How to Read a Fertilizer Label

  • Difficulty Rating: Beginner


Fertilizer bags

It’s easy to get overwhelmed by all the different lawn and garden fertilizers on the store shelves. Believe me, I understand. Here’s some basic advice about these products, and some questions to consider before buying a fertilizer:

What does your garden soil really need?

Before you start fertilizing your garden, it’s helpful to determine the condition of your soil. Over-fertilizing can cause plenty of gardening problems, plus you’re wasting your money and time.

Consider getting a soil test from your local cooperative extension office. When we moved into our house, we had our soil’s pH and nutrient levels tested. It helped us fertilize the soil properly, without wasting money on unnecessary products.


Fertilizer bag with glove


 What are those symbols “N-P-K” on fertilizer boxes?

On most commercial fertilizers you’ll see three numbers, separated by dashes. An example is 4-10-7, shown above. This is the analysis or percentage of three important plant nutrients: (N) nitrogen, (P) phosphorus, and (K) potassium.

If you purchased a 10-pound bag of fertilizer labeled 4-10-7, it would contain 4% nitrogen, 10% phosphorus, and 7% potassium. The remaining amounts would contain other nutrients, fillers and perhaps ingredients like beneficial bacteria that improve the quality of your soil.

Personally, I find it easiest to look at the ratio, and how the numbers relate to each other. In other words, a 5-10-5 fertilizer has the ratio of 1-2-1.  That information becomes more important, when you need a phosphorus-rich fertilizer to encourage flowering, or a nitrogen-rich fertilizer for leaf growth.

So, what do N-P-K actually do?

  • Nitrogen (N) stimulates vigorous growth, particularly foliage growth, among other things. Too much nitrogen, however, can cause your plants to have lush foliage but few, if any, flowers and fruit.  That’s just one reason why you want to pay attention to the fertilizer’s dosage recommendations.
  • Phosphorus (P) helps root growth, stimulates blooming and fruit development, and improves the plant’s ability to absorb water and other nutrients. Many edibles, such as tomatoes and peppers, require phosphorus in the beginning of their growing season, so they flower and produce fruit.  Older, more established plants in your garden may need smaller amounts. Follow directions for best results.
  • Potassium (K) contains different enzymes that contribute to the overall health and vigor of plants. Some soils in the United States are naturally high in this mineral and need little added. This is another reason why a soil test is a good starting point.
  • Micro nutrients: Although N-P-K are the three most essential plant nutrients, there are other micronutrients – such as iron, zinc, copper and magnesium – which are needed in smaller quantities. Your soil test will often identify any mineral-deficiency diseases. 

Is that fertilizer organic or conventional?

Fertilizers are made with either chemically-based or natural-based ingredients. Organic fertilizers will specify which ingredients are organic, and the percentage of synthetic and natural ingredients contained inside in the product.

Chemical-based fertilizers come from ingredients like ammonium nitrate, superphosphate and potassium sulfate. They often come in higher concentrations and release more quickly into the garden, unless they are slow-release products. These fertilizers can provide a solid nutrient source, but they don’t improve the soil like a natural-based fertilizer. They also can burn plants, if applied improperly.


Side pannel of fertilizer


 Natural-based fertilizers come from ingredients like fish waste, bone meal, poultry manure, compost or bat guano. When you go shopping, look for these organic ingredients in your garden fertilizers. These fertilizers tend to release slower into the garden over time, and they also contain beneficial bacteria and ingredients that encourage healthy microbial growth, improve your soil, encourage worms and add micronutrients.

These are the type of fertilizers I like to use in my garden, and I supplement these feedings with annual applications of dried manure and compost. The soil amendments also improve the quality and texture of my garden soil.

What are you growing?

Unfortunately, there is no one-size-fits-all fertilizer out there. It really depends on the condition of your soil, and what you are trying to grow.

Since flowers and many vegetables need phosphorus in the early part of their growing cycle, look for a ratio around 1-2-1. Calcium is an important micronutrient when growing tomatoes, eggplants and peppers, so look for that growing information on the label.  Lettuces and leafy greens like a nitrogen-rich fertilizer for rich, green foliage.

Most fertilizers will state right on the packaging what type of plants will benefit most from their product. The products are formulated to handle specific growing conditions for different plants, such as roses, citrus trees, bulbs, tomatoes and flowering shrubs.

Regardless of the brand you buy, always water well after applying fertilizers. Read the label instructions carefully too.  Even organic products can be dangerous if applied inappropriately.