Here's How to Understand the Numbers on Fertilizer Bags (2024)

Whether you’re trying to green up your lawn or raise award-winning pumpkins, it's important to feed your plants.

But the numbers and letters on a bag of fertilizer can be confusing. Is 10-10-10 different from 13-13-13? Which numbers are important, and which can be too much?

Unless you've done a soil test (which we recommend), you have no idea what nutrients are already in your soil—or not. Here’s how to take the mystery out of fertilizing to avoid wasting time and money.

The three numbers on every bag of fertilizer refer to the three most important nutrients for growing plants. They are, listed in order, N, P, and K. Each of those are also elements.

If you don't remember the periodic table from your high school chemistry class, it's okay. N stands for Nitrogen, P for Phosphorus, and K for potassium.

In a nutshell, the total listing of NPK numbers represent the percentage of each specific element found within a fertilizer formula. So, a fertilizer that contains 5-10-10 means it has 5 percent nitrogen, 10 percent phosphorus (phosphate), and 10 percent potassium (potash). A “complete” fertilizer contains all three.

Each nutrient plays a different role. Nitrogen boosts green leafy growth.

“It’s like the gas pedal,” says John Esslinger, a retired horticulture educator at Penn State Extension. “That’s why many lawn fertilizers are high in nitrogen to promote leafy growth.”

Phosphorus helps strong roots form. Plants lacking in phosphorus may be purple-ish and slow-growing.

Potassium helps promote vigorous growth and hardiness, so a deficit may result in wimpy fruit or spindly plants that fall prey to pests and diseases.

“Before you add anything to your lawn or garden, get a soil test,” says Esslinger. “Otherwise, you have no idea where you’re starting from and what your soil needs or doesn’t need.”

In fact, some nutrients, such as phosphorus, tend to stay in the soil. You may not want to add them every time you fertilize. And more is not always better. Not only is it a waste of money, it’s bad for the plants.

For example, too much nitrogen will grow monster-sized tomato plants, but they may not fruit. And really, then, what’s the point?

Get your soil test kit from a local garden center or your area’s coop extension service; they’re typically about $10 to $20, and you only need to do one every few years.

The master gardener at the county extension service can help you decipher the results and explain how much of each nutrient you need to apply.

A balanced fertilizer (with all the same numbers, such as 5-5-5) may be okay for many situations such as flower and vegetable gardens, but only a soil test will tell for sure.

Finally, your test also should include your soil’s pH, a measure of how acidic or alkaline your soil is.

It’s another important aspect of soil fertility. If your pH is too high or too low, your plants may not be able to use the nutrients you've applied, says Esslinger.

Based on your test results, an application of lime to raise pH or sulfur to lower pH may be recommended.

Aside from understanding the numbers, there are several other decisions to make, when it comes to buying fertilizers. Namely, you'll want to decide if organic or synthetic fertilizers are right for you, and choose between wet and dry fertilizers.

Organic food, clothing, and skincare products have a reputation for being cleaner, so you might think that organic fertilizer is better for your garden. But that's not always the case. Before you jump into buying a box of fertilizer just because it says "organic" on the packaging, it pays to know a little about what makes a fertilizer organic.

“Organic means it’s derived from animal or plant-based sources, such as manure or compost,” says Glen Harris, PhD, professor and extension agronomist at the Department of Crop and Soil Sciences at the University of Georgia.

Organic fertilizers typically include ingredients like animal manure, alfalfa, bone meal, and fish emulsion. They are distinctly different from synthetic commercial fertilizers that primarily include a concentrated formula of nitrogen, phosphorus, and potassium.

Plants primarily absorb nutrients in certain forms. While synthetic fertilizers are often ready to be absorbed, organic fertilizer formulas must typically first be broken down by soil microbes through a process called mineralization. This process converts nutrients into forms plants can use, says Harris.

Furthermore, organic fertilizers typically contain nutrients in lower concentrations. “One of the challenges with organic fertilizers is that the nitrogen is often around 3 or 4 percent,” says Clint Waltz, PhD, a turfgrass specialist at the University of Georgia’s Turfgrass Research and Education Center. “It takes a lot more of these products to get the results you want, especially on lawns. When you calculate cost, synthetics are less expensive and you don’t have to apply as much.”

Here's everything you need to know about using organic fertilizers.

The biggest difference between wet and dry fertilizers is the rate at which they are absorbed by plants.

In general, you can expect wet fertilizers to be absorbed quickly, while dry fertilizers will be absorbed over time. There are other advantages and drawbacks of both: While wet fertilizer can be simply applied to each plant, dry fertilizer must be mixed into the soil.

Dry fertilizer tends to be easier to store and lasts longer, however. And it can be easily blended together with other kinds of dry fertilizer to achieve a specific N-P-K ratio.

Whether you're fertilizing a lawn, a vegetable garden, or your houseplants, the best time to fertilize is when the plants are growing. This means that early spring through late summer, when the most growth is happening, is the absolute best time. And deep winter, when plants are dormant, is the least effective time.

If you're using dry fertilizer outdoors, it's important to avoid spreading it when plants are wet; the granules can stick to wet leaves or stalks, and burn, or damage them.

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