What are Fertilizer Numbers?

What are Fertilizer Numbers


If you love to grow plants, whether it is in your backyard, your farm or in a greenhouse, you must be aware of the fact that giving them just the water is not enough for their health. Plants need food in the form of nutrients that they can use to grow their plant parts. Gardeners and plant hobbyists do so by adding fertilizers to the soil from time to time. 

If you are a beginner, choosing the right fertilizer can get a bit confusing if you are unaware of all the numbers, terms and percentages mentioned on the fertilizer packages.

 In this article, we will help you not only in identifying the fertilizer numbers but also in understanding how to choose a number based on your plant’s needs.

What Is a Plant Fertilizer?

What are Fertilizer Numbers

Plant fertilizers contain the nutrients that are necessary for the plants to grow and bloom. Gardeners use fertilizers as additives to the soil to replenish the nutrients in the soil. All fertilizers contain macronutrients, micronutrients and some fillers.

Plants need a total of 13 nutrients in addition to oxygen, carbon and hydrogen for optimum growth.

There are three nutrients that are needed in considerable amounts namely, Nitrogen (N), Phosphorus (P) and Potassium (K). These are known as macronutrients. In addition, many fertilizers also contain micronutrients, that are needed by the plants in smaller quantities such as iron, calcium, zinc, boron etc

Plant fertilizers are only necessary to use at particular stages of plant growth and in specific calculated amounts. Using too much at the wrong time can cause more harm than good. Hence, it is necessary to understand what plant numbers are and how to use them to get the best results.

Fertilizer Numbers, What They Mean and How to Use Them to Grow Better

Understanding fertilizer labels

Whenever you visit the fertilizer aisle in your local supermarket or you search for them online, you will notice that all of the packages have some numbers and letters that might seem alien at first. But, if you recall the periodic table from school, you might recognize these letters as N for Nitrogen, P for Phosphorus and K for Potassium. These are known as macronutrients that all fertilizers contain. 

How to Read Fertilizer Numbers

All three of these are present in specific percentages inside the fertilizer. If you see that a fertilizer has a number 4-6-2 written on it, it means that the fertilizer contains 4% Nitrogen, 6% Phosphorus and 2% Potassium. This is known as the NPK ratio of a fertilizer.

Different fertilizers have different numbers of nutrient content. If the NPK numbers are indicated as 10-10-10 that means that the fertilizer has 30% of total macronutrients, an estimated 10% of micronutrients and 60% of filler content. Filler is necessary as it prevents the fertilizer from burning plant parts.

How to Use Fertilizer Numbers

Once you know how to read these numbers and identify percentages, you can use the information to calculate how much fertilizer your soil needs.

The first step to finding out the appropriate fertilizer is to have a soil test done. It will tell you the exact amount of nutrient content by weight that the soil needs. You can easily calculate the weight of nutrients from the total weight of the bag and NPK percentage.

If a 20 pound fertilizer has NPK numbers 8-4-6 it means that it has 1.6 pounds of total nitrogen (0.08×20=1.6). Based on the soil test result, you can use the amount of nitrogen phosphorus and potassium by weight to promote better growth of your plant.

What Do Plants Use the N, P, and K in the Fertilizer Numbers For?

All three nutrients, nitrogen phosphorus and potassium, serve different purposes for the plant.


Nitrogen in soil promotes green leafy growth. It is essential for the plants that produce green leaves all year long. Nitrogen is also a component of chlorophyll hence it promotes vegetation and healthier leaves and shoots.

You will need to use nitrogen for leafy vegetables like spinach, or for house plants such as pathos, fiddle leaf and monstera plant. It is also needed for grass beds to stay green all year long. But, if you use it for your fruit and vegetable plants such as tomatoes, you might get a beautiful, huge, leafy growth but no fruit.


Phosphorus is essential for cell division and new growth in plants. It is an essential nutrient needed by seedlings and flourishing plants. It also promotes root , flower and fruit growth.

It is best for root crops such as turnip, carrots and reddish etc. It is also needed by flowering and fruiting plants like tomatoes to promote fruit production.


