Fertilizer Numbers Meaning: What the 3 Numbers Mean 

Correctly feeding your plants is an important part of caring for them. Without a regular dose of fertilizer your plants may struggle to grow and develop or even worse, set flowers. Consistently applying the wrong type of plant food could harm or kill your plants.

You may assume that every fertilizer does the same job, nourishing your plants. But this is not the case. There are different types of fertilizer for different purposes.

Some plant foods promote growth while others encourage flowering or fruit to develop. Other products are designed for a specific purpose, such as nourishing your lawn or boosting the growth of your houseplants.

Plant food helps to promote healthy growth and flowering.

But how do you know which plant food you should be using on your plants? The answer lies in the meaning of the fertilizer numbers which are printed on the packet.

The amount of different plant foods available can seem overwhelming, particularly if you are unsure of the meaning of the numbers. During the course of this article we will explore the meaning of fertilizer numbers as well explaining how the different combinations can benefit, or harm, your garden.

What is the Meaning of Fertilizer Numbers?

Look at a packet of fertilizer. You will see 3 numbers on the packet. Each of these 3 numbers has a specific meaning.

The 3 numbers refer to a certain nutrient. They also tell you how much of each nutrient is in the plant food mix.

The 3 numbers on the packet refer to nutrient content. Source: Photo-of-Fertilizer by Linda Barkovsky / CC 2.0

While the numbers are a percentage of the mix, they don’t add up to 100%. This is because there are other elements that also make up the mix. These can include:

• Minerals,
• Vitamins,
• Water,
• Sand,
• Natural materials.

All these materials are combined together to make a balanced, effective plant food.

Understanding the Meaning of the 3 Fertilizer Numbers

The meaning of the 3 numbers is revealed by their more formal name, the N-P-K rating.

N-P-K refers to the 3 macro-nutrients which are contained in the plant foodr. These main elements are key to healthy growth, providing the main nutrients that plants need to consistently absorb in order to grow and thrive.

The meaning of the term “complete fertilizer” is one that contains all 3 of the key nutrients.

These key N-P-K nutrients are:

• Nitrogen,
• Phosphorus,
• Potassium.

These 3 main elements are always listed in this order.

Potassium, sometimes referred to as potash, is a key plant food ingredient.

The numbers on the packet have a specific meaning. They are designed to tell you how much of each macronutrient is in the product.

The numbers also tell you what the purpose of the plant food is meaning that by looking at the numbers we can see that a product such as Down to Earth Organic Citrus Fertilizer 6-3-3 mix has more nitrogen in it than phosphorus and potassium while the Better Gro Orchid Bloom Booster has a 11-35-15 mix, meaning that it is rich in phosphorus.

Each of these key elements has a different purpose.

Nitrogen

The first of the numbers in the N-P-K rating is nitrogen. This promotes foliage and leaf development. Nitrogen levels in the soil are often depleted more quickly than phosphorus and potassium levels meaning that many products contain a higher amount of nitrogen than other elements.

Phosphorus

The second of the numbers tells you how much phosphorus is in the product. Phosphorus promotes the development of both flowers and fruit. It also encourages strong root growth. A complete plant food for vegetables or fruit plants typically has a higher phosphorus level than products designed for grass or leafy plants.

Phosphorus promotes flowering. Photo-of-Fertilizer-2 by Linda Barkovsky / CC 2.0

Potassium

The last of the 3 numbers refers to potassium levels in the plant food. Also known as potash, potassium helps plants to stay healthy and vigorous, meaning that its presence helps plants resist disease.

Potassium works by triggering certain plant enzymes and regulating the carbon dioxide intake of the plant. This is done by controlling the pores on the leaf surface, known as stomata. It is through stomata that gasses pass.

The letter K is used to denote potassium. The element’s chemical symbol, it is derived from Kalium, the Latin word for potash.

If you want to boost flowering a product high in phosphorus is usually recommended, while growers of leafy vegetables such as lettuce will be better off applying a regular dose of high nitrogen plant food.

Warning, just because a plant food is described as complete this doesn’t mean that it is an all purpose product. The amount of each element affects what the product is best used for meaning that a tomato plant food is not suitable for applying on lawns or other plants such as potatoes.

What Else is in Plant Food?

Now that we have established the meaning behind the 3 main fertilizer numbers, it is important to also understand what else is in plant food.

