The numbers on the front of a container of fertilizer tell you a lot about how the fertilizer helps plants.

Fertilizer numbers: What they mean and how to use them to grow better

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Garden centers and nurseries are beautiful places, but they can also be very confusing, especially for new gardeners. Shelves filled with fertilizer choices are enough to make your head spin. What’s in all of those bags and bottles of fertilizer anyway? What do the fertilizer numbers on the package mean? And does it really matter which one you choose for your garden? Let’s take a close look at plant fertilizer numbers and how they can help you grow your best garden ever.

The best NPK ratio for your garden.

A lush, healthy garden is the result of improving the soil and feeding plants the right way.

What is a plant fertilizer?

Let’s start at the beginning. By definition, a plant fertilizer is either a chemical or a natural material added to the soil to increase its fertility and aid in plant growth. Your mother may have drenched her plants with a blue, water-soluble chemical fertilizer every week and maybe you think you should do the same. But, there’s been a major shift in thinking over the past decade when it comes to how to fertilize plants. We’ve moved away from the idea of “feeding plants” and toward the idea of “feeding soil.” Let me explain.

Organic liquid fertilizers help feed your soil, and in turn, your plants.

Organic liquid fertilizers work to feed your soil, which in turn, feeds your plants. Skip the chemical-based “blue stuff” and opt for natural fertilizers instead.

When you use naturally-derived fertilizers instead of those derived from chemical salts, your plants are provided with a much more balanced nutrient source. Natural fertilizers provide nutrition for growing plants by feeding the soil’s living organisms. In turn, these microscopic critters (most of which are fungi and bacteria) process these fertilizers, breaking them down into the nutrients plants use to grow. When we feed the soil, our plants reap the benefits.

Fertilize with compost first

Compost is one of the best plant fertilizers and soil amendments because it contains a broad diversity of nutrients essential for plant growth and acts as a great food source for soil microbes. Encouraging healthy, biologically active soil by adding compost to the garden every year is the best way to promote optimum plant growth.

Adding compost to the garden improves the health of the soil, but sometimes extra fertilizer is needed.

Adding compost is always the best option for feeding the soil, but sometimes additional fertilizer is needed.

However, there are times when our plants need more nutrition, such as when the nutrients contained in the compost are depleted or unavailable. Or, when our back or our budget can’t afford to spread wheelbarrow after wheelbarrow of compost. For those times, there are a number of easy-to-use natural fertilizers on the market that also do an excellent job of feeding the soil. These fertilizers can be added to your garden throughout the growing season. However, it’s important to understand what’s in each bag or bottle of fertilizer and how these products influence plant growth. Doing so enables gardeners to get the most bang for their buck while keeping excessive nutrients from polluting waterways and potentially harming our plants.

Every package of fertilizer has 3 fertilizer numbers on the label. Learn what they are for before using.

Every package of liquid or granular fertilizer has three fertilizer numbers on the package. They represent the N-P-K ratio.

What do fertilizer numbers mean?

When shopping for fertilizers, spend some time reading the labels. It’s important to understand the fertilizer numbers you see on the front of the package, as well as the ingredients that are used to make the fertilizer.

The three numbers you see on the label of every bagged or bottled fertilizer represent the product’s N-P-K ratio. The N in the ratio stands for nitrogen, the P stands for phosphorous, and the K stands for potassium. The fertilizer numbers found on the package represent the percentage (by weight) of these three macro-nutrients as they are found in that package. Though plants use many different nutrients to fuel their growth, these three are used in the largest amounts.

For example, a bag of fertilizer with the fertilizer numbers 10-5-10 on the label, holds 10% N, 5% P, and 10% K. The remaining 75% of the bag’s weight is carrier product.

Fertilizer packages display the ratio of nitrogen, phosphorous, and potassium on the label.

When shopping at the garden center, read the label of a fertilizer for a clear list of ingredients along with the product’s N-P-K ratio.

Organic fertilizer numbers vs synthetic fertilizer numbers

For natural-based organic fertilizers, the numbers in the N-P-K ratio are often smaller (2-3-2 or 1-1-6, for example). This is due to the fact that the label percentages are based on levels of immediately-available nutrients and many of the nutrients in natural fertilizers are not available immediately upon application. It takes some time for the soil microbes to process these nutrients and release them for plant use. This may seem like a negative when in fact, it isn’t. Instead, it means the nutrients are released slowly, over a long period of time, serving as a slow-release fertilizer over the course of many weeks.

Bonemeal is a natural fertilizer that's high in phosphorous.

Natural fertilizers offer a slow release of nutrients over a long period of time.

A study out of the University of Massachusetts noted that synthetic chemical fertilizers release their nitrogen (of which only 40-60% is actually useable by plants) within 3-6 weeks, while organic, fish-based liquid fertilizer releases its nitrogen (of which a whopping 90% is useable by plants) over the course of 15 weeks. Although the organic products may seem more expensive, you’ll actually be getting more nutrients over a longer period of time, making them more than worth the extra cash.

What do plants use the N, P, and K in the fertilizer numbers for?

It’s also important to understand how plants use these three nutrients.

Nitrogen

Nitrogen is a component of the chlorophyll molecule, and it promotes optimum shoot and leaf growth. Adding a fertilizer high in nitrogen (such as 6-2-1 or 10-5-5) to a fruiting or flowering plant, like a tomato or a petunia, will result in excessive green growth often at the expense of flower and fruit production. But adding it to a green, leafy vegetable plant, such as spinach or lettuce, makes much more sense.

