Feeding Your Lawn and Garden: What Is Fertilizer and Why Do You Need It?

Ginny Bartolone
Written by Ginny Bartolone
Updated February 1, 2022
Contemporary Craftsman-style home
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  • Fertilizer contains three macronutrients known as NPK.

  • You'll find NPK ratios on bags of fertilizer.

  • Choose fast or slow-release depending on landscape goals.

  • Avoid overfertilizing to protect plants and groundwater.

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In the plant world, fertilizer is a bit like taking your daily multivitamin. The variety of grass, shrubs, flowers, and trees in your yard can all use a little help from time to time to grow faster and more resilient against what nature throws its way. 

Let's take a look at what fertilizer is made of, which types of fertilizer to use for each part of your lawn, and how to make sure you're making the most eco-conscious choice when applying it.

What Is Fertilizer and What Is It For?

Fertilizing your lawn and garden is a common and often crucial step in maintaining a healthy landscape. Its three main ingredients—nitrogen, phosphorus, and potassium (NPK)—enrich the soil and the plant itself to grow taller and stronger. 

You'll also find organic fertilizer and synthetic fertilizer. Organic comes from manure, compost, or other organic material, while synthetic includes compounds from raw materials.

So why do we need to fertilize plants in our yards while plants in the wild do just fine on their own? Lawns come under a lot of pressure from water runoff, lack of biodiversity, leaf removal, and additives like mulch that can lower a soil's nitrogen levels. Heavily compacted or clay-heavy soil may not support certain grasses or plants as well as others as well.

Home gardeners typically apply fertilizer between two and five times a year to ensure a green, lush, and disease-free yard.

NPK: The Main Ingredients of Fertilizer

woman holding fertilizer in her hands
Photo: Deidre / Adobe Stock

Let's take a deeper look at the three main ingredients of fertilizer. When you grab a bag from the garden store, you'll either choose an all-purpose variety or one specifically targeting certain types of grass, plants, or trees.

How to Decipher NPK Ratios

You'll also spot three important numbers on the front of the bag known as the NPK ratio. NPK ratios may look something like 10-10-10, 5-2-9, or 10-0-3, etc. 

The numbers line up with the percent of macronutrients in the mixtures. So, a 50-pound bag of fertilizer with a 5-5-5 NPK ratio means it is 5% nitrogen, 5% phosphorus, and 5% potassium. 

The remaining percent typically come from small amounts of micronutrients like calcium, zinc, sulfur, iron, and magnesium, as well as stabilizing fillers and microorganisms.

Why the different ratios? Too much phosphorus and nitrogen can damage a plant and add to damaging runoff into our water system. Plants also need different levels of these nutrients depending on their age, issues, and climate.

Plant illustration and description on how nitrogen, potassium, and phosphorus relate to plant health

Choosing the Right Fertilizer

man sprinkling fertilizer on lawn
Photo: ImagESine / Adobe Stock

In some cases, fortifying your healthy lawn with a slow-release, all-purpose fertilizer is just fine as long as you read the instructions on the bag and avoid overuse. However, narrowing your choice down a bit further can both boost plant growth and avoid common mistakes like patchy grass or wasted nutrients.

To cut out the guesswork, we recommend chatting with a local landscaper or requesting a soil test company in your area check the pH balance and nutrient levels in your lawn. 

Both steps provide a baseline for the NPK ratio you need and the type of fertilizer for your current goals. Let's take a deeper look at each type.

Fast Release vs. Slow Release

Fertilizers typically come in slow-release and fast-release forms. Fast-release fertilizers send out their nutrients over several weeks to maintain an already-lush lawn that needs a boost before the summer at the end of the fall.

Fast-release fertilizers step in when you need immediate care for common lawn problems—a bit like chicken soup when you have a cold. These fast-acting fertilizers release the nutrients in just a few days or weeks to battle disease, wilting grass, or enhance color.

The makeup of your chosen fertilizer also plays a hand in its release speed. Many organic fertilizers naturally release quite slowly since they are often not water-soluble.

Fertilizer for Grass

Depending on the current health of your grass, you'll likely choose an NPK with a higher nitrogen and potassium percentage and only a small touch of phosphorus. 

Too much of either, particularly in quick-release fertilizers, can burn your grass, so follow the instructions to a T. Keep in mind that warm-season grasses and cool-season grasses have individual fertilization schedules and requirements as well.

Fertilizer for Gardens

Flowering plants typically benefit from a slow-release fertilizer with something like a 3-5-4 ratio that can sit right on the leaves, as well as sink into its roots. Vegetable gardens often enjoy an initial boost from a fast-acting 10-10-10 fertilizer at the start of the season to promote seed growth.

Fertilizer for Trees

The NPK for tree fertilizers typically relies on a nitrogen-rich and concentrated formula that you must dilute before use. Trees don't always need fertilization, but a professional arborist can diagnose signs of disease, pests, and growth issues stemming from the soil.

Too Much of a Good Thing: Avoiding Overfertilization

The EPA alerts us that using too much nitrogen or phosphorus can lead to runoff detrimental to the environment. When either of these nutrients leaches into groundwater, they kill fish, create an imbalance of algae growth, and release harmful nitrogen-based compounds into the air.

For these reasons, and for the protection of your plants, it's important to stick to a fertilizing regimen. Most healthy soil only requires one to two fertilization applications a year, particularly during their high growth season at the end of spring or the start of fall, depending on the plant. 

While you may need more applications to support a lush landscape, be sure to work with a local landscaper to ensure you're not over or under-feeding your landscape with these helpful boosts of nutrients.

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