Figuring Out Fertilizer — Seattle’s Favorite Garden Store Since 1924 – Swansons Nursery

THE NUMBERS: N-P-K

The three numbers you see on most fertilizers are the analysis of essential elements, or “macronutrients” in the fertilizer and include:

• N – Nitrogen: The most basic element essential to plant life, evident in the growth of foliage.

• P – Phosphorus: Fosters flower and fruit production.

• K – Potassium: (Latin term: Kalium) Supports root development, overall vigor, and disease resistance.

The numbers are the percentage of these macronutrients within the total mix, so 6-3-4 indicates 6%-3%-4% respectively, of N-P-K. Most fertilizers include a balance of these three, but some are skewed toward one for a specific purpose. Examples are E.B. Stone® Organics Lawn Food, (10-1-4), or Espoma® Flower Tone Bloom Booster, (3-4-5).

For most (and especially newer) gardeners, we encourage using balanced fertilizers with all three macronutrients included. But there are some preferences among different plant types:

• Green, leafy plants such as lawns and salad greens are generally heavy feeders of nitrogen.

• Fruiting plants such as tomatoes, cucumbers, and melons like a balance of nutrients but need fairly frequent feeding.

• Perennials, trees and shrubs (incluing fruiting types) also prefer a balance of nutrients but at lower or less-frequent levels.

Higher numbers indicate higher concentrations of these elements. Is that better for your plants? Not necessarily. They are often found in synthetic fertilizers, such as lawn food with high nitrogen. High concentrations can bring quick, dramatic results, such as quick greening of your heavy-feeding lawn for that summer wedding. But the effect can be temporary and not so good for long-term health. If applied too heavily or not sufficiently watered in, high-number fertilizers can “burn” plants similar to salt toxicity. In most cases, it’s best to add nutrients slowly for the long term. For a healthy lawn, supplement it in fall and spring with a slower release lawn food.

So look for the proportion of the numbers, rather than their volume.

The package ingredients might also list several “micronutrients” such as calcium and magnesium. A deficiency of one or a combination of these can cause problems. When diagnosed, they can often be corrected by adding single ingredient fertilizers or supplements. If not in combination fertilizers, you can find them in specialized products such as magnesium sulfate or dolomite lime (calcium carbonate).