Even if you make your own compost to feed your plants and amend the soil with plenty of organic matter, you need to fertilize your plants to keep them healthy. Luckily, commercial fertilizer contains guaranteed amounts of the essential nutrients that plants need.But, the abundance of different fertilizer products on the market can make it a bit challenging to pick the right fertilizer.
In this simple guide, you learn everything you need to know about fertilizer numbers, what they indicate, and how to choose the right product for your garden.
| Numbers | Fertilizer Type | Best for |
| 10-10-10 | Complete balanced fertilizer | Any stage of plant growth |
| 10-20-10 | High in phosphorus | Flowers, fruit, and roots |
| 20-10-10 | High in nitrogen | Leaf growth and color |
| 10-10-20 | High in potassium | Roots and stems |
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.
:max_bytes(150000):strip_icc()/GettyImages-136349695-9c12342f5ec044b6bdc8d80da1d62a6c.jpg "Choosing a Fertilizer Can Be Confusing—How to Select the Right Numbers for Your Plants (1)")
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.
Fast Facts
- A fertilizer with high numbers isn’t necessarily better than a fertilizer with low numbers, and vice versa.
- A fertilizer with higher numbers, such as 10-20-10, simply has a higher concentration of nutrients than a 5-10-5 fertilizer, so you will need more of it.
- Which fertilizer works better always depends on what you are fertilizing, when, and how often. Always follow the label directions.
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.
:max_bytes(150000):strip_icc()/GettyImages-1354927999-f82be57567b54239a4929be143f87d70.jpg "Choosing a Fertilizer Can Be Confusing—How to Select the Right Numbers for Your Plants (2)")
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).
Avoid Over-Fertilization
If in doubt, start with a weaker fertilizer and lower quantities. Over-fertilizing your plants, such as an over-fertilized lawn, can be just as detrimental as too little fertilizer.
The following quick checklist will help you determine which fertilizer you need:
- 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).
- 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.
- 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.
How to Fix an Over-Fertilized Lawn
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
A homeowner's guide to fertilizer. North Carolina Department of Agriculture.
Razaq, Muhammad, et al. Influence of nitrogen and phosphorous on the growth and root morphology of Acer mono.PLoS One, vol. 12, no. 2, 2017, pp. e0171321. doi:10.1371/journal.pone.0171321
Fertilizers. Clemson Cooperative Extension.
Taliman, Nissar Ahmad, et al. Effect of Phosphorus Fertilization on the Growth, Photosynthesis, Nitrogen Fixation, Mineral Accumulation, Seed Yield, and Seed Quality of a Soybean Low-Phytate Line.Plants (Basel), vol. 8, no. 5, 2019, pp. 119. doi:10.3390/plants8050119
Understanding soil pH. PennState Extension.
