Why Are Nitrogen, Phosphorus, and Potassium in Plant Fertilizer?

Why Are Nitrogen, Phosphorus, and Potassium in Plant Fertilizer?

Whether it’s lettuces or hollyhocks, all the plants in your garden require certain essential nutrients (17 of them) to grow properly. However, you usually only have to worry about the Big 3, called the primary or macronutrients: nitrogen (N), phosphorus (P), and potassium (K).

Look at the label on any fertilizer package, and you’ll see three numbers separated by dashes, which correspond to the amounts of primary nutrients in the product. For example, one with three of the same number, 4-4-4, is called a “balanced” fertilizer because it has equal amounts of the Big 3 N-P-K (always shown in that order). A container of tomato food ($12, The Home Depot) might be labeled 2-5-3, which indicates higher amounts of P and less N and K. Why do these levels matter, and what do nitrogen, phosphorus, and potassium do for plants anyway? Here’s what you need to know.

Close up of someone mixing fertilizer into soil

What Nitrogen, Phosphorus, and Potassium Do for Plants

While all the Big 3 nutrients work together in a plant, each has some specific jobs. A simple trick for remembering what each component of N-P-K does is “head-arms-legs” for “leaves-flowers or fruit-roots.”

Nitrogen (N) gets the growth show on the road. It’s a building block for growing new stems and leaves, plus it is a necessary part of chlorophyll, which makes the leaves green and helps plants photosynthesize.

Phosphorus (P) is needed for developing flowers, fruits, and root systems.

Potassium (K) keeps roots healthy and also aids flowers and fruits. It helps plants tolerate stress, such as drought.

What Happens When Plants Don’t Get Enough N-P-K?

Annuals such as petunias and marigolds and most vegetables live out their lives during the warmer months of a single year. They’re often called “heavy feeders” because they pull a lot of N, P, and K out of the soil to fuel all their rapid growth during their short lives. Because of this, they’re usually the first types of plants to show signs that the soil is low in nutrients. So keep your eyes peeled for these symptoms:

  • Low nitrogen (N): Pale green or yellowing older leaves, undersized leaves, or short or weak stems.
  • Low phosphorus (P): Red or purple tinges to leaves that are supposed to be green or leaves with twisted or irregular shapes.
  • Low potassium (K): Lower leaves that are dead at the edges or in spots or are wilting.

A soil test will help determine which nutrients are missing if you see these symptoms. You can get a quick and rough measurement of N-P-K with an inexpensive soil test kit from a garden center. For more detailed information and guidelines on how much fertilizer to add to the soil, mail a soil test to your state Cooperative Extension service.

How to Choose the Right Fertilizer

Once you know what’s missing, you can add the proper nutrients back into the soil with fertilizer. The numbers on the fertilizer label show the percentage of a nutrient in terms of the total volume in the fertilizer. So, a bag of rose food ($8, The Home Depot) that says 12-6-10 means that it has 12% nitrogen, 6% phosphorus, and 10% potassium.

You can give a plant a nudge in the direction you want it to grow by increasing one of the primary nutrients. For instance, if your roses have grown lots of leaves but aren’t pushing out many blooms, try adding fertilizer with higher P to boost flower production but lower N, which stimulates leaf growth.

The Best Fertilizers for Your Grass Are Actually Organic—And These 8 Are Our Favorites

You have many choices for adding fertilizers to the soil, from slow-release granules to powders, liquids or even sprays you put on the leaves. All will provide what your plants need, but the important thing is to follow the label instructions so that you don’t overdo it in terms of quantity and frequency. It usually doesn’t take much to replenish your plants’ nutrients to thrive!

Most Popular Flowers & Plants in the USA

RegionPopular FlowersPopular Plants
Northeast (e.g., New York, Massachusetts)Rose, Purple Lilac, Mountain LaurelSugar Maple, American Elm, White Pine
Southeast (e.g., Florida, Georgia)Orange Blossom, Cherokee Rose, Southern MagnoliaLive Oak, Spanish Moss, Saw Palmetto
Midwest (e.g., Ohio, Illinois)Carnation, Violet, Purple ConeflowerBur Oak, Prairie Grasses, Wild Bergamot
Southwest (e.g., Texas, Arizona)Bluebonnet, Saguaro Cactus Flower, Indian PaintbrushJoshua Tree, Agave, Mesquite
West (e.g., California, Washington)California Poppy, Coast Rhododendron, BitterrootGiant Sequoia, Redwood, Manzanita
Rocky Mountain (e.g., Colorado, Montana)Rocky Mountain Columbine, Bitterroot, Indian PaintbrushBlue Spruce, Aspen, Sagebrush
Great Plains (e.g., Kansas, Nebraska)Sunflower, Goldenrod, Purple ConeflowerCottonwood, Bluestem Grasses, Buffalo Grass
Pacific Northwest (e.g., Oregon, Alaska)Oregon Grape, Forget-me-not, Pacific RhododendronDouglas Fir, Western Hemlock, Ferns

This table includes the most popular flowers and plants in the USA by region, which considers a range of botanical species, including native and widely cultivated varieties.

Most Popular Flowers & Plants in Australia

RegionPopular FlowersPopular Plants
New South WalesWaratah, Bottlebrush, Flannel FlowerEucalyptus, Acacia, Banksia
VictoriaCommon Heath, Waxflower, Pink HeathMountain Ash, Silver Wattle, Victorian Blue Gum
QueenslandCooktown Orchid, Golden Penda, Umbrella Tree FlowerMoreton Bay Fig, Macadamia Nut, Queensland Bottle Tree
South AustraliaSturt's Desert Pea, Kangaroo Paw, Eucalyptus BlossomAdelaide Blue Gum, South Australian Blue Gum, Saltbush
Western AustraliaRed and Green Kangaroo Paw, Mangles Kangaroo Paw, Swan River DaisyJarrah, Marri, Karri
TasmaniaTasmanian Blue Gum, Leatherwood Flower, Tasmanian WaratahHuon Pine, Tasmanian Oak, Myrtle Beech
Northern TerritorySturt's Desert Rose, Frangipani, Desert RoseBoab, Gidgee, Spinifex
Australian Capital TerritoryRoyal Bluebell, Australian Daisy, CorreaSnow Gum, River Red Gum, Black Mountain

This table offers a basic overview of popular flowers and plants in each Australian region, focusing on a combination of state flowers, native species, and other characteristic plants. It's important to note that specific species' popularity and prevalence can vary. This table is a simplified representation. Consulting local botanical gardens or regional horticultural societies in Australia would be ideal for more detailed and accurate information.

Charlotte Gammon

By Charlotte Gammon

Meet Charlotte Gammon, our expert and author. She's our true treasure, as she has got 20+ years of experience in gardening, winery and house design. In early 2000s, she worked for today.com magazine and was in charge of the gaardening section. Later on, Charlotte opened her own designer agency and worked as a designer and decorator. We are happy to have Charlotte with us, as she is our good friend. We value her experience and we're sure you will love the articles she created for our blog.