Choosing a Fertilizer Can Be Confusing—How to Select the Right Numbers for Your Plants (2024)

The numbers on commercially produced fertilizers indicate the percentage of each main ingredient in the mixture. The first number indicates the percentage amount of nitrogen in the fertilizer, the second number indicates the amount of phosphorous, and the third number indicates the amount of potassium.

What Does NPK Stand For?

NPK in fertilizer stands for nitrogen (N), phosphorous (P), and potassium (K). These are the three main nutrients, also called primary or macronutrients, that plants need to grow healthy leaves, stems, roots, flowers, and fruit.

Nitrogen, phosphorous, and potassium can be included in equal or varied amounts depending on the intended purpose of the fertilizer.Nitrogen promotes healthy leaves and green foliage growth. Phosphorous helps roots, flowers, and fruit growth. Potassium encourages stronger stems and fast growth.

What Do Fertilizer Numbers Mean?

The actual numbers on the fertilizer label often vary slightly from the chart above. Organic fertilizers are typically less concentrated than synthetic fertilizers, so the numbers will be lower. For example, an organic bulb fertilizer with the numbers 3-5-3 contains more phosphorus than nitrogen and potassium.

Fertilizers are made with different nutrient ratios to suit specific plant needs. For many plants, a fertilizer that contains all three nutrients in equal amounts—this is known as a complete balanced fertilizer—is just fine.

Depending on the type of plant and/or its growth stage, it is important to select a fertilizer with a higher amount of one of the nutrients. For flowering or fruiting plants, a fertilizer with a high phosphorus content is often better, and for targeting leafy growth (like on a lawn), a fertilizer with mostly nitrogen is the best choice.

What Nutrients Should You Look For?

Most fertilizer contains nitrogen, phosphorous, and potassium with additional micronutrients and fillers. If you purchase a 10-10-10 fertilizer, the remaining 70 percent contains other ingredients that are necessary for the main nutrients to combine and absorb into the soil properly—plus, they provide extra benefits to your plants.

Fertilizers often contain some of the more than a dozen other nutrients. Plants only need those macronutrients in small amounts but they play an important role because deficiency causes a wide range of problems. Important micronutrients include:

• Magnesium: Both too little and too much magnesium in the soil is harmful to plants. Too much magnesium interferes with calcium uptake whereas too little can lead to reduced leaf growth, chlorosis in the leaf margins, and yellowing between the leaf veins, as well as leaf cupping.Magnesium is a macronutrient often lacking in vegetable gardens.
• Calcium: A deficiency in calcium leads to poor bud growth, dying root tips, blossom end rot, and overall weak growth.
• Copper: If the new growth on your plants is small, misshaped, or wilted, it might be due to copper deficiency.
• Zinc: This macronutrient is responsible for healthy leaf growth. Zinc deficiency shows up as small and distorted leaves as well as chlorosis.
• Manganese: Chlorosis between the leaf veins and later brown spots are a sign of manganese deficiency. Too much manganese, however, also hampers plant growth.
• Boron: Without boron, plants are unable to set seed.

How to Pick the Right Fertilizer Numbers

Before you start shopping for a fertilizer, make sure to get your soil tested. Without a soil test, you can only guess what you actually need.

A complete soil test will also tell you the pH of your soil, which is crucial because it impacts the availability of nutrients to plants. It is best to get a soil test done by a professional soil laboratory (you can often get soil test kits through your localcooperative extension office).

The following quick checklist will help you determine which fertilizer you need:

1. Match the fertilizer to your plants. Determine whether it is sufficient to feed them a complete balanced fertilizer, or whether they need a special boost for leaf growth (nitrogen) or root growth, flowering, and fruiting (phosphorus).
2. Determine whether you need liquid or dry fertilizer. While liquid fertilizer offers a quick nutrient boost, dry fertilizer will give a slow-release of nutrients. Consider which is better for your plant's needs.
3. Identify the plants with specific fertilizer needs. For example, there are specially formulated fertilizers for acid-loving plants. Or, if you’ve had problems with blossom end rot of tomatoes in the past, make sure to feed your plants tomato fertilizers with an adequate amount of calcium.
4. Carefully read the label before buying fertilizer.You might not find a fertilizer with the exact ratio recommended in your soil test but understanding what fertilizer numbers mean lets you pick a similar product.

Organic vs. Conventional Fertilizer

The final thing you'll need to consider when choosing a fertilizer is whether to buy organic or conventional. Conventional fertilizer can come from a variety of different sources, but it is made from inorganic material. They are usually fast-acting and water-soluble. While they can help give your plant a nutrient boost, they won't necessarily improve the soil quality. If you apply too much conventional fertilizer, you could risk burning/damaging your plants.

Organic fertilizer, on the other hand, can provide nutrients to plants as well as improve the soil quality. They have a lower NPK than conventional fertilizers, which means that they are slower-acting and it may take more time to see results from an organic fertilizer. However, they will not damage your plants.