted by soil bacteria to a gaseous state and released into the atmosphere before the plant has time to absorb it and use it. To avoid denitrification, apply fertilizer on a cool day or right before an expected rainfall.
Definitions
  • Incomplete Fertilizer: A fertilizer that contains only one or two of the three major nutrients (N-P-K). Example: 46-0-0 or 0-20-20
  • N-P-K: The elemental shorthand for the three major nutrients in fertilizers. “N” stands for Nitrogen, “P” stands for Phosphate (P2O2) and “K” stands for Potash (K2O). These are the chemical forms of nitrogen, phosphorus and potassium that are present in fertilizers. The three numbers on a fertilizer label directly correspond to these nutrients.
 
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