Potassium promotes overall vigor and health of the plant. It ensures that the fruits are plump and vegetables are healthy and nutritious. It helps in carrying out and improving metabolic processes of the plants, hence ensuring healthier stems and foliage, robust roots and flavorful fruits and vegetables. If you are growing vegetables, you might want to use it regularly to replenish soil as it is used up quickly.

Which Fertilizer Numbers Are Right for Your Plants?

Fertilizer analysis

Choosing the right number of fertilizer nutrients depends upon the kind of plant you own, its stage of growth and season of the year.

You should start with a balanced fertilizer when you first plant. A ratio of 10:10:10 might be suitable to put in the soil bed as foundation. As the plant grows you might have to use different ratios.

If your plant needs to grow in size and vigor, and a foliage growth is needed, use a high nitrogen fertilizer. You need only small amounts of nitrogen as it is used up quite slowly. You can use slow release granular fertilizer for this purpose.

If your plant is in the flowering or fruiting stage of development it will need adequate levels of phosphorus so choose a nitrogen phosphorus and potassium ratio that is higher in phosphorus such as 4:6:4.

It should also be used if you notice phosphorus deficiency that can be identified by dull leaves and purplish stems. You can use phosphorus rich liquid organic fertilizers for this purpose.

Use a potassium rich fertilizer if you are growing vegetables and need an abundant harvest. It can improve the overall health of plants.

Using the right fertilizer type for your plant is also very important. If you need to replenish soil nutrients quickly, consider buying synthetic fertilizers as they act faster. Organic fertilizers are very healthy but they are slow in action and might not act in time to save your plant.

Even though the fertilizer numbers are higher in synthetic fertilizers they are cheaper than organic fertilizers so they might as well be a better investment if you are looking for nutrients only. 

What Is Low or High-Analysis Fertilizer?

Garden fertilizer guide

An analysis of fertilizer generally represents the total number of available nutrients N P and K  in the plant fertilizer.

High Analysis Fertilizer

If the NPK numbers of a plant fertilizer added together constitute more than 30% of total fertilizer bag content, it is known as a high analysis fertilizer. An example would be a fertilizer of 10-10-10 NPK ratio or 26-5-10 ratio.

Low Analysis Fertilizer

If the total added NPK content of plant fertilizer is less than 30% of fertilizer content, it is known as low analysis fertilizer. A fertilizer with a 2-4-6 ratio is low analysis fertilizer.

What Fertilizer Numbers Are Best?

Good lawn fertilizers can be used in spring with NPK 15-0-6 or 20-0-5. As grass is all but leaves, high nitrogen content will ensure healthy leaf growth through the season. As grass does not bear fruit or flowers, it does not need phosphorus.

Tree and shrub plant fertilizers can be used in NPK numbers 16-08-8 in any season to promote foliage or flower production.

For growing vegetables, an equal ratio is recommended, for example 10-10-10. You should use twice the phosphorus in harvest seasons, for example 10-20-10.

How Do You Know If Your Plants Need Fertilizer?

Your plants will start showing signs if they are malnourished. Pale yellow leaves indicate that nitrogen deficiency is present. Dark green veins on yellow leaves indicate potassium deficiency and pale green or purplish colored leaves indicate that the plant is low on phosphorus.

Additionally, If you notice blossom end rot, it is a clear indication of calcium deficiency, so you will have to buy a package with calcium.

Final Thoughts

Seeing all those numbers on the fertilizer packaging might have puzzled you in the past but you no longer have to be intimidated. Plant numbers may seem confusing at first but once you know the basics, you are good to go. It is best to educate yourself before you make a purchase and start fertilizing. After all, gardeners that know their fertilizer numbers are always equipped with the finest tools to have the best growing season ever.

Johan Perez
Johan Perez is an experienced agriculturalist with over twenty years in the field. He holds a Ph.D. in Agricultural Sciences and has contributed extensively to research on sustainable farming practices. Johan has also written for numerous agricultural periodicals, offering expert advice on farming technologies and methods. In his free time, he enjoys outdoor adventures, which often inform his professional insights into ecological agriculture.

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