One of the largest components is plant materials. These are ingredients derived from plants, such as corn gluten meal, alfalfa meal, kelp meal, cottonseed meal.

Natural materials such as manure are a key part of plant food. 

Manure materials are also a large part of the product. These can be manures such as:

• pelletized poultry manure,
• cricket manure,
• bat guano,
• dehydrated cow manure,
• worm castings.

Animal by-products are components derived from by-products of food such as fish emulsion, feather meal, blood meal, bone meal and crab meal. These are also a key ingredient, providing vital nutrients and boosting growth.

Finally, many natural plant foods also contain mined minerals such as rock phosphate, crushed limestone, sulfate of potash or greensand.

When selecting a fertilizer, try to choose products which combine all these elements.

Which Product Should I Use?

Now that you know the meaning of fertilizer numbers and when and where to use each product, you must decide what type of plant food you want to apply.

In general your choice will be between a complete granular blend or a liquid plant food. Both provide a natural and balanced blend of vital nutrients.

As we have already noted, the meaning of a “Complete granular fertilizer” is one that blends all the vital NPK nutrients along with all the other necessary nutrients, trace nutrients, vitamins and other nutritious elements necessary for healthy plant growth.

Just because a product is described as complete does not mean that it is an all purpose product. It may not be suitable for use on every part of your garden.

Remember the meaning of the fertilizer numbers tells you the formulation and composition of the individual product. The composition changes depending on the purpose of the product.

Each plant food has a different mixture depending on its purpose. This means that they are often tailored for specific crops or areas of the garden. A bloom boosting plant food may have the same ingredients as Miracle-Gro Lawn Food but the amounts of each ingredient varies greatly. It is the quantity of each ingredient that dictates how the plant food should be used.

What are Liquid Plant Foods?

One of the easiest ways to nourish your plants, a liquid plant food can be easily incorporated into your watering routine.

Liquid plant foods can be mixed in watering cans and incorporated into watering routines. 

Liquid plant foods can be applied directly into the soil, enabling the plants to absorb their nutrients via roots and foliage.

Liquid products tend to work more quickly than granular plant foods. This is because plants are able to absorb liquid nutrients more quickly.

Instead of chemical liquid plant foods which are often salt based, look for organic or natural options. These include products such as:

• Fish Emulsion,
• Liquid Kelp,
• Liquid bone meal,
• Compost tea.

https://www.youtube.com/watch?v=OirUxg5xuVE

As we have already noted these are less likely to burn or scorch your plants than synthetic products. They also provide a more complex source of plant nutrition than chemical plant foods.

As well as purchasing, you can make your own liquid plant food at home.

Now that you know the meaning behind the fertilizer numbers the next step is to work out what you want the plant food for.

Why Are You Fertilizing?

Once you understand the meaning of fertilizer numbers, the next stage, before purchasing plant food, is to work out what you want it for.

A tired or underdeveloped plant has different needs to one bearing fruit. Work out what you want before you purchase. If you just want to give your growing plants a nutritious boost there are a number of DIY options, such as compost tea or manure, that may be more effective.

Tired flowers can be revived with a dose of plant food.  

If you are solving a specific problem observe the plant and work out what the trouble is before you buy and apply a product. Don’t hastily apply plant food. Applying the wrong type of plant food can do more harm than good.

Use a soil test kit to measure the nutrient balance of your soil. If you know that your soil is lacking in phosphorus, for example, you can select an appropriate plant food to amend the balance.

As well as the plant, observe the environment it is growing in. If your soil has lots of weeds it may mean that it is acidic. Weeds tend to favor an acidic soil.

Pine trees growing close by can also be a sign that your soil is high in acidity. Remember, different plants thrive in different types of soil, meaning that leafy herbs and greens thrive in acidic soils while root crops such as beets or radishes enjoy only moderate success.

Nitrogen is the one element that gardens tend to devour. Look for signs of nitrogen deficiency such as poor growth, pale green leaves. Stunted or straggly plants can also be a sign of a nitrogen deficiency in the soil. If your garden is displaying signs of nitrogen deficiency, there are a few ways that you can quickly boost nitrogen levels.

If your soil is more alkaline in nature, nitrogen rich plant foods are a good way to raise acidity levels. Amending the soil with well-composted manure from chickens, sheep or goats also boosts nitrogen levels.

