Not all fertilizers are the same. Here's what you should know before you buy.

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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 Three, called the primary or macro nutrients: nitrogen (N), phosphorus (P), and potassium (K). Look at the label on any package of fertilizer and you’ll see three numbers separated by dashes, which correspond to the amounts of primary nutrients in the product. One with three of the same number, 4-4-4 for example, is called a “balanced” fertilizer because it has equal amounts of the Big Three N-P-K (which are always shown in that order). A container of tomato food 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
Credit: Greg Scheidemann

What Nitrogen, Phosphorus, and Potassium Do for Plants

While all of the Big Three nutrients work together in a plant, each does have 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. 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.

If you see these symptoms, a soil test will help you figure out exactly which nutrients are missing. You can get a quick and rough measurement of N-P-K with an inexpensive soil test kit ($15, The Home Depot) 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 right 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 ($6, 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 upping 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.

You have lots of 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, both in terms of quantity and frequency. It usually doesn't take much to replenish the nutrients your plants need to thrive!


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