Which fertilizer numbers are best for each different crop.

Green leafy vegetables, like this lettuce, use more nitrogen than fruiting or flowering crops.

Phosphorus

Phosphorous, on the other hand, is used for cell division and to generate new plant tissue. It promotes good root growth and is used to encourage fruit and flower production. Phosphorous is particularly important for root crops, like beets, carrots, and onions, as well as for encouraging flower and fruit production. That’s why fertilizers that contain bonemeal and rock phosphate are often recommended for use on root crops; both are rich in phosphorous. Choose a fertilizer that’s higher in phosphorous for plants that produce flowers, fruits (like tomatoes and cucumbers), or edible roots.

The fertilizer numbers for fruiting crops, like squash or tomatoes, should be slightly higher in phosphorous.

The fertilizer numbers for fruiting crops, like squash or tomatoes, should be slightly higher in phosphorous.

Potassium

This plant nutrient helps trigger certain plant enzymes and regulates a plant’s carbon dioxide uptake by controlling the pores on a leaf’s surface, called stomata, through which gasses pass. Potassium levels influence a plant’s heartiness and vigor.

Now that you know what the fertilizer numbers stand for, let’s look at what ingredients you might find in the bag or bottle.

What’s in a package of fertilizer?

While synthetic chemical-based fertilizers are made from salts synthesized in a factory, along with lots of filler ingredients that you won’t find on the label, natural fertilizers are made from blends of naturally occurring materials. Natural fertilizers have four main ingredient sources.

1. Plant materials
These are fertilizer ingredients derived from plants. A few examples include corn gluten meal, alfalfa meal, kelp meal, and cottonseed meal.

2. Manure materials
You may also see pelletized poultry manure, dehydrated cow manure, cricket manure, bat guano, and worm castings on the label of a natural fertilizer.

The NPK ratio of natural fertilizers varies depending on the ingredients.

Dehydrated manure is a common ingredient in natural fertilizers.

3. Animal by-products
Fertilizer components found in this category are derived from by-products of our food industry. They include things like fish emulsion, bonemeal, feather meal, blood meal, and crab meal.

4. Mined minerals
Natural fertilizers for plants may also include mined minerals, such as greensand, rock phosphate, crushed limestone, and sulfate of potash.

Using fertilizers containing a combination of these ingredients is a terrific way to feed your soil when nutrients become depleted and adding more compost isn’t an option.

The fertilizer numbers on the bag, combined with the ingredient list, tell you all you need to know about a fertilizer.

Which fertilizer numbers should you choose?

To make your decision easier, you have two basic choices when it comes to natural fertilizers for your garden.

1. Complete Granular Fertilizer Blends

There are literally dozens of different brands of complete granular fertilizer blends. Most of these products combine the assorted plant-, manure-, animal-, and mineral-based ingredients I mentioned above, in addition to others, to create a complete granular fertilizer. Depending on the brand, they may have N-P-K fertilizer numbers of 4-5-4 or 3-3-3 or something similar. What makes them “complete” is that they contain a combination of ingredients that provides some amount of all three of these nutrients, in addition to many trace nutrients, vitamins, and other “goodies.”

All of these products have different formulations and compositions, so be sure to chose appropriately according to what plants you’re growing in your garden. Some complete granular fertilizer blends are even tailored for specific crops, such as tomatoes or flowers or bulbs, and are labeled as such.

Complete organic fertilizers have all three macronutrients present.

Complete organic granular fertilizers contain some amount of all three of the primary macronutrients, N,P, and K.

2. Liquid Fertilizers

Liquid fertilizer products are absorbed into plants via both their roots and their foliage. In general, nutrients provided to plants via a liquid solution are more readily, and more rapidly, available for plant use.

Instead of turning to chemical salt-based liquid fertilizers, look for organic or natural-based liquid fertilizers to feed your garden. They’re often made of ingredients like liquid kelp, fish emulsion, liquid bonemeal, and compost “tea.” Using them means a reduced risk of fertilizer burn and a more complex source of nutrition for your plants.

Liquid fertilizers offer great nutrition for the soil and plants.

Liquid organic fertilizers are an excellent, and quickly available, nutrient source.

Know before you throw

Before you start tossing fertilizer onto your garden, make sure it’s actually needed. A simple soil test will tell you where your soil stands in regards to its nutrient content. You can purchase a soil test kit from an independent laboratory, or from a nearby land-grant university extension service if you live in the U.S. Once the existing nutrient levels are determined, calculate what’s needed by using the fertilizer numbers on the bag. Testing is a necessary practice to determine the true state of your soil. Do this every few years. A soil test also tells you about another critical aspect of soil health: the pH. Read more about the importance of optimum soil pH here.

Though starting with a soil test is always the best way to determine how much fertilizer to add, at the bare minimum, follow the application rates listed on the label of the fertilizer.

Now that you know all about fertilizer numbers and how to use them to grow a better garden, you’re sure to have your best growing season ever!

For more on caring for your garden, please visit the following articles:

Mulching basics
Organic weed control tips
Vegetable garden pest control guide
Mulch calculator
Making a perfect compost pile
Soil pH and why it’s so important

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