Understanding Fertilizer Numbers: N-P-K

Daniel Overcash

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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?

To look their best, roses require nitrogen for foliage and blooms, phosphorus for good root development and proper growth, and potassium to stimulate growth. The N-P-K formulation you select for your plants depends on your specific gardening needs.

If you don’t have time to test the soil and you’re unsure of your garden’s needs, use a balanced fertilizer (such as 10-10-10), which means it has equal quantities of each main ingredient.

No matter what your plant types or climate, every garden benefits when given the proper amount of fertilizer. The key is to know which nutrients your plants need the most.

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

Photo Credit: Daniel Overcash

And what the heck do those numbers mean anyway?

It may seem intimidating, but you can figure it out. When selecting a fertilizer – whether it’s for annuals, vegetable gardens, trees and shrubs, perennials or lawns – 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 lawn 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 a new tree, 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.

Now, 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 conduct a soil test. Most states offer soil samples through their Cooperative Extension Service at no charge (or 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 on the package intimidate you. Just consider what your plants need and match their needs to the numbers. You, and your yard, will be fine!

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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.

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.

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.

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.

Do you have a question about fertilizers or fertilizing certain plants in your garden? Come to The Garden Party and post it in the Learn2Grow Forums for our expert gardeners to answer.

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