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Secret Sauce of Gardening: How to Read the Fertilizer Code

You don't have to be a chemist to figure out how to boost lawn and garden growth

spinner image man reads information on the back of a bag of lawn fertilizer
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The fertilizer aisle of the garden store might make you wish you'd paid attention in science class. But a basic understanding of fertilizer labels, along with some updated marketing by manufacturing companies, can ease the confusion.

spinner image Vegetable garden in late summer
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"The gardener of today doesn't have to be … a chemist or a mad scientist to figure out each individual thing that they need for their garden,” says Oscar Fortis, head of new business development for Dr. Earth, a California-based company that sells organic fertilizers and soils.

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That's because many companies now package fertilizers based on how they will be used, labeling them for specific plants like tomatoes or azaleas, for example. But all fertilizers are based on a proportion of three basic elements, known as the NPK:

  • nitrogen (N), to promote green growth
  • phosphorus (P), for strong roots and blooms
  • potassium, or potash (K), for overall health and structure

Somewhere on the package, because it's required by law, there will be the percentage by weight of each element. To use an example from the University of Maryland Extension, a 5-pound bag of 10-5-5 fertilizer contains .5 pounds of nitrogen, .25 pounds of phosphorus and .25 pounds of potassium. If you want a stronger lawn, you know to look for a higher proportion of nitrogen. For more blooms, pick a fertilizer with more phosphorus.

Organic fertilizers include manures, compost and bonemeal and come directly from plant or animal sources, according to the Oregon State University Extension Service. Inorganic fertilizers, such as ammonium sulfate or ammonium phosphate, are usually called commercial or synthetic fertilizers and go through a manufacturing process.

Before you put anything in your garden or on your lawn, however, first determine whether you even need fertilizer, experts say.

"No one should ever assume they have to fertilize,” says C.L. Fornari, a gardening author, speaker and consultant based on Cape Cod in Massachusetts. “And you shouldn't assume you have to fertilize every year. … Have a soil test done.”

You can use a home kit available at garden centers, but most state extension services offer a more thorough analysis, Fornari and others say, so consider getting a lab test at least every few years. The results will show how to improve your soil, so you'll spend less on fertilizer and won't harm the environment.

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"If you have soil that's already high in phosphorus or potassium, it's ludicrous to be dumping more phosphorus or potassium,” says Fornari, “because not only does that phosphorus become a pollutant in our waterways, but if you get levels of any particular element or mineral too high in your soil, you can throw the whole thing off balance. ... Your plants won't grow well or can't absorb other nutrients that are there.”

Growing healthier lawns

When it comes to lawns, Chrissie Segars, an extension turf grass specialist with Texas A&M AgriLife Extension in Dallas, says homeowners make three common fertilizer mistakes: not getting a soil test, applying too much fertilizer, and applying fertilizer at the wrong time of year.

"We want to make sure they're applying the fertilizer when the grass is actively growing,” she says. In other words, if the lawn isn't growing enough to mow, don't fertilize.

Next consider how you use your lawn. Is it ornamental or a heavily used play area? “The higher the use, the more nitrogen you'll need, because nitrogen promotes shoot growth for recovery,” she says. And think about how much you want to spend, she adds. “The more nitrogen you put down, the more you've got to mow it, right?”

Enhancing garden output

If you're growing vegetables, get a soil test at least every three years, Fornari says. “If you're growing a large vegetable garden that you are depending on for a good portion of the food that may come to your table, it's good to know, right?"

Don't assume that slow growth, stunting, pale leaves or low yields indicate a need to fertilize. These symptoms can be caused by other factors, such as crowding, low sunlight, compacted soil or pests, according to the University of Maryland Extension. And beware of overfertilizing, or you might get lush green plants but no fruit.

Do not use lawn fertilizers on gardens, according to the Texas A&M AgriLife Extension. They may be too high in nitrogen and might also include chemicals for weed control that can damage vegetables.

More fertilizer guidance

Your state extension service is the best source for questions about fertilizers and application, and may have helpful fact sheets. But here are five more tips to start.

  • Synthetic fertilizers, which are designed to act quickly, usually have higher proportions of nutrients. The NPK for Miracle-Gro Water Soluble All Purpose Plant Food, a synthetic fertilizer, for example, is 24-8-16, compared to Dr. Earth's All-Purpose Fertilizer, which is organic, at 2-2-2. You might use less of the synthetic, but because it is water-soluble, nutrients may be wasted when they leach through the soil. Fornari suggests thinking of synthetics as “fast food” — a quick hit for annual flowers or plants in containers, for example, but not as helpful as slower-acting organic materials for building the long-term quality of your soil.
  • Don't only rely on specialty labeling; check the NPK so you don't buy products you don't need. For example, Dr. Earth's organic Pansy Food has an NPK of 3-7-4. and its Flower Girl Bud and Bloom Booster is 3-9-4. Both are high in phosphorus to promote blooms and would have similar results.
  • Some organic fertilizers note on their labels that they include microbes, which help turn organic matter into plant food, and/or mycorrhiza, a fungus that can help plants pull nutrients from the soil. But Fornari says some of that is marketing. “What fertilizer companies do — and even soil companies — is throw in a broad spectrum, figuring that one or two of these will be helpful on whatever plant is there,” she says. They might be helpful, depending on the weather, the circumstances, the quality of the product and the plants, but “very possibly a lot of them won't be useful.”
  • Never use a synthetic product when the soil is still cold and the plants aren't growing, because they're not going to use it, says Fornari.
  • Finally, remember that fertilizers can make plants bigger and stronger, but not necessarily healthier. “We kind of think of fertilizer like vitamins, but that's not the way to think about fertilizer,” Fornari says. “People going with less is probably better. … Nature grows very strong plants without the help of supplements. And we homeowners need to take a hint.”

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