If you’ve ever tried your hand at gardening, chances are you’ve come across the word “fertilizer.” But what exactly is it—and why should you use it?

In simple terms, fertilizer is any substance that gives plants the nutrients they need to grow strong and healthy. These nutrients include the big three—nitrogen (N), phosphorus (P), and potassium (K)—as well as others like magnesium (Mg), calcium (Ca), sulfur (S), and trace elements like iron (Fe), zinc (Zn), and copper (Cu). These can come from natural sources like compost and manure, or from mined minerals and synthetic products.

Organic fertilizers are made from plant and animal materials—think grass clippings, leaves, kitchen scraps, or animal byproducts like manure and bone meal. On the other hand, mineral-based fertilizers like rock phosphate and potash ore are mined and sometimes refined to make nutrients more available to plants. Once refined, they’re considered synthetic.

One of the most common synthetic nutrients is nitrogen, which is produced using the Haber-Bosch process. This method pulls nitrogen from the air using natural gas and an iron catalyst—pretty cool science behind your garden’s green leaves!

Reading a Fertilizer Label

All fertilizers—whether organic or synthetic—are labeled with the nutrients they contain and instructions for proper application. The three large numbers on the label represent the percentages of nitrogen (N), phosphorus (P), and potassium (K) in the product. In this example, since the label reads 10-30-20, the first number (10) indicates that 10% of the fertilizer is nitrogen. In a 20-pound bag, this means 2 pounds of the total weight is nitrogen.

Why do my plants need fertilizer?

Most home gardens don’t exactly mimic nature. Construction often strips away topsoil, and we tend to remove leaves, twigs, and other organic matter that would naturally break down and feed the soil. So, when we clean up our yards for aesthetics, we’re also removing nutrients that plants rely on. Consider leaving the leaves—they help enrich the soil and provide shelter for beneficial critters that overwinter in garden spaces.

The types of plants you grow also matter. Lawns, especially tall fescue (the go-to grass in the Mid-Atlantic area), are nutrient-hungry. Without enough nutrients, you’ll start seeing bare patches, which can lead to erosion and even water pollution from runoff. Plus, those bare spots are prime real estate for weeds.

As fallen leaves and spent perennials decompose over the winter, they help to return nutrients to the soil and provide habitat for overwintering critters.

Vegetable gardens are another big one. Many of our favorite veggies come from regions with richer soils than what we have locally. And every time you harvest a tomato or a head of lettuce, you’re taking nutrients out of the soil. If you don’t put them back in, your plants will eventually struggle.

Tomatoes and other common garden vegetables are considered “heavy feeders”—they require large amounts of nutrients from the soil to thrive. To support healthy growth and high yields, these plants often benefit from multiple fertilizer applications throughout the growing season.

Potted plants? They’re the divas of the plant world when it comes to fertilizer. Since they grow in limited space and often in artificial soil mixes, they need regular feeding. Plus, frequent watering washes away nutrients, especially nitrogen. But don’t worry—there are fertilizers designed to help with that.

What are slow-release and controlled-release fertilizers?

These types of fertilizers are like the slow cookers of the gardening world—they release nutrients gradually over time, which means less waste and less work for you.

Slow-release fertilizers can be either organic or synthetic. They release nutrients based on things like temperature, moisture, or microbial activity. If you’re shopping for one, check the label for “water-insoluble nitrogen” (WIN)—that’s your clue that it’s slow-release.

Controlled-release fertilizers are usually synthetic and release nutrients based on soil temperature. They’re super predictable, which makes them great for planning out your garden care.

Water-soluble fertilizers are mixed with water before being applied to the soil. This method delivers nutrients quickly, giving plants an immediate boost. However, any nutrients not absorbed right away may be washed out of the soil during subsequent watering.

Slow-release fertilizers typically come in the form of small granules or pellets. These are sprinkled on the soil surface around plants or mixed into potting soil for container-grown plants. As they are exposed to heat and moisture, the pellets gradually release nutrients over time, providing a steady supply to the plant.

How do I know if my plants need fertilizer?

The best way to find out? A soil test. And don’t worry—you don’t have to be a scientist to do one. Your local cooperative extension, university, or a private lab can walk you through how to collect a sample. They’ll test it and send back a report with recommendations tailored to your plants.

For potted plants, soil testing isn’t usually practical. But your plants will often “talk” to you—just not with words. Look for signs like yellowing leaves, weird growth, or dead patches. These can be clues that your plant is missing something. Reliable online resources, especially from cooperative extensions, can help you figure out what’s going on and what to do about it.

Plants often show signs when they need a nutrient boost. Discolored leaves—such as yellowing or bronzing—are common indicators of a deficiency. To determine the cause, consider conducting a soil test or consulting a trusted professional. This can help identify whether the issue is due to a lack of nutrients or something else, like pests or disease.

Wrapping It Up

Now that you’ve got the basics down, you’re ready to make informed choices about fertilizing your plants. Whether you’re growing a lush lawn, a veggie-packed garden, or a few cheerful potted plants on your balcony, the right fertilizer can make a big difference. It’s all about giving your plants what they need—when they need it—so they can thrive.

And remember, gardening is a journey. Don’t stress if things aren’t perfect right away. With a little observation, some trial and error, and the right nutrients, you’ll be well on your way to growing healthier, happier plants. So grab your gloves, dig in, and let your garden grow!