Chicken manure can be used to amend the soil.

If your soil is rich in nitrogen, avoid plant food with high N numbers. Too much nitrogen harms fruit production. Instead apply products with higher P and K levels, such as a 10-20-20 product. Bone meal is a good organic option if you want a plant food rich in phosphorus while kelp meal boosts potassium.

When amending your soil and fertilizing try to use products that balance the pH levels.

When you know what your plants need, determine which plant food is appropriate. Some areas or planters may have different needs to other parts of your garden. For example, the plant food your vegetable plot requires is not the same product that you should apply to your lawn.

Apply the right type of plant food to promote healthy growth and development.  

The right plant food depends on everything from the quality or texture of your soil to how soon you want to see results. A slow release organic product is a good long term nourishment option. Chemical plant foods provide more instant results. However, they are not the best for regular applications.

Should I Use a Chemical or Organic Plant Fertilizer?

As we have already noted, plant food is a product that is made to boost plant growth and increase soil fertility.

There are two types of plant food:

• Organic,
• Chemical.

Organic plant foods are slower to act than chemical products. These work by enriching the soil which, in turn, boosts your plants. Organic products deliver long term, sustainable results.

Organic plant foods don’t need to be certified organic to be organic. The meaning of an organic plant food is simply one that is made up of living matter. Meaning that the ingredients were not necessarily grown without the use of artificial agents of pesticides.

Organic plant foods enrich the soil.  

For example, your compost tumbler can be filled with certified organic and non-certified organic vegetables. These break down and compost together meaning that if you use this compost in your garden, or make a compost tea from it, then it can be termed an organic fertilizer. This is because it is made from living matter. It is not a certified organic product.

Organic options are increasingly affordable. Some, such as manure or compost from your garden or a local dairy farm, may even be free.

Organic products use natural sources of NPK, allowing them to build up in soil. This provides plants with a long term nutritious meal and improves garden beds.

Unlike chemical products, you can use organic plant foods without fear of burning or harming your plants. The only risk is over feeding.

A University of Massachusetts study reported that synthetic types of plant food release their nitrogen within 3 to 6 weeks. Only 40-60% of this is usable by plants. In contrast an organic fish based liquid plant food releases nitrogen over the course of 15 weeks. Around 90% of this is usable by plants.

Compost is one of the best natural plant foods. A reliable way to amend your soil, compost contains a broad range of essential nutrients that supports both plant growth and feed the soil. This encourages a healthy, biologically active soil which helps to sustain plant growth in the long term.

A reliable plant food, compost can contain both organic and non-organic vegetables. 

Chemical products provide a more instant boost, instantly impacting your plant. Unlike organic products there is no long term benefit with many chemical fertilizers because they fail to nourish the soil.

In fact regular use of chemical products can deplete soil nutrients, creating unhealthy soil. This is because they destroy humus, or living matter which is present in the soil.

Without humus, soil is less likely to be rich and healthy meaning that it struggles to sustain plant growth. If you have unhealthy soil, unless you are prepared to amend it before planting, you will need to regularly use chemical products.

Additionally, synthetic products need to be applied carefully. Applying the product directly onto the plant, particularly if it hasn’t been diluted, can burn the foliage. The products can also kill the roots if applied incorrectly.

Long term use of chemical products can do more harm than good. Source: Self-Growing-fertilizer by Tom Ray / CC 2.0

For these reasons, gardeners are increasingly trying to avoid chemical or synthetic products.

The Dangers of Over Feeding

Too much fertilizer, whether it is chemical or organic, can harm your plants.

Some plants such as rosemary require very little fertilizer.  

How much food a plant requires varies depending on the plant. Some, such as aloes or rosemary, are quickly over fertilized. Additionally, some plants prefer low nutrient poor soil while others are better in rich soil. Planting specimens with similar preferences together can help to simplify plant care and feeding.

If you are uncertain how  much fertilizer to apply, remember that it is always better to under fertilizer than apply too much. You can add further nutrients and amendments at a later date.

A regular dose of organic fertilizer should slowly enrich the soil. Over time the soil becomes rich and fertile.

Adding too much plant food is impossible to correct.

Before applying any product, take the time to research your plants and growing area. Pay close attention to the meaning of fertilizer numbers. This numbers provide valuable information which can help you to decide what is the best product for